War Poem
The Earth’s atmosphere has changed over billions of years, but for the past 200 million years it has been much as it is today. We are, however, causing our atmosphere to change by human activity. Burning fossil fuels and deforestation are two examples of human effect on the environment.
Composition of the Earth's atmosphere
The composition of air
You need to know the proportions of the main gases in the atmosphere.
The Earth's atmosphere has remained much the same for the past 200 million years. The pie chart shows the proportions of the main gases in the atmosphere.
It is clear that the main gas is nitrogen. Oxygen - the gas that allows animals and plants to respire, and fuels to burn - is the next most abundant gas. These two gases are both elements and account for about 99% of the gases in the atmosphere.
The remaining gases, such as carbon dioxide, water vapour and noble gases such as argon, are found in much smaller proportions.
Evolution of the atmosphere
The early atmosphere
Scientists believe that the Earth was formed about 4.5 billion years ago. Its early atmosphere was probably formed from the gases given out by volcanoes. It is believed that there was intense volcanic activity for the first billion years of the Earth's existence.
The early atmosphere was probably mostly carbon dioxide, with little or no oxygen. There were smaller proportions of water vapour, ammonia and methane. As the Earth cooled down, most of the water vapour condensed and formed the oceans.
It is thought that the atmospheres of Mars and Venus today, which contain mostly carbon dioxide, are similar to the early atmosphere of the Earth.
Scientists can’t be sure about the early atmosphere and can only draw evidence from other sources: for example, volcanoes on other planets release high quantities of carbon dioxide or nitrogen and iron-based compounds which are present in very old rocks that could have formed only if there was little or no oxygen.
Changes in the atmosphere
So how did the proportion of carbon dioxide in the atmosphere go down, and the proportion of oxygen go up?
The proportion of oxygen went up because of photosynthesis by plants.
The proportion of carbon dioxide went down because: * it was locked up in sedimentary rocks such as limestone, and in fossil fuels * it was absorbed by plants for photosynthesis * it dissolved in the oceans.
The burning of fossil fuels is adding carbon dioxide to the atmosphere faster than it can be removed. This means that the level of carbon dioxide in the atmosphere is increasing.
A simple carbon cycle
The level of carbon dioxide in the atmosphere is maintained by several processes, including photosynthesis, respiration and combustion.
Green plants remove carbon dioxide from the atmosphere by photosynthesis. Living organisms - including all plants and animals - release energy from their food using respiration. Respiration and combustion both release carbon dioxide into the atmosphere.
These processes form a carbon cycle in which the proportion of carbon dioxide in the atmosphere remains about the same. The animation should help you to understand how the cycle works. (Note that you do not need to know about decomposition and fossilisation.)
Volcanic activity also increases the amount of carbon dioxide in the atmosphere.
Human influences
Carbon dioxide is produced by burning fossil fuels. Increased energy consumption is leading to a rise in the use of fossil fuels, which in turn increases the release of carbon dioxide into the atmosphere.
The rising human population is adding to atmospheric carbon dioxide in other ways too. When land is cleared for timber and farms (deforestation), there are fewer trees to remove carbon dioxide from the atmosphere by photosynthesis. If the fallen trees are burned or left to rot, additional carbon dioxide is released into the atmosphere. This is particularly important when forests are cleared to make way for farms: not only are there then fewer trees to absorb carbon dioxide, butthe burning of the trees releases carbon dioxide.
Rocks are classified (organised) into three main groups: igneous rocks, sedimentary rocks and metamorphic rocks. This classification is based on how they were formed and their characteristics.
Igneous rock
Igneous rocks are formed by magma from the molten interior of the Earth. Whenmagma erupts it cools to form volcanic landforms. When it cools inside the Earth it forms intrusive rock, which may later be exposed by erosion and weathering. Intrusive rock will have large crystals as it has cooled slowly. Magma that has cooled on the surface is known as extrusive rock. This will have small crystals as it has cooled quickly.
Examples of igneous rocks include basalt and granite.
Examples of igneous rocks Type of rock | Example | Basalt | Basalt | Granite | Granite |
Sedimentary rocks
Formation of sedimentary rocks
A river carries, or transports, pieces of broken rock as it flows along. When the river reaches a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks are deposited. The deposited rocks build up in layers, called sediments. This process is called sedimentation.
The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. The water is squeezed out from between the pieces of rock and crystals of different salts form.
The crystals form a sort of glue that sticks or cements the pieces of rock together. This process is called cementation.
These processes eventually make a type of rock called sedimentary rock. It may take millions of years for sedimentary rocks to form.
These are the different processes in order: sedimentation → compaction → cementation
Sedimentary rock formation
Deposited rocks build up to form sediments
The weight of each additional sedimentary layer causes compaction
The pieces of rock stick together through the process of cementation
Examples of sedimentary rock
Sedimentary rocks contain rounded grains in layers. Examples of sedimentary rock are:
Sedimentary rocks like sandstone have layers * chalk * limestone * sandstone * shale.
The oldest layers are at the bottom and the youngest layers are at the top. Sedimentary rocks may contain fossils of animals and plants trapped in the sediments as the rock was formed. Sedimentary rocks are often quite soft and are susceptible to erosion.
Metamorphic rocks
Formation of metamorphic rock
Slate quarry, Cullipool
Metamorphic rock has been subjected to tremendous heat and / or pressure, which caused it to change into another type of rock. It is usually resistant to weathering and erosion and is therefore very hard wearing.
Examples of metamorphic rock
Examples include marble - which originates from chalk or limestone, slate - which originates from clay, and schists formed from sandstone or shale (sedimentary rocks).
Limestone is mainly calcium carbonate, CaCO3. When heated, it breaks down to form calcium oxide and carbon dioxide. Calcium oxide reacts with water to produce calcium hydroxide.
Limestone and its products have many uses: for example, in mortar, cement, concrete and glass.
Limestone, chalk and marble
Limestone, chalk and marble are all forms of calcium carbonate. They exist naturally in the Earth’s crust. Limestone is a very common building material and many tonnes are quarried in the UK every year. It is used for building - making concrete and cement - and the manufacture of glass, steel and iron.
Advantages and disadvantages of various building materials
Limestone, cement and mortar slowly react with carbon dioxide dissolved in rainwater, and wear away. This damages walls made from limestone, and it leaves gaps between bricks in buildings. These gaps must be filled in or "pointed". Pollution from burning fossil fuels makes the rain more acidic than it should be, and this rain makes these problems worse.
Concrete is easily formed into different shapes before it sets hard. It is strong when squashed, but weak when bent or stretched. However, concrete can be made much stronger by reinforcing it with steel. Some people think that concrete buildings and bridges are unattractive.
Glass is usually brittle and easily shattered, but toughened glass can be used for windows. While glass is transparent and so lets light into a building, the use of too much glass can make buildings very hot in the summer.
Uses of calcium oxide
Limestone can be broken down using heat to produce calcium oxide, which has lots of uses. * Add water and a vigorous exothermic reaction takes place. This forms calcium hydroxide. * Calcium hydroxide is soluble in water and forms a solution known as limewater which is used to test for carbon dioxide. * Calcium oxide, calcium hydroxide and calcium carbonate can be used to neutralise soil acidity. * Calcium carbonate can be used to remove acidic gases from coal-fired power station chimneys reducing harmful emissions and acid rain.
Quarrying
The limestone industry: environmental, social and economic considerations
Quarrying limestone is big business but the need for limestone has to be balanced against the economic, environmental and social effects. Some factors that have to be considered include: * effect on employment – increased job opportunities * pollution – noise, sound and air * traffic levels * visual effects of having a quarry
You need to be able to evaluate some of the effects of the limestone industry.
The main advantages and disadvantages of the limestone industry Advantages | Disadvantages | Limestone is a valuable natural resource, used to make things such as glass and concrete. | Limestone quarries are visible from long distances and may permanently disfigure the local environment. | Limestone quarrying provides employment opportunities that support the local economy in towns around the quarry. | Quarrying is a heavy industry that creates noise and heavy traffic, which damages people's quality of life. |
Thermal decomposition
Metal carbonates such as calcium carbonate break down when heated strongly. This is called thermal decomposition. Here are the equations for the thermal decomposition of calcium carbonate: calcium carbonate calcium oxide + carbon dioxide
CaCO3 CaO + CO2
Other metal carbonates decompose in the same way. Here are the equations for the thermal decomposition of copper carbonate: copper carbonate copper oxide + carbon dioxide
CuCO3 CuO + CO2
Notice that in both examples the products are a metal oxide and carbon dioxide. The carbon dioxide gas can be detected using limewater. Limewater turns cloudy white when carbon dioxide is bubbled through it.
Metals high up in the reactivity series - such as calcium - have carbonates that take a lot of energy to decompose them. Metals low down in the reactivity series - such as copper - have carbonates that are easily decomposed. This is why copper carbonate is often used at school to show these reactions. It is easily decomposed, and its colour change, from green copper carbonate to black copper oxide, is easy to see.
Atoms are the smallest particle of an element. A chemical reaction involves atoms in the reactants being rearranged to form the products. No material is lost or gained. A precipitation reaction is a good way of showing this conservation of mass.
Conservation of mass
Atoms are the smallest particles of an element that can take part in a chemical reaction. During any chemical reaction no particles are created or destroyed: the atoms are simply rearranged from the reactants to the products. The products may have different properties to the reactants.
Mass is never lost or gained in chemical reactions. We say that mass is alwaysconserved. In other words, the total mass of products at the end of the reaction is equal to the total mass of the reactants at the beginning.
This fact allows you to work out the mass of one substance in a reaction if the masses of the other substances are known. For example:
In practice, it is not always possible to get all the calculated amount of product from a reaction because: * reversible reactions may not go to completion * some product may be lost when it is removed from the reaction mixture * some of the reactants may react in an unexpected way.
Precipitation reactions
A simple example of conservation of mass is a precipitation reaction. Transition metals form coloured compounds with other elements. Many of these are soluble in water, forming coloured solutions. If sodium hydroxide solution is then added, a transition metal hydroxide is formed. These are insoluble. They do not dissolve but instead form solid precipitates. As all the reactants and products remain in the sealed reaction container then it is easy to show that the total mass is unchanged.
Here are the equations for copper sulfate solution reacting with sodium hydroxide solution: copper sulfate + sodium hydroxide → copper hydroxide + sodium sulfate
CuSO4 + 2NaOH → Cu(OH)2 + Na2SO4
(blue solution + colourless solution → blue precipitate + colourless solution)
Copper solutions form a blue precipitate with sodium hydroxide
Copper solutions form a blue precipitate with sodium hydroxide.
Different transition metals form different coloured precipitates.
Some common transition metals and the colours of their precipitates. transition metal | colour of precipitate | iron(II) | green - turns orange-brown when left standing | iron(III) | orange-brown | copper | pale blue | zinc | white |
Example of a precipitation reaction
How could you tell if an unknown substance contained iron(II) nitrate or iron(III) nitrate? You would add a few drops of sodium hydroxide solution. If you got a dark green precipitate it would show that the unknown substance was iron(II) nitrate; if you got an orange-brown precipitate it would show that the unknown substance was iron(III) nitrate.
Acids have a variety of applications for the industrial and domestic markets. Acids can be neutralised using an alkali or base and used to make salts.
What are acids?
Corrosive Irritant
All acids: * have a low pH (1-6) – the lower the number the stronger the acid * react with bases to form neutral compounds * are corrosive when they are strong * are an irritant when they are weak.
Indigestion remedies
Hydrochloric acid is used in the body to help digestion and kill bacteria.
However too much acid can cause indigestion and we use indigestion remedies to neutralise excess acids.
An indigestion remedy contains a base such as magnesium hydroxide, which reacts to form a neutral compound and raises the pH of the stomach.
Neutralisation
You need to be able to describe the reactions of hydrochloric acid and sulfuric acid with metal hydroxides, metal oxides and metal carbonates.
Metal hydroxides
Metal hydroxides, such as sodium hydroxide, usually dissolve in water to form clear, colourless solutions. When an acid reacts with a metal hydroxide, the only products formed are a salt plus water. Here is the general word equation for the reaction: acid + metal hydroxide → a salt + water
You usually observe these things during the reaction: * there is a temperature rise * the pH of the reaction mixture changes
Metal oxides
Some metal oxides, such as sodium oxide, dissolve in water to form clear, colourless solutions. Many of them are not soluble in water, but they will react with acids. Copper(II) oxide is like this. When an acid reacts with a metal oxide, the only products formed are a salt plus water. Here is the general word equation for the reaction: acid + metal oxide → a salt + water
You usually observe the same things during the reaction that you observe with metal hydroxides.
Metal carbonates
Although sodium carbonate can dissolve in water, most metal carbonates are not soluble. Calcium carbonate (chalk, limestone and marble) is like this. When an acid reacts with a metal carbonate, the products formed are a salt plus water, but carbon dioxide is also formed. Here is the general word equation for the reaction: acid + metal carbonate → a salt + water + carbon dioxide
You usually observe bubbles of gas being given off during the reaction. You can show that the gas is carbon dioxide by bubbling it through limewater: this turns cloudy white when it reacts with carbon dioxide.
Naming salts
The name of a salt is in two parts: * The first part of the name comes from the metal in the metal oxide, hydroxide or carbonate. * The second part of the name comes from the acid used to make it. The names of salts made from hydrochloric acid end in chloride, while the names of salts made from sulfuric acid end in sulfate.
How salts are named metal involved | acid | salt | sodium hydroxide reacts with | hydrochloric acid | to make sodium chloride | potassium hydroxide reacts with | sulfuric acid to make | potassium sulfate | copper oxide reacts with | hydrochloric acid to make | copper chloride | zinc oxide reacts with | sulfuric acid to make | zinc sulfate | calcium carbonate reacts with | hydrochloric acid to make | calcium chloride | sodium carbonate reacts with | sulfuric acid to make | sodium sulfate |
Here are the word equations and balanced formulae equations for the reactions involving hydrochloric acid in the table: sodium hydroxide + hydrochloric acid → sodium chloride + water
NaOH + HCl → NaCl + H2O copper oxide + hydrochloric acid → copper chloride + water
CuO + 2HCl → CuCl2 + H2O calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
CaCO3 + 2HCl → CaCl2 + H2O + CO2
Here are the word equations and balanced formulae equations for the reactions involving sulfuric acid in the table: potassium hydroxide + sulphuric acid → potassium sulfate + water
2KOH + H2SO4 → K2SO4 + 2H2O zinc oxide + sulfuric acid → zinc sulfate + water
ZnO + H2SO4 → ZnSO4 + H2O sodium carbonate + sulfuric acid → sodium sulfate + water + carbon dioxide
Na2CO3 + H2SO4 → Na2SO4 + H2O + CO2
Electrolysis is the process by which ionic substances are broken down into simpler substances using electricity. During electrolysis, metals and gases may form at the electrodes.
Electrolysis of hydrochloric acid and water
To understand electrolysis, you need to know what an ionic substance is.
Ionic substances form when a metal reacts with a non-metal. They contain charged particles called ions. For example, sodium chloride forms when sodium reacts with chlorine. It contains positively charged sodium ions and negatively charged chloride ions. Ionic substances can be broken down by electricity.
Electrolysis is the process by which ionic substances are decomposed (broken down) into simpler substances when an electric current is passed through them.
For electrolysis to work, the ions must be free to move. Ions are free to move when an ionic substance is dissolved in water or molten (melted). For example, if electricity is passed through copper chloride solution, the copper chloride is broken down to form copper metal and chlorine gas.
Electrolysis
The process of electrolysis
Here is what happens during electrolysis: * Positively charged ions move to the negative electrode during electrolysis. They receive electrons and are reduced. * Negatively charged ions move to the positive electrode during electrolysis. They lose electrons and are oxidised.
Many substances are commonly electrolysed, but here are two examples:
Hydrochloric acid * Produces chlorine at the positive electrode * Produces hydrogen at the negative electrode * If the gas produces a squeaky pop from a lighted splint, it is hydrogen * If the gas turns blue litmus paper red then white (bleached) it is chlorine.
Water
* Produces oxygen at the positive electrode * Produces hydrogen at the negative electrode * If the gas relights a glowing splint, it is oxygen.
The chlor-alkali industry
Brine is concentrated sodium chloride solution. If an electric current is passed through brine, hydrogen gas forms at the negative electrode and chlorine gas forms at the positive electrode, and a solution of sodium hydroxide forms.
You might have expected sodium metal to be deposited at the negative electrode, but sodium is too reactive for this to happen, so hydrogen is given off instead.
Electrolysis of sodium chloride solution
This table shows how three products - hydrogen, chlorine and sodium hydroxide - have important uses in the chemical industry:
Important uses of products in the chemical industry Product | Test | Uses | Problems | chlorine | damp blue litmus paper turns red (as chlorine is acidic) then white (chlorine is a bleach) | manufacture of bleach and PVC (polyvinylchloride) Water treatment | toxic gas | hydrogen | lighted splint gives a squeaky pop | many uses including as a fuel | flammable | sodium hydroxide | turns red litmus blue | cleaning products | corrosive |
Ores are naturally occurring rocks that contain metal or metal compounds in sufficient amounts to make it worthwhile extracting them. The method used to extract a given metal from its ore depends upon the reactivity of the metal and so how stable the ore is. The uses of metals depend on their properties. Alloys are made by mixing a metal with another material in order to improve the properties.
Metal ores
Oxidation and reduction
Oxidation is the gain of oxygen by a substance. For example, magnesium is oxidised when it reacts with oxygen to form magnesium oxide: magnesium + oxygen → magnesium oxide
2Mg + O2 → 2MgO
Reduction is the loss of oxygen from a substance. For example, copper oxide can be reduced to form copper if it is reacted with hydrogen: copper oxide + hydrogen → copper + water
CuO + H2 → Cu + H2O
Many ores contain metal oxides, therefore many metals can be extracted from their ores by reduction reactions. The method used to extract a given metal depends on how reactive it is.
Methods of extracting metals
The method used to extract a metal from its ore depends upon the stability of its compound in the ore, which in turn depends upon the reactivity of the metal: * the oxides of very reactive metals, such as aluminium, form stable oxides and other compounds. A lot of energy is needed to reduce them to extract the metal. * the oxides of lesser reactive metals, such as iron, form less stable oxides and other compounds. Relatively little energy is needed to reduce them to extract the metal.
So, the method of extraction of a metal from its ore depends on the metal's position in the reactivity series.
Reactivity and extraction method Metal | Reactivity | * potassium * sodium * calcium * magnesium * aluminium | extract by electrolysis | carbon | | * zinc * iron * tin * lead | extract by reaction with carbon or carbon monoxide | hydrogen | | * copper * silver * gold * platinum | extracted by various chemical reactions |
Reactive metals such as aluminium are extracted by electrolysis, while a less-reactive metal such as iron may be extracted by reduction with carbon. Gold, because it is so unreactive, is found as the native metal and not as a compound, so it does not need to be chemically separated. However, chemical reactions may be needed to remove other elements that might contaminate the metal.
Oxidation and reduction
Oxidation is the gain of oxygen by a substance. For example, magnesium is oxidised when it reacts with oxygen to form magnesium oxide: magnesium + oxygen → magnesium oxide
2Mg + O2→ 2MgO
Reduction is the loss of oxygen from a substance. For example, copper oxide can be reduced to form copper if it is reacted with hydrogen: copper oxide + hydrogen → copper + water
CuO + H2→ Cu + H2O
Many ores contain metal oxides, therefore many metals can be extracted from their ores by reduction reactions. The method used to extract a given metal depends on how reactive it is: * very reactive metals – electrolysis * less reactive metals - reduction
Rusting
Iron and steel rust when they come into contact with water and oxygen: this is a form of corrosion. Both water and oxygen are needed for rusting to occur. Rusting is an oxidation reaction. The iron reacts with water and oxygen to formhydrated iron(III) oxide, which we see as rust. Here is the word equation for the reaction: iron + water + oxygen → hydrated iron(III) oxide
In the experiment below, the nail does not rust when air - containing oxygen - or water is not present:
Calcium chloride absorbs water in the right-hand test tube
Salt dissolved in water does not cause rusting, but it does speed it up, as doesacid rain.
Aluminium does not rust (corrode) because its surface is protected by a natural layer of aluminium oxide which prevents the metal below from coming into contact with air and oxygen. Unlike rust, which can flake off the surface of iron and steel objects, the layer of aluminium oxide does not flake off.
More reactive elements are more likely to oxidise.
Uses of metals
We use different metals for different jobs as they have different properties: it’s important to choose the right metal for the job.
Choosing the right metal for the job Metal | Properties | Uses | aluminium | low density, does not corrode | suitable for the bodies of planes | copper | good conductor of electricity, does not react with water | electrical wires as it is a good conductorwater pipes due to its low reactivity | gold | very good conductor of electricity, unreactive | electrical connections on circuit boards - due to its conductivityjewellery - due to its lack of reactivity | steel | cheap and strong | suitable for building material |
When you answer questions on properties of metals it’s important to make sure that the property you give is relevant to the use you’ve been asked about: eg, copper is unreactive with water but that is not relevant if the question asks you about its use in electrical wires.
Alloys
The properties of a metal are changed by including other elements, such as carbon. A mixture of two or more elements, where at least one element is a metal, is called an alloy. Alloys contain atoms of different sizes, which distort the regular arrangements of atoms. This makes it more difficult for the layers to slide over each other, so alloys are harder than the pure metal.
It is more difficult for layers of atoms to slide over each other in alloys
Copper, gold and aluminium are too soft for many uses. They are mixed with other metals to make them harder for everyday use. For example: * Brass, used in electrical fittings, is 70 per cent copper and 30 per cent zinc. * 18 carat gold, used in jewellery, is 75 per cent gold and 25 per cent copper and other metals. * Duralumin, used in aircraft manufacture, is 96 per cent aluminium and 4 per cent copper and other metals.
Smart alloys can return to their original shape after being bent. They are useful for spectacle frames and dental braces.
Alloys are used in everyday life. Scientists are developing new alloys to fit a range of new applications.
Alloys in everyday life Alloy | Reason for alloy | Used for | gold with copper, nickel, silver or platinum | increases strength | jewellery | nitinol | returns to its original shape when squashed | spectacle frames | nitinol | returns to its original shape when warmed | nitinol tubes are used to open up damaged arteries |
Crude oil is a mixture of compounds called hydrocarbons. Many useful materials can be produced from crude oil. It can be separated into different fractions using fractional distillation, and some of these can be used as fuels. Unfortunately, there are environmental consequences when fossil fuels such as crude oil and its products are used.
Hydrocarbons and alkanes
Hydrocarbons
Most of the compounds in crude oil are hydrocarbons. This means that they only contain hydrogen and carbon atoms, joined together by chemical bonds. There are different types of hydrocarbon, but most of the ones in crude oil are alkanes.
Alkanes
The alkanes are a family of hydrocarbons that share the same general formula. This is:
CnH2n+2
The general formula means that the number of hydrogen atoms in an alkane is double the number of carbon atoms, plus two. For example, methane is CH4 and ethane is C2H6. Alkane molecules can be represented by displayed formulae in which each atom is shown as its symbol (C or H) and the chemical bonds between them by a straight line.
Structure of alkanes alkane | formula | chemical structure | ball-and-stick model | methane | CH4 | | | propane | C3H8 | | | butane | C4H10 | | |
Notice that the molecular models on the right show that the bonds are not really at 90º.
Alkanes are saturated hydrocarbons. This means that their carbon atoms are joined to each other by single bonds. This makes them relatively unreactive, apart from their reaction with oxygen in the air, which we call burning or combustion.
Boiling point and state at room temperature
Hydrocarbons have different boiling points, and can be either solid, liquid or gas at room temperature: * small hydrocarbons with only a few carbon atoms have low boiling points and are gases * hydrocarbons with between five and 12 carbon atoms are usually liquids * large hydrocarbons with many carbon atoms have high boiling points and aresolidsstrong.
Distillation
Distillation is a process that can be used to separate a pure liquid from a mixture of liquids. It works when the liquids have different boiling points. Distillation is commonly used to separate ethanol (the alcohol in alcoholic drinks) from water.
Distillation process to separate ethanol from water
Step 1 - water and ethanol solution are heated
Step 2 - the ethanol evaporates first, cools, then condenses
Step 3 - the water left evaporates, cools, then condenses
The mixture is heated in a flask. Ethanol has a lower boiling point than water so it evaporates first. The ethanol vapour is then cooled and condensed inside the condenser to form a pure liquid. The thermometer shows the boiling point of the pure ethanol liquid. When all the ethanol has evaporated from the solution, the temperature rises and the water evaporates.
This is the sequence of events in distillation: heating → evaporating → cooling → condensing
Combustion of fuels
Complete combustion
Fuels burn when they react with oxygen in the air. The hydrogen in hydrocarbons is oxidised to water (remember that water, H2O, is an oxide of hydrogen). If there is plenty of air, we get complete combustion and the carbon in hydrocarbons is oxidised to carbon dioxide: hydrocarbon + oxygen → water + carbon dioxide
The test to show carbon dioxide is limewater it turns from clear to cloudy in the presence of carbon dioxide.
Incomplete combustion
If there is insufficient air for complete combustion, we get carbon monoxide. Particles of carbon, seen as soot or smoke, are also released. Carbon monoxide is a problem because it reduces the amount of oxygen that the haemoglobin part of the blood can carry around the body. Every year many people are admitted to hospital due to carbon monoxide poisoning and some die.
Sulfur
Most hydrocarbon fuels naturally contain some sulfur compounds. When the fuel burns, the sulfur it contains is oxidised to sulfur dioxide.
Summary
Clouds of smoke and other combustion products are emitted from chimneys
The combustion of a fuel may release several gases into the atmosphere, including: * water vapour * carbon dioxide * carbon monoxide * particles * sulfur dioxide
These products may be harmful to the environment.
Factors influencing the choice of a fuel
The fossil fuels include coal, oil and natural gas. Various factors need to be considered when deciding how to use a fossil fuel. These include: * the energy value of the fuel in J/g of fuel * the availability of the fuel * how the fuel can be stored * the cost of the fuel * the toxicity of the fuel - whether it is poisonous * any pollution caused when the fuel is used, such as acid rain * how easy it is to use the fuel
Factories can cause air pollution
In general, solids such as coal are easier to store than liquids and gases. But they are often more difficult to light. Liquids and gases ignite more easily. They also flow, which means they can be transported through pipelines.
The table shows some approximate energy values of the different fossil fuels, and the typical mass of carbon dioxide they produce when they burn. Carbon dioxide is a greenhouse gas that contributes to global warming.
Energy values of fuel fuel | energy content (kJ/g) | mg of carbon dioxide produced for each kJ | natural gas | 52 | 53 | petrol | 43 | 71 | coal | 24 | 93 |
Coal releases the least amount of energy per gram of fuel. It also produces the most carbon dioxide for a given amount of energy released when it burns.
You are expected to be able to list factors such as the ones above. You should be able to interpret data to choose the best fuel for a particular purpose.
Problems with fuels: sulfur dioxide
Sulfur dioxide
Sulfur dioxide is produced when fuels that contain sulfur compounds burn. It is a gas with a sharp, choking smell. When sulfur dioxide dissolves in water droplets in clouds, it makes the rain more acidic than normal. This is called acid rain.
Effects of acid rain
Acid rain reacts with metals and rocks such as limestone. Buildings and statues are damaged as a result. Acid rain damages the waxy layer on the leaves of trees and makes it more difficult for trees to absorb the minerals they need for healthy growth. They may die as a result. Acid rain also makes rivers and lakes too acidic for some aquatic life to survive.
Reducing acid rain
Sulfur dioxide can be removed from waste gases after combustion of the fuel. This happens in power stations. The sulfur dioxide is treated with powdered limestone to form calcium sulfate. This can be used to make plasterboard for lining interior walls, so turning a harmful product into a useful one.
The process of removing sulfur dioxide
Sulfur can be removed from fuels at the oil refinery. This makes the fuel more expensive to produce, but it prevents sulfur dioxide being produced. You may have noticed 'low sulfur' petrol and diesel on sale at filling stations.
Problems with fuels: carbon dioxide
Global warming
Carbon dioxide from burning fuels causes global warming, a process capable of changing the world’s climate significantly.
Carbon dioxide in the atmosphere has risen at a higher rate since the 19th century
The temperature of the earth has risen over the years
As you can see from the graphs, the amount of carbon dioxide in the atmosphere has increased steadily over the past 150 years, and so has the average global temperature. Some of this is due to human activity.
Along with other gases such as methane and water vapour, carbon dioxide is a greenhouse gas. It absorbs heat energy and prevents it escaping from the Earth’s surface into space. The greater the amount of carbon dioxide in the atmosphere, the more heat energy is absorbed and the hotter the Earth becomes.
Greenhouse effect
1. The Sun’s rays enter the Earth’s atmosphere 2. Heat is reflected back from the Earth’s surface 3. Heat is absorbed by greenhouse gases, such as carbon dioxide, and as a result becomes trapped in the Earth’s atmosphere. 4. The Earth becomes hotter as a result
Results of global warming
A rise of just a few degrees in world temperatures will have a dramatic impact on the climate: * global weather patterns will change, causing drought in some places and flooding in others. * polar ice caps will melt, raising sea levels and causing increased coastal erosion and flooding of low-lying land – including land where major cities lie
The Triftgletscher glacier, Switzerland, 2002
The Triftgletscher glacier, Switzerland, 2003. As the glacier melts further, the lake's water level rises.
Scientists are trying to control the amount of carbon dioxide in the atmosphere by: * iron seeding of oceans * converting carbon dioxide into hydrocarbons
However, some scientists do not believe that the global temperature increase and the carbon dioxide increase are caused by human activities.
Biofuels
Biofuels come from the products of living organisms, such as methane biogas from decaying manure and sewage. Vegetable oils are also used as fuels for vehicles. Some of this biodiesel is made from waste cooking oil and rapeseed oil.
Ethanol
Ethanol is the type of alcohol found in alcoholic drinks such as wine and beer. It is also useful as a fuel. For use in cars and other vehicles it is usually mixed with petrol.
Ethanol can be made by a process called fermentation. This converts sugar from sugar cane or sugar beet into ethanol and carbon dioxide. Single-celled fungi, called yeast, contain enzymes that are natural catalysts for making this process happen:
C6H12O6 2C2H5OH + 2CO2
Advantages of using biofuels
Biofuels are carbon neutral, which means that they release only as much carbon dioxide when they burn as was used to make the original oil by photosynthesis.
This helps to reduce global warming.
However, some people are concerned about whether it is ethical to use food crops in this way, instead of using them to feed hungry people.
Hydrogen
Hydrogen is often seen as an environmentally friendly alternative to fossil fuels and biofuels. When hydrogen burns, the only product formed is water: hydrogen + oxygen → water 2H2 + O2 → 2H2O
Problems with hydrogen
Hydrogen powered car
Some hydrogen-powered vehicles fitted with hydrogen fuel cells have already been made and are on the road, but there are few of them because of difficulties involved in making and handling hydrogen.
Making hydrogen
At the moment, most hydrogen is made by reacting steam with coal or natural gas, which are non-renewable resources.
Hydrogen can also be made by passing electricity through water. Unfortunately, most electricity is generated using coal and other fossil fuels: any pollution from burning these fuels just happens at the power station instead of at the vehicle itself.
Handling hydrogen
Hydrogen gas is very flammable and may explode if handled incorrectly. It must be compressed and chilled, then stored in tough, insulated tanks. It is not as convenient as petrol and diesel.
Cracking
Fuels made from oil mixtures containing large hydrocarbon molecules are not efficient. They do not flow easily and are difficult to ignite. Crude oil often contains too many large hydrocarbon molecules and not enough small hydrocarbon molecules to meet demand - this is where cracking comes in.
Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules. Fractions containing large hydrocarbon molecules are vaporised and passed over a hot catalyst. This breaks chemical bonds in the molecules, and forms smaller hydrocarbon molecules.
Cracking is an example of a thermal decomposition reaction.
Some of the smaller molecules formed by cracking are used as fuels, and some of them are used to make polymers in plastics manufacture.
Alkenes
The products of cracking include alkenes (for example ethene and propene). The alkenes are a family of hydrocarbons that share the same general formula. This is CnH2n.
The general formula means that the number of hydrogen atoms in an alkene is double the number of carbon atoms. For example, ethene is C2H4 and propene is C3H6. Alkene molecules can be represented by displayed formulae, in which each atom is shown as its symbol (C or H) and the chemical bonds between them by a straight line.
Structure of alkenes alkene | formula | chemical structure | ball-and-stick model | ethene | C2H4 | | | propene | C3H6 | | |
Alkenes are unsaturated hydrocarbons. They contain a double bond, which is shown as two lines between two of the carbon atoms. The presence of this double bond allows alkenes to react in ways that alkanes cannot. They can react with oxygen in the air, so they could be used as fuels. But they are more useful than that. They can be used to make ethanol (alcohol) and polymers (plastics), two crucial products in today's world.
Testing for alkenes
Bromine water is used to tell the difference between an alkane and an alkene. An alkene will turn brown bromine water colourless as it reacts with the double bond. Bromine water remains brown in the presence of an alkane as there is no double bond.
Polymers
Alkenes can be used to make polymers. Polymers are very large molecules made when many smaller molecules join together, end-to-end. The smaller molecules are called monomers. In general: lots of monomer molecules → a polymer molecule
The animation shows how several chloroethene monomers can join end-to-end to make poly(chloroethene), which is also called PVC.
Alkenes can act as monomers because they have a double bond: * Ethene can polymerise to form poly(ethene), which is also called polythene. * Propene can polymerise to form poly(propene), which is also called polypropylene.
Different polymers have different properties, so they have different uses. The table below gives some examples.
Examples of polymers and their uses polychloroethene | water pipes and insulation on electricity cables | Uses of polymersPolymers have many different uses. The use of a polymer is related to its properties.Polymer properties and uses Monomer | Polymer | Properties | Uses | ethene | poly(ethene) | flexible, cheap, electrical insulator | plastic bags and bottles, coating on electrical wires | propene | poly(propene) | flexible and strong | buckets and crates | chloroethene | poly(chloroethene) or PVC | tough, cheap and long lasting | window frames | tetrafluoroethene | poly(tetrafluoroethene) or PTFE | tough and non-stick | non-stick coating on pans |
Polymer problemsOne of the useful properties of polymers is that they are unreactive, so they aresuitable for storing food and chemicals safely. Unfortunately, this property makes it difficult to dispose of polymers.BiodegradableMost polymers, including poly(ethene) and poly(propene) are not biodegradable. This means that microorganisms cannot break them down, so they may last for many years in rubbish dumps. However, it is possible to include chemicals that cause the polymer to break down more quickly. Carrier bags and refuse bags made from such degradable polymers are already available.IncinerationPolymers can be burnt or incinerated. They release a lot of heat energy when they burn and this can be used to heat homes or to generate electricity.There are problems with incineration. Carbon dioxide is produced, which adds to global warming. Toxic gases are produced unless the polymers are incinerated at high temperatures.RecyclingPolymers have recycling symbols like this one for PVC to show what they areMany polymers can be recycled. This reduces the disposal problems and the amount of crude oil used. But the different polymers must be separated from each other first, and this can be difficult and expensive to do. | |
Composition of the Earth's atmosphere
The composition of air
You need to know the proportions of the main gases in the atmosphere.
The Earth's atmosphere has remained much the same for the past 200 million years. The pie chart shows the proportions of the main gases in the atmosphere.
It is clear that the main gas is nitrogen. Oxygen - the gas that allows animals and plants to respire, and fuels to burn - is the next most abundant gas. These two gases are both elements and account for about 99% of the gases in the atmosphere.
The remaining gases, such as carbon dioxide, water vapour and noble gases such as argon, are found in much smaller proportions.
Evolution of the atmosphere
The early atmosphere
Scientists believe that the Earth was formed about 4.5 billion years ago. Its early atmosphere was probably formed from the gases given out by volcanoes. It is believed that there was intense volcanic activity for the first billion years of the Earth's existence.
The early atmosphere was probably mostly carbon dioxide, with little or no oxygen. There were smaller proportions of water vapour, ammonia and methane. As the Earth cooled down, most of the water vapour condensed and formed the oceans.
It is thought that the atmospheres of Mars and Venus today, which contain mostly carbon dioxide, are similar to the early atmosphere of the Earth.
Scientists can’t be sure about the early atmosphere and can only draw evidence from other sources: for example, volcanoes on other planets release high quantities of carbon dioxide or nitrogen and iron-based compounds which are present in very old rocks that could have formed only if there was little or no oxygen.
Changes in the atmosphere
So how did the proportion of carbon dioxide in the atmosphere go down, and the proportion of oxygen go up?
The proportion of oxygen went up because of photosynthesis by plants.
The proportion of carbon dioxide went down because: * it was locked up in sedimentary rocks such as limestone, and in fossil fuels * it was absorbed by plants for photosynthesis * it dissolved in the oceans.
The burning of fossil fuels is adding carbon dioxide to the atmosphere faster than it can be removed. This means that the level of carbon dioxide in the atmosphere is increasing.
A simple carbon cycle
The level of carbon dioxide in the atmosphere is maintained by several processes, including photosynthesis, respiration and combustion.
Green plants remove carbon dioxide from the atmosphere by photosynthesis. Living organisms - including all plants and animals - release energy from their food using respiration. Respiration and combustion both release carbon dioxide into the atmosphere.
These processes form a carbon cycle in which the proportion of carbon dioxide in the atmosphere remains about the same. The animation should help you to understand how the cycle works. (Note that you do not need to know about decomposition and fossilisation.)
Volcanic activity also increases the amount of carbon dioxide in the atmosphere.
Human influences
Carbon dioxide is produced by burning fossil fuels. Increased energy consumption is leading to a rise in the use of fossil fuels, which in turn increases the release of carbon dioxide into the atmosphere.
The rising human population is adding to atmospheric carbon dioxide in other ways too. When land is cleared for timber and farms (deforestation), there are fewer trees to remove carbon dioxide from the atmosphere by photosynthesis. If the fallen trees are burned or left to rot, additional carbon dioxide is released into the atmosphere. This is particularly important when forests are cleared to make way for farms: not only are there then fewer trees to absorb carbon dioxide, butthe burning of the trees releases carbon dioxide.
Rocks are classified (organised) into three main groups: igneous rocks, sedimentary rocks and metamorphic rocks. This classification is based on how they were formed and their characteristics.
Igneous rock
Igneous rocks are formed by magma from the molten interior of the Earth. Whenmagma erupts it cools to form volcanic landforms. When it cools inside the Earth it forms intrusive rock, which may later be exposed by erosion and weathering. Intrusive rock will have large crystals as it has cooled slowly. Magma that has cooled on the surface is known as extrusive rock. This will have small crystals as it has cooled quickly.
Examples of igneous rocks include basalt and granite.
Examples of igneous rocks Type of rock | Example | Basalt | Basalt | Granite | Granite |
Sedimentary rocks
Formation of sedimentary rocks
A river carries, or transports, pieces of broken rock as it flows along. When the river reaches a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks are deposited. The deposited rocks build up in layers, called sediments. This process is called sedimentation.
The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. The water is squeezed out from between the pieces of rock and crystals of different salts form.
The crystals form a sort of glue that sticks or cements the pieces of rock together. This process is called cementation.
These processes eventually make a type of rock called sedimentary rock. It may take millions of years for sedimentary rocks to form.
These are the different processes in order: sedimentation → compaction → cementation
Sedimentary rock formation
Deposited rocks build up to form sediments
The weight of each additional sedimentary layer causes compaction
The pieces of rock stick together through the process of cementation
Examples of sedimentary rock
Sedimentary rocks contain rounded grains in layers. Examples of sedimentary rock are:
Sedimentary rocks like sandstone have layers * chalk * limestone * sandstone * shale.
The oldest layers are at the bottom and the youngest layers are at the top. Sedimentary rocks may contain fossils of animals and plants trapped in the sediments as the rock was formed. Sedimentary rocks are often quite soft and are susceptible to erosion.
Metamorphic rocks
Formation of metamorphic rock
Slate quarry, Cullipool
Metamorphic rock has been subjected to tremendous heat and / or pressure, which caused it to change into another type of rock. It is usually resistant to weathering and erosion and is therefore very hard wearing.
Examples of metamorphic rock
Examples include marble - which originates from chalk or limestone, slate - which originates from clay, and schists formed from sandstone or shale (sedimentary rocks).
Limestone is mainly calcium carbonate, CaCO3. When heated, it breaks down to form calcium oxide and carbon dioxide. Calcium oxide reacts with water to produce calcium hydroxide.
Limestone and its products have many uses: for example, in mortar, cement, concrete and glass.
Limestone, chalk and marble
Limestone, chalk and marble are all forms of calcium carbonate. They exist naturally in the Earth’s crust. Limestone is a very common building material and many tonnes are quarried in the UK every year. It is used for building - making concrete and cement - and the manufacture of glass, steel and iron.
Advantages and disadvantages of various building materials
Limestone, cement and mortar slowly react with carbon dioxide dissolved in rainwater, and wear away. This damages walls made from limestone, and it leaves gaps between bricks in buildings. These gaps must be filled in or "pointed". Pollution from burning fossil fuels makes the rain more acidic than it should be, and this rain makes these problems worse.
Concrete is easily formed into different shapes before it sets hard. It is strong when squashed, but weak when bent or stretched. However, concrete can be made much stronger by reinforcing it with steel. Some people think that concrete buildings and bridges are unattractive.
Glass is usually brittle and easily shattered, but toughened glass can be used for windows. While glass is transparent and so lets light into a building, the use of too much glass can make buildings very hot in the summer.
Uses of calcium oxide
Limestone can be broken down using heat to produce calcium oxide, which has lots of uses. * Add water and a vigorous exothermic reaction takes place. This forms calcium hydroxide. * Calcium hydroxide is soluble in water and forms a solution known as limewater which is used to test for carbon dioxide. * Calcium oxide, calcium hydroxide and calcium carbonate can be used to neutralise soil acidity. * Calcium carbonate can be used to remove acidic gases from coal-fired power station chimneys reducing harmful emissions and acid rain.
Quarrying
The limestone industry: environmental, social and economic considerations
Quarrying limestone is big business but the need for limestone has to be balanced against the economic, environmental and social effects. Some factors that have to be considered include: * effect on employment – increased job opportunities * pollution – noise, sound and air * traffic levels * visual effects of having a quarry
You need to be able to evaluate some of the effects of the limestone industry.
The main advantages and disadvantages of the limestone industry Advantages | Disadvantages | Limestone is a valuable natural resource, used to make things such as glass and concrete. | Limestone quarries are visible from long distances and may permanently disfigure the local environment. | Limestone quarrying provides employment opportunities that support the local economy in towns around the quarry. | Quarrying is a heavy industry that creates noise and heavy traffic, which damages people's quality of life. |
Thermal decomposition
Metal carbonates such as calcium carbonate break down when heated strongly. This is called thermal decomposition. Here are the equations for the thermal decomposition of calcium carbonate: calcium carbonate calcium oxide + carbon dioxide
CaCO3 CaO + CO2
Other metal carbonates decompose in the same way. Here are the equations for the thermal decomposition of copper carbonate: copper carbonate copper oxide + carbon dioxide
CuCO3 CuO + CO2
Notice that in both examples the products are a metal oxide and carbon dioxide. The carbon dioxide gas can be detected using limewater. Limewater turns cloudy white when carbon dioxide is bubbled through it.
Metals high up in the reactivity series - such as calcium - have carbonates that take a lot of energy to decompose them. Metals low down in the reactivity series - such as copper - have carbonates that are easily decomposed. This is why copper carbonate is often used at school to show these reactions. It is easily decomposed, and its colour change, from green copper carbonate to black copper oxide, is easy to see.
Atoms are the smallest particle of an element. A chemical reaction involves atoms in the reactants being rearranged to form the products. No material is lost or gained. A precipitation reaction is a good way of showing this conservation of mass.
Conservation of mass
Atoms are the smallest particles of an element that can take part in a chemical reaction. During any chemical reaction no particles are created or destroyed: the atoms are simply rearranged from the reactants to the products. The products may have different properties to the reactants.
Mass is never lost or gained in chemical reactions. We say that mass is alwaysconserved. In other words, the total mass of products at the end of the reaction is equal to the total mass of the reactants at the beginning.
This fact allows you to work out the mass of one substance in a reaction if the masses of the other substances are known. For example:
In practice, it is not always possible to get all the calculated amount of product from a reaction because: * reversible reactions may not go to completion * some product may be lost when it is removed from the reaction mixture * some of the reactants may react in an unexpected way.
Precipitation reactions
A simple example of conservation of mass is a precipitation reaction. Transition metals form coloured compounds with other elements. Many of these are soluble in water, forming coloured solutions. If sodium hydroxide solution is then added, a transition metal hydroxide is formed. These are insoluble. They do not dissolve but instead form solid precipitates. As all the reactants and products remain in the sealed reaction container then it is easy to show that the total mass is unchanged.
Here are the equations for copper sulfate solution reacting with sodium hydroxide solution: copper sulfate + sodium hydroxide → copper hydroxide + sodium sulfate
CuSO4 + 2NaOH → Cu(OH)2 + Na2SO4
(blue solution + colourless solution → blue precipitate + colourless solution)
Copper solutions form a blue precipitate with sodium hydroxide
Copper solutions form a blue precipitate with sodium hydroxide.
Different transition metals form different coloured precipitates.
Some common transition metals and the colours of their precipitates. transition metal | colour of precipitate | iron(II) | green - turns orange-brown when left standing | iron(III) | orange-brown | copper | pale blue | zinc | white |
Example of a precipitation reaction
How could you tell if an unknown substance contained iron(II) nitrate or iron(III) nitrate? You would add a few drops of sodium hydroxide solution. If you got a dark green precipitate it would show that the unknown substance was iron(II) nitrate; if you got an orange-brown precipitate it would show that the unknown substance was iron(III) nitrate.
Acids have a variety of applications for the industrial and domestic markets. Acids can be neutralised using an alkali or base and used to make salts.
What are acids?
Corrosive Irritant
All acids: * have a low pH (1-6) – the lower the number the stronger the acid * react with bases to form neutral compounds * are corrosive when they are strong * are an irritant when they are weak.
Indigestion remedies
Hydrochloric acid is used in the body to help digestion and kill bacteria.
However too much acid can cause indigestion and we use indigestion remedies to neutralise excess acids.
An indigestion remedy contains a base such as magnesium hydroxide, which reacts to form a neutral compound and raises the pH of the stomach.
Neutralisation
You need to be able to describe the reactions of hydrochloric acid and sulfuric acid with metal hydroxides, metal oxides and metal carbonates.
Metal hydroxides
Metal hydroxides, such as sodium hydroxide, usually dissolve in water to form clear, colourless solutions. When an acid reacts with a metal hydroxide, the only products formed are a salt plus water. Here is the general word equation for the reaction: acid + metal hydroxide → a salt + water
You usually observe these things during the reaction: * there is a temperature rise * the pH of the reaction mixture changes
Metal oxides
Some metal oxides, such as sodium oxide, dissolve in water to form clear, colourless solutions. Many of them are not soluble in water, but they will react with acids. Copper(II) oxide is like this. When an acid reacts with a metal oxide, the only products formed are a salt plus water. Here is the general word equation for the reaction: acid + metal oxide → a salt + water
You usually observe the same things during the reaction that you observe with metal hydroxides.
Metal carbonates
Although sodium carbonate can dissolve in water, most metal carbonates are not soluble. Calcium carbonate (chalk, limestone and marble) is like this. When an acid reacts with a metal carbonate, the products formed are a salt plus water, but carbon dioxide is also formed. Here is the general word equation for the reaction: acid + metal carbonate → a salt + water + carbon dioxide
You usually observe bubbles of gas being given off during the reaction. You can show that the gas is carbon dioxide by bubbling it through limewater: this turns cloudy white when it reacts with carbon dioxide.
Naming salts
The name of a salt is in two parts: * The first part of the name comes from the metal in the metal oxide, hydroxide or carbonate. * The second part of the name comes from the acid used to make it. The names of salts made from hydrochloric acid end in chloride, while the names of salts made from sulfuric acid end in sulfate.
How salts are named metal involved | acid | salt | sodium hydroxide reacts with | hydrochloric acid | to make sodium chloride | potassium hydroxide reacts with | sulfuric acid to make | potassium sulfate | copper oxide reacts with | hydrochloric acid to make | copper chloride | zinc oxide reacts with | sulfuric acid to make | zinc sulfate | calcium carbonate reacts with | hydrochloric acid to make | calcium chloride | sodium carbonate reacts with | sulfuric acid to make | sodium sulfate |
Here are the word equations and balanced formulae equations for the reactions involving hydrochloric acid in the table: sodium hydroxide + hydrochloric acid → sodium chloride + water
NaOH + HCl → NaCl + H2O copper oxide + hydrochloric acid → copper chloride + water
CuO + 2HCl → CuCl2 + H2O calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
CaCO3 + 2HCl → CaCl2 + H2O + CO2
Here are the word equations and balanced formulae equations for the reactions involving sulfuric acid in the table: potassium hydroxide + sulphuric acid → potassium sulfate + water
2KOH + H2SO4 → K2SO4 + 2H2O zinc oxide + sulfuric acid → zinc sulfate + water
ZnO + H2SO4 → ZnSO4 + H2O sodium carbonate + sulfuric acid → sodium sulfate + water + carbon dioxide
Na2CO3 + H2SO4 → Na2SO4 + H2O + CO2
Electrolysis is the process by which ionic substances are broken down into simpler substances using electricity. During electrolysis, metals and gases may form at the electrodes.
Electrolysis of hydrochloric acid and water
To understand electrolysis, you need to know what an ionic substance is.
Ionic substances form when a metal reacts with a non-metal. They contain charged particles called ions. For example, sodium chloride forms when sodium reacts with chlorine. It contains positively charged sodium ions and negatively charged chloride ions. Ionic substances can be broken down by electricity.
Electrolysis is the process by which ionic substances are decomposed (broken down) into simpler substances when an electric current is passed through them.
For electrolysis to work, the ions must be free to move. Ions are free to move when an ionic substance is dissolved in water or molten (melted). For example, if electricity is passed through copper chloride solution, the copper chloride is broken down to form copper metal and chlorine gas.
Electrolysis
The process of electrolysis
Here is what happens during electrolysis: * Positively charged ions move to the negative electrode during electrolysis. They receive electrons and are reduced. * Negatively charged ions move to the positive electrode during electrolysis. They lose electrons and are oxidised.
Many substances are commonly electrolysed, but here are two examples:
Hydrochloric acid * Produces chlorine at the positive electrode * Produces hydrogen at the negative electrode * If the gas produces a squeaky pop from a lighted splint, it is hydrogen * If the gas turns blue litmus paper red then white (bleached) it is chlorine.
Water
* Produces oxygen at the positive electrode * Produces hydrogen at the negative electrode * If the gas relights a glowing splint, it is oxygen.
The chlor-alkali industry
Brine is concentrated sodium chloride solution. If an electric current is passed through brine, hydrogen gas forms at the negative electrode and chlorine gas forms at the positive electrode, and a solution of sodium hydroxide forms.
You might have expected sodium metal to be deposited at the negative electrode, but sodium is too reactive for this to happen, so hydrogen is given off instead.
Electrolysis of sodium chloride solution
This table shows how three products - hydrogen, chlorine and sodium hydroxide - have important uses in the chemical industry:
Important uses of products in the chemical industry Product | Test | Uses | Problems | chlorine | damp blue litmus paper turns red (as chlorine is acidic) then white (chlorine is a bleach) | manufacture of bleach and PVC (polyvinylchloride) Water treatment | toxic gas | hydrogen | lighted splint gives a squeaky pop | many uses including as a fuel | flammable | sodium hydroxide | turns red litmus blue | cleaning products | corrosive |
Ores are naturally occurring rocks that contain metal or metal compounds in sufficient amounts to make it worthwhile extracting them. The method used to extract a given metal from its ore depends upon the reactivity of the metal and so how stable the ore is. The uses of metals depend on their properties. Alloys are made by mixing a metal with another material in order to improve the properties.
Metal ores
Oxidation and reduction
Oxidation is the gain of oxygen by a substance. For example, magnesium is oxidised when it reacts with oxygen to form magnesium oxide: magnesium + oxygen → magnesium oxide
2Mg + O2 → 2MgO
Reduction is the loss of oxygen from a substance. For example, copper oxide can be reduced to form copper if it is reacted with hydrogen: copper oxide + hydrogen → copper + water
CuO + H2 → Cu + H2O
Many ores contain metal oxides, therefore many metals can be extracted from their ores by reduction reactions. The method used to extract a given metal depends on how reactive it is.
Methods of extracting metals
The method used to extract a metal from its ore depends upon the stability of its compound in the ore, which in turn depends upon the reactivity of the metal: * the oxides of very reactive metals, such as aluminium, form stable oxides and other compounds. A lot of energy is needed to reduce them to extract the metal. * the oxides of lesser reactive metals, such as iron, form less stable oxides and other compounds. Relatively little energy is needed to reduce them to extract the metal.
So, the method of extraction of a metal from its ore depends on the metal's position in the reactivity series.
Reactivity and extraction method Metal | Reactivity | * potassium * sodium * calcium * magnesium * aluminium | extract by electrolysis | carbon | | * zinc * iron * tin * lead | extract by reaction with carbon or carbon monoxide | hydrogen | | * copper * silver * gold * platinum | extracted by various chemical reactions |
Reactive metals such as aluminium are extracted by electrolysis, while a less-reactive metal such as iron may be extracted by reduction with carbon. Gold, because it is so unreactive, is found as the native metal and not as a compound, so it does not need to be chemically separated. However, chemical reactions may be needed to remove other elements that might contaminate the metal.
Oxidation and reduction
Oxidation is the gain of oxygen by a substance. For example, magnesium is oxidised when it reacts with oxygen to form magnesium oxide: magnesium + oxygen → magnesium oxide
2Mg + O2→ 2MgO
Reduction is the loss of oxygen from a substance. For example, copper oxide can be reduced to form copper if it is reacted with hydrogen: copper oxide + hydrogen → copper + water
CuO + H2→ Cu + H2O
Many ores contain metal oxides, therefore many metals can be extracted from their ores by reduction reactions. The method used to extract a given metal depends on how reactive it is: * very reactive metals – electrolysis * less reactive metals - reduction
Rusting
Iron and steel rust when they come into contact with water and oxygen: this is a form of corrosion. Both water and oxygen are needed for rusting to occur. Rusting is an oxidation reaction. The iron reacts with water and oxygen to formhydrated iron(III) oxide, which we see as rust. Here is the word equation for the reaction: iron + water + oxygen → hydrated iron(III) oxide
In the experiment below, the nail does not rust when air - containing oxygen - or water is not present:
Calcium chloride absorbs water in the right-hand test tube
Salt dissolved in water does not cause rusting, but it does speed it up, as doesacid rain.
Aluminium does not rust (corrode) because its surface is protected by a natural layer of aluminium oxide which prevents the metal below from coming into contact with air and oxygen. Unlike rust, which can flake off the surface of iron and steel objects, the layer of aluminium oxide does not flake off.
More reactive elements are more likely to oxidise.
Uses of metals
We use different metals for different jobs as they have different properties: it’s important to choose the right metal for the job.
Choosing the right metal for the job Metal | Properties | Uses | aluminium | low density, does not corrode | suitable for the bodies of planes | copper | good conductor of electricity, does not react with water | electrical wires as it is a good conductorwater pipes due to its low reactivity | gold | very good conductor of electricity, unreactive | electrical connections on circuit boards - due to its conductivityjewellery - due to its lack of reactivity | steel | cheap and strong | suitable for building material |
When you answer questions on properties of metals it’s important to make sure that the property you give is relevant to the use you’ve been asked about: eg, copper is unreactive with water but that is not relevant if the question asks you about its use in electrical wires.
Alloys
The properties of a metal are changed by including other elements, such as carbon. A mixture of two or more elements, where at least one element is a metal, is called an alloy. Alloys contain atoms of different sizes, which distort the regular arrangements of atoms. This makes it more difficult for the layers to slide over each other, so alloys are harder than the pure metal.
It is more difficult for layers of atoms to slide over each other in alloys
Copper, gold and aluminium are too soft for many uses. They are mixed with other metals to make them harder for everyday use. For example: * Brass, used in electrical fittings, is 70 per cent copper and 30 per cent zinc. * 18 carat gold, used in jewellery, is 75 per cent gold and 25 per cent copper and other metals. * Duralumin, used in aircraft manufacture, is 96 per cent aluminium and 4 per cent copper and other metals.
Smart alloys can return to their original shape after being bent. They are useful for spectacle frames and dental braces.
Alloys are used in everyday life. Scientists are developing new alloys to fit a range of new applications.
Alloys in everyday life Alloy | Reason for alloy | Used for | gold with copper, nickel, silver or platinum | increases strength | jewellery | nitinol | returns to its original shape when squashed | spectacle frames | nitinol | returns to its original shape when warmed | nitinol tubes are used to open up damaged arteries |
Crude oil is a mixture of compounds called hydrocarbons. Many useful materials can be produced from crude oil. It can be separated into different fractions using fractional distillation, and some of these can be used as fuels. Unfortunately, there are environmental consequences when fossil fuels such as crude oil and its products are used.
Hydrocarbons and alkanes
Hydrocarbons
Most of the compounds in crude oil are hydrocarbons. This means that they only contain hydrogen and carbon atoms, joined together by chemical bonds. There are different types of hydrocarbon, but most of the ones in crude oil are alkanes.
Alkanes
The alkanes are a family of hydrocarbons that share the same general formula. This is:
CnH2n+2
The general formula means that the number of hydrogen atoms in an alkane is double the number of carbon atoms, plus two. For example, methane is CH4 and ethane is C2H6. Alkane molecules can be represented by displayed formulae in which each atom is shown as its symbol (C or H) and the chemical bonds between them by a straight line.
Structure of alkanes alkane | formula | chemical structure | ball-and-stick model | methane | CH4 | | | propane | C3H8 | | | butane | C4H10 | | |
Notice that the molecular models on the right show that the bonds are not really at 90º.
Alkanes are saturated hydrocarbons. This means that their carbon atoms are joined to each other by single bonds. This makes them relatively unreactive, apart from their reaction with oxygen in the air, which we call burning or combustion.
Boiling point and state at room temperature
Hydrocarbons have different boiling points, and can be either solid, liquid or gas at room temperature: * small hydrocarbons with only a few carbon atoms have low boiling points and are gases * hydrocarbons with between five and 12 carbon atoms are usually liquids * large hydrocarbons with many carbon atoms have high boiling points and aresolidsstrong.
Distillation
Distillation is a process that can be used to separate a pure liquid from a mixture of liquids. It works when the liquids have different boiling points. Distillation is commonly used to separate ethanol (the alcohol in alcoholic drinks) from water.
Distillation process to separate ethanol from water
Step 1 - water and ethanol solution are heated
Step 2 - the ethanol evaporates first, cools, then condenses
Step 3 - the water left evaporates, cools, then condenses
The mixture is heated in a flask. Ethanol has a lower boiling point than water so it evaporates first. The ethanol vapour is then cooled and condensed inside the condenser to form a pure liquid. The thermometer shows the boiling point of the pure ethanol liquid. When all the ethanol has evaporated from the solution, the temperature rises and the water evaporates.
This is the sequence of events in distillation: heating → evaporating → cooling → condensing
Combustion of fuels
Complete combustion
Fuels burn when they react with oxygen in the air. The hydrogen in hydrocarbons is oxidised to water (remember that water, H2O, is an oxide of hydrogen). If there is plenty of air, we get complete combustion and the carbon in hydrocarbons is oxidised to carbon dioxide: hydrocarbon + oxygen → water + carbon dioxide
The test to show carbon dioxide is limewater it turns from clear to cloudy in the presence of carbon dioxide.
Incomplete combustion
If there is insufficient air for complete combustion, we get carbon monoxide. Particles of carbon, seen as soot or smoke, are also released. Carbon monoxide is a problem because it reduces the amount of oxygen that the haemoglobin part of the blood can carry around the body. Every year many people are admitted to hospital due to carbon monoxide poisoning and some die.
Sulfur
Most hydrocarbon fuels naturally contain some sulfur compounds. When the fuel burns, the sulfur it contains is oxidised to sulfur dioxide.
Summary
Clouds of smoke and other combustion products are emitted from chimneys
The combustion of a fuel may release several gases into the atmosphere, including: * water vapour * carbon dioxide * carbon monoxide * particles * sulfur dioxide
These products may be harmful to the environment.
Factors influencing the choice of a fuel
The fossil fuels include coal, oil and natural gas. Various factors need to be considered when deciding how to use a fossil fuel. These include: * the energy value of the fuel in J/g of fuel * the availability of the fuel * how the fuel can be stored * the cost of the fuel * the toxicity of the fuel - whether it is poisonous * any pollution caused when the fuel is used, such as acid rain * how easy it is to use the fuel
Factories can cause air pollution
In general, solids such as coal are easier to store than liquids and gases. But they are often more difficult to light. Liquids and gases ignite more easily. They also flow, which means they can be transported through pipelines.
The table shows some approximate energy values of the different fossil fuels, and the typical mass of carbon dioxide they produce when they burn. Carbon dioxide is a greenhouse gas that contributes to global warming.
Energy values of fuel fuel | energy content (kJ/g) | mg of carbon dioxide produced for each kJ | natural gas | 52 | 53 | petrol | 43 | 71 | coal | 24 | 93 |
Coal releases the least amount of energy per gram of fuel. It also produces the most carbon dioxide for a given amount of energy released when it burns.
You are expected to be able to list factors such as the ones above. You should be able to interpret data to choose the best fuel for a particular purpose.
Problems with fuels: sulfur dioxide
Sulfur dioxide
Sulfur dioxide is produced when fuels that contain sulfur compounds burn. It is a gas with a sharp, choking smell. When sulfur dioxide dissolves in water droplets in clouds, it makes the rain more acidic than normal. This is called acid rain.
Effects of acid rain
Acid rain reacts with metals and rocks such as limestone. Buildings and statues are damaged as a result. Acid rain damages the waxy layer on the leaves of trees and makes it more difficult for trees to absorb the minerals they need for healthy growth. They may die as a result. Acid rain also makes rivers and lakes too acidic for some aquatic life to survive.
Reducing acid rain
Sulfur dioxide can be removed from waste gases after combustion of the fuel. This happens in power stations. The sulfur dioxide is treated with powdered limestone to form calcium sulfate. This can be used to make plasterboard for lining interior walls, so turning a harmful product into a useful one.
The process of removing sulfur dioxide
Sulfur can be removed from fuels at the oil refinery. This makes the fuel more expensive to produce, but it prevents sulfur dioxide being produced. You may have noticed 'low sulfur' petrol and diesel on sale at filling stations.
Problems with fuels: carbon dioxide
Global warming
Carbon dioxide from burning fuels causes global warming, a process capable of changing the world’s climate significantly.
Carbon dioxide in the atmosphere has risen at a higher rate since the 19th century
The temperature of the earth has risen over the years
As you can see from the graphs, the amount of carbon dioxide in the atmosphere has increased steadily over the past 150 years, and so has the average global temperature. Some of this is due to human activity.
Along with other gases such as methane and water vapour, carbon dioxide is a greenhouse gas. It absorbs heat energy and prevents it escaping from the Earth’s surface into space. The greater the amount of carbon dioxide in the atmosphere, the more heat energy is absorbed and the hotter the Earth becomes.
Greenhouse effect
1. The Sun’s rays enter the Earth’s atmosphere 2. Heat is reflected back from the Earth’s surface 3. Heat is absorbed by greenhouse gases, such as carbon dioxide, and as a result becomes trapped in the Earth’s atmosphere. 4. The Earth becomes hotter as a result
Results of global warming
A rise of just a few degrees in world temperatures will have a dramatic impact on the climate: * global weather patterns will change, causing drought in some places and flooding in others. * polar ice caps will melt, raising sea levels and causing increased coastal erosion and flooding of low-lying land – including land where major cities lie
The Triftgletscher glacier, Switzerland, 2002
The Triftgletscher glacier, Switzerland, 2003. As the glacier melts further, the lake's water level rises.
Scientists are trying to control the amount of carbon dioxide in the atmosphere by: * iron seeding of oceans * converting carbon dioxide into hydrocarbons
However, some scientists do not believe that the global temperature increase and the carbon dioxide increase are caused by human activities.
Biofuels
Biofuels come from the products of living organisms, such as methane biogas from decaying manure and sewage. Vegetable oils are also used as fuels for vehicles. Some of this biodiesel is made from waste cooking oil and rapeseed oil.
Ethanol
Ethanol is the type of alcohol found in alcoholic drinks such as wine and beer. It is also useful as a fuel. For use in cars and other vehicles it is usually mixed with petrol.
Ethanol can be made by a process called fermentation. This converts sugar from sugar cane or sugar beet into ethanol and carbon dioxide. Single-celled fungi, called yeast, contain enzymes that are natural catalysts for making this process happen:
C6H12O6 2C2H5OH + 2CO2
Advantages of using biofuels
Biofuels are carbon neutral, which means that they release only as much carbon dioxide when they burn as was used to make the original oil by photosynthesis.
This helps to reduce global warming.
However, some people are concerned about whether it is ethical to use food crops in this way, instead of using them to feed hungry people.
Hydrogen
Hydrogen is often seen as an environmentally friendly alternative to fossil fuels and biofuels. When hydrogen burns, the only product formed is water: hydrogen + oxygen → water 2H2 + O2 → 2H2O
Problems with hydrogen
Hydrogen powered car
Some hydrogen-powered vehicles fitted with hydrogen fuel cells have already been made and are on the road, but there are few of them because of difficulties involved in making and handling hydrogen.
Making hydrogen
At the moment, most hydrogen is made by reacting steam with coal or natural gas, which are non-renewable resources.
Hydrogen can also be made by passing electricity through water. Unfortunately, most electricity is generated using coal and other fossil fuels: any pollution from burning these fuels just happens at the power station instead of at the vehicle itself.
Handling hydrogen
Hydrogen gas is very flammable and may explode if handled incorrectly. It must be compressed and chilled, then stored in tough, insulated tanks. It is not as convenient as petrol and diesel.
Cracking
Fuels made from oil mixtures containing large hydrocarbon molecules are not efficient. They do not flow easily and are difficult to ignite. Crude oil often contains too many large hydrocarbon molecules and not enough small hydrocarbon molecules to meet demand - this is where cracking comes in.
Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules. Fractions containing large hydrocarbon molecules are vaporised and passed over a hot catalyst. This breaks chemical bonds in the molecules, and forms smaller hydrocarbon molecules.
Cracking is an example of a thermal decomposition reaction.
Some of the smaller molecules formed by cracking are used as fuels, and some of them are used to make polymers in plastics manufacture.
Alkenes
The products of cracking include alkenes (for example ethene and propene). The alkenes are a family of hydrocarbons that share the same general formula. This is CnH2n.
The general formula means that the number of hydrogen atoms in an alkene is double the number of carbon atoms. For example, ethene is C2H4 and propene is C3H6. Alkene molecules can be represented by displayed formulae, in which each atom is shown as its symbol (C or H) and the chemical bonds between them by a straight line.
Structure of alkenes alkene | formula | chemical structure | ball-and-stick model | ethene | C2H4 | | | propene | C3H6 | | |
Alkenes are unsaturated hydrocarbons. They contain a double bond, which is shown as two lines between two of the carbon atoms. The presence of this double bond allows alkenes to react in ways that alkanes cannot. They can react with oxygen in the air, so they could be used as fuels. But they are more useful than that. They can be used to make ethanol (alcohol) and polymers (plastics), two crucial products in today's world.
Testing for alkenes
Bromine water is used to tell the difference between an alkane and an alkene. An alkene will turn brown bromine water colourless as it reacts with the double bond. Bromine water remains brown in the presence of an alkane as there is no double bond.
Polymers
Alkenes can be used to make polymers. Polymers are very large molecules made when many smaller molecules join together, end-to-end. The smaller molecules are called monomers. In general: lots of monomer molecules → a polymer molecule
The animation shows how several chloroethene monomers can join end-to-end to make poly(chloroethene), which is also called PVC.
Alkenes can act as monomers because they have a double bond: * Ethene can polymerise to form poly(ethene), which is also called polythene. * Propene can polymerise to form poly(propene), which is also called polypropylene.
Different polymers have different properties, so they have different uses. The table below gives some examples.
Examples of polymers and their uses polychloroethene | water pipes and insulation on electricity cables | Uses of polymersPolymers have many different uses. The use of a polymer is related to its properties.Polymer properties and uses Monomer | Polymer | Properties | Uses | ethene | poly(ethene) | flexible, cheap, electrical insulator | plastic bags and bottles, coating on electrical wires | propene | poly(propene) | flexible and strong | buckets and crates | chloroethene | poly(chloroethene) or PVC | tough, cheap and long lasting | window frames | tetrafluoroethene | poly(tetrafluoroethene) or PTFE | tough and non-stick | non-stick coating on pans |
Polymer problemsOne of the useful properties of polymers is that they are unreactive, so they aresuitable for storing food and chemicals safely. Unfortunately, this property makes it difficult to dispose of polymers.BiodegradableMost polymers, including poly(ethene) and poly(propene) are not biodegradable. This means that microorganisms cannot break them down, so they may last for many years in rubbish dumps. However, it is possible to include chemicals that cause the polymer to break down more quickly. Carrier bags and refuse bags made from such degradable polymers are already available.IncinerationPolymers can be burnt or incinerated. They release a lot of heat energy when they burn and this can be used to heat homes or to generate electricity.There are problems with incineration. Carbon dioxide is produced, which adds to global warming. Toxic gases are produced unless the polymers are incinerated at high temperatures.RecyclingPolymers have recycling symbols like this one for PVC to show what they areMany polymers can be recycled. This reduces the disposal problems and the amount of crude oil used. But the different polymers must be separated from each other first, and this can be difficult and expensive to do. | |
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