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OCR GCSE Chemistry Textbook C6 Chemical Synthesis

By emsciency123 Feb 16, 2014 9898 Words


C6: Chemical Synthesis

Why study chemical
We use chemicals to preserve food, treat disease, and decorate our homes. Many of these chemicals do not occur naturally: they are synthetic. Developing new products, such as drugs to treat disease, depends on chemists who synthesise and test new chemicals.

What you already know
Atoms are rearranged during chemical reactions.
The number of atoms of each element stays the same
in a chemical reaction.
Raw materials can be used to make synthetic
Alkalis neutralise acids to make salts.
Chemical reactions can be represented by word
equations and balanced symbol equations.
Some substances are made up of electrically charged
particles called ions.
Data is more reliable if it can be repeated.

Find out about
the importance of the chemical industry
a theory to explain acids and alkalis
reactions that give out and take in energy
techniques for controlling the rate of chemical change
the steps involved in the synthesis of a new chemical
ways to measure the efficiency of chemical synthesis.

The Science
Chemists who synthesise
new chemicals need practical
skills and an understanding
of science explanations.
They must control reactions
so that they are neither too
slow nor too fast. They must
calculate how much of the
reactants to use to make the
amount of product required.
Chemists also take into
account any energy changes.
Acids are important reactants
in synthesis. Ionic theory
explains the characteristic
behaviours of these chemicals.

Ideas about
Chemists make sure they use
the right grade of chemical for
a reaction. Technical chemists
test chemicals from suppliers
to check the purity. They take
measurements and make
sure the data they collect is
as accurate and reliable as
possible. They can then make
the best estimate of the true
value of the purity.


A  The chemical industry
Find out about
DD the chemical industry
DD bulk and fine chemicals
DD the importance of
chemical synthesis

The chemical industry converts raw materials, such as crude oil, natural gas, minerals, air and water, into useful products. The products include chemicals for use as food additives, fertilisers, pigments, dyes, paints, and pharmaceutical drugs.

The industry makes bulk chemicals on a scale of thousands or even millions of tonnes per year. Examples are ammonia, sulfuric acid, sodium hydroxide, chlorine, and ethene.
On a much smaller scale, the industry makes fine chemicals such as drugs, herbicides, and pesticides. It also makes small quantities of speciality chemicals needed by other manufacturers for particular purposes. These include such things as flame retardants, food additives, and the liquid crystals for flat-screen televisions and computer displays.

The chemical industry converts raw
materials into pure chemicals, which
are then used in synthesis to make a
wide range of products.

The range of products made by the chemical industry in the UK by value of sales.

Industrial chemists work in the plant
and the laboratory.

Key words
DD chemical industry
DD bulk chemicals
DD fine chemicals
DD plant
DD pilot plant
DD scale up


The part of a chemical works that produces a chemical is called a plant. Some of the chemical reactions occur at a high temperature, so a source of energy is needed. Also, a lot of electrical power is needed for pumps to move reactants and products from one part of the plant to another. Sometimes the energy to produce this power can be supplied by chemical reactions that give out energy.

Sensors monitor the conditions, such as temperature and pressure, in all areas of the plant. The data is fed to computers in the control centre, where the technical team controls the plant.

C6: Chemical Synthesis

People in the chemical industry
People with many different skills are needed in the chemical industry. Research chemists work in laboratories to find new processes and develop new products.
The industry needs new processes so that it can be more competitive and more sustainable. The aim is to use smaller amounts of raw materials and energy while creating less waste.
People devising new products have to work closely with people in the marketing and sales department. They are able to say if the novel product is wanted. If the new product is promising, it may first be tried out by making small amounts of it in a pilot plant.

As part of the market research, possible new products are given to customers for trial. At the same time, financial experts estimate the value of the new product in the market. They then compare this with the cost of making the product to check that the new process will be profitable.

Chemical engineers have to scale up the process and design a full-scale plant. This can cost hundreds of millions of pounds.
Some chemicals from the industry go directly on sale to the public, but

1 Classify the following as
raw materials or products
of the chemical industry:
air, ammonia, aspirin,
water, crude oil, polythene.
2 Give the name and chemical

formula of a bulk chemical.
3 List these chemicals under
two headings: ‘bulk chemical’
and ‘fine chemical’.
the drug aspirin
the hydrocarbon ethene
the perfume chemical
the acid sulfuric acid
the herbicide glyphosate
the alkali sodium
the food dye carotene

most of them are used to make other products. Transport workers carry the chemicals to the industry’s customers.
Every chemical plant needs managers and administrators to control the whole operation. There are also people in service departments who look after the needs of the people working in the plant. These include medical and catering staff, and training and safety officers.

Plant operators monitor the processes from a control room.

Maintenance workers help to keep the plant running.

A: The chemical industry


B  Acids and alkalis
Find out about
DD acids and alkalis
DD the pH scale
DD reactions of acids

The word acid sounds dangerous. Nitric, sulfuric, and hydrochloric acids are very dangerous when they are concentrated. You must handle them with great care. These acids are less of a hazard when diluted with water. Dilute hydrochloric acid, for example, does not hurt the skin if you wash it away quickly, but it stings in a cut and rots clothing. It is in fact present in our stomachs where it helps to break down food and kill bacteria. Our stomachs are lined with a protective layer of mucus. Not all acids are dangerous to life. Many acids are part of life itself. Biochemists have discovered the citric acid cycle. This is a series of reactions in all cells. The cycle harnesses the energy from respiration for movement and growth in living things.

Organic acids

Key words
DD acid
DD alkali

Organic acids are molecular. They are made of groups of atoms. Their molecules consist of carbon, hydrogen, and oxygen atoms. The acidity of these acids arises from the hydrogen in the —COOH group of atoms. Citric and tartaric acids are examples of solid organic acids. Ethanoic acid is a liquid organic acid.

Acetic acid (chemical name: ethanoic acid) is a
liquid. It is the acid in vinegar. Most white
vinegar is just a dilute solution of acetic acid.
Brown vinegars have other chemicals in the
solution that give the vinegar its colour and
flavour. Most microorganisms cannot survive in
acid, so vinegar is used as a preservative in
pickles (E260).


Citric acid (a solid acid) is found in citrus fruits like oranges and lemons. The human body processes about 2 kg of citric
acid a day during respiration. Citric acid and its salts are added to food to prevent them reacting with oxygen in the
air, and to give a tart taste to drinks and sweets.

C6: Chemical Synthesis

Mineral acids
Sulfuric, hydrochloric, and nitric acids come from inorganic or mineral sources. The pure acids are all molecular. Sulfuric acid and nitric acid are liquids at room temperature. Hydrogen chloride is a gas, and becomes hydrochloric acid when it dissolves in water.

Pharmacists sell antacids in tablets to control heartburn and indigestion. The chemicals in these medicines are the chemical opposites of acids. They are designed to neutralise excess acid produced in the stomach – hence the name ‘antacids’.

Antacids in medicines are usually insoluble in water. Other chemical antacids are soluble in water and give a solution with a pH above 7. Chemists call them alkalis. Common alkalis are sodium hydroxide, NaOH; potassium hydroxide, KOH; and calcium hydroxide, Ca(OH)2. The traditional name for sodium hydroxide is caustic soda. The word caustic means that the chemical attacks living tissue, including skin. Alkalis can do more damage to delicate tissues than dilute acids. Caustic alkalis are used in the strongest oven and drain cleaners. They have to be used with great care.

Sulfuric acid, H2SO4, is
manufactured from sulfur, oxygen,
and water. The pure, concentrated
acid is an oily liquid. The chemical
industry in the UK makes about
2 million tonnes of the acid each
year. The acid is essential for the
manufacture of other chemicals,
including detergents, pigments,
dyes, plastics, and fertilisers.

1 From the pictures of
molecules work out the
formulae of:
a acetic acid
b citric acid.
2 What are the formulae of
these antacids?
aMagnesium hydroxide
made up of magnesium
ions, Mg2+, and
hydroxide ions, OH–.
bAluminium hydroxide
made up of aluminium
ions, Al3+, and
hydroxide ions, OH–.
Hydrogen chloride forms when
concentrated sulfuric acid is added
to salt (sodium chloride) crystals.
Hydrogen chloride, HCl, is a gas that
fumes in moist air and is very soluble
in water.

Oven cleaners often contain
caustic alkalis.

B: Acids and alkalis


Indicators and the pH scale
Indicators change colour to show whether a solution is acidic or alkaline. Blue litmus turns red in acid solution and red litmus turns blue in alkalis. Special mixed indicators, such as universal indicator, show a range of colours and can be used to estimate pH values.

pH values can also be measured electronically using a pH meter with an electrode that dips into the solution. The meter can be read directly from the display or it may be connected to a datalogger or computer. The term pH appears on many cosmetic, shampoo, and food labels. It is a measure of acidity. The pH scale is a number scale that shows the acidity or alkalinity of a solution in water. Most laboratory solutions have a pH in the range 1–14.

The pH scale.

A pH meter can be used to
measure pH values.

Hydrangea flowers contain natural indicators – they are blue if grown on acid soil and pink on alkaline soil. Note that this is the opposite of the litmus colours.

Reaction of acids
Acids with metals
Using a feather to brush away
hydrogen bubbles while etching
a metal plate with acid.

Acids react with metals to produce salts. The other product is hydrogen gas.



For example:  2HCl(aq) Mg(s)





Not all metals will react in this way. You may remember the list of metals in order of reactivity in C5, G. Metals below lead in the list do not react with acids, and even with lead it is hard to detect any change in a short time.


C6: Chemical Synthesis

Acids with metal oxides or hydroxides
An acid reacts with a metal oxide or hydroxide to form a salt and water. No gas forms.


metal oxide
(or hydroxide)

For example: 2HCl(aq)



The reaction between an acid and a metal oxide is often a vital step in making useful chemicals from ores.

Acids with carbonates
Acids react with carbonates to form a salt, water, and bubbles of carbon dioxide gas.
salt  water  carbon dioxide

Geologists can test for carbonates by dripping hydrochloric acid onto rocks. If they see any fizzing, the rocks contain a carbonate. This is likely to be calcium carbonate or magnesium carbonate.
The word equation is:



 water 

The balanced equation is:



DD pH scale
DD metal oxide
DD indicators DD metal
DD metals
DD carbonates
DD salts



acid  metal carbonate

Key words

CaCl2(aq) H2O(l) CO2(g)

This is a foolproof test for the carbonate ion. So the term ‘the acid test’ has come to be used to describe any way of providing definite proof.

3 pattern can be etched
onto a zinc plate using
hydrochloric acid to react
with the zinc, forming
soluble zinc chloride, ZnCl2.
Write a word equation
and a balanced symbol
equation for the reaction.
4 Magnesium hydroxide,
Mg(OH)2, is an antacid
used to neutralise excess
stomach acid, HCl. Write a
word equation and a
balanced symbol equation
for the reaction.
5 There is a volcano in
Tanzania, Africa, whose lava
contains sodium carbonate,
Na2CO3. The cooled lava
fizzes with hydrochloric
acid. Write a word equation
and a balanced symbol
equation for the reaction.
6 Limescale forms in kettles
where hard water is heated.
Limescale consists of
calcium carbonate. Three
acids are often used to
remove limescale:
citric acid, acetic acid (in
vinegar), and dilute
hydrochloric acid. Which acid
would you use to de-scale an
electric kettle and why?

Testing for carbonate using hydrochloric acid.

B: Acids and alkalis


C  Salts from acids
What makes an acid an acid?

Find out about

Chemists have a theory to explain why all the different compounds that are acids behave in a similar way when they react with indicators, metals, carbonates, metal oxides, and metal hydroxides.

DD an ionic explanation for
neutralisation reactions
DD salts and their formulae

It turns out that acids do not simply mix with water when they dissolve. They react, and when they react with water they produce hydrogen ions (H1). For example, hydrochloric acid is a solution of hydrogen chloride in water. The HCl molecules react with the water to produce hydrogen ions and chloride ions.







H+(aq) 1 Cl–(aq)

The theory of acids is an ionic theory. Any compound is an acid if it produces hydrogen ions when it dissolves in water.


All acids contain hydrogen in their formula. Nitric acid, HNO3, and phosphoric acid, H3PO4, both contain hydrogen. But not all chemicals that contain hydrogen are acids. Ethane, C2H6, and ethanol, C2H5OH, are not acids.

chloride gas

In an organic acid it is only the hydrogen atom in the —COOH group that can ionise when the acid dissolves in water.

What makes a solution alkaline?



Alkalis such as the soluble metal hydroxides are ionic compounds. They consist of metal ions and hydroxide ions (OH–). When they dissolve, they add hydroxide ions to water. It is these ions that make the solution alkaline.




Na+(aq) 1 OH–(aq)



Hydrogen chloride dissolves in
water to make hydrochloric acid.
The HCl molecules react with water
to form ions.

Key words
DD hydrogen ions
DD hydroxide ions
DD neutralisation reaction


Sodium hydroxide and hydrochloric acid react to produce a salt (sodium chloride) and water.
Na+(aq) 1 OH–(aq) 1 H+(aq) 1 Cl–(aq)

Na+(aq) 1 Cl–(aq) 1 H2O(l)

During a neutralisation reaction the hydrogen ions from an acid react with hydroxide ions from the alkali to make water.
H+(aq) + OH–(aq)


The remaining ions in the solution make a salt.

C6: Chemical Synthesis

Salts form when a metal oxide, or hydroxide, neutralises an acid. So every salt can be thought of as having two parents. Salts are related to a parent metal oxide or hydroxide and to a parent acid.

Salts are ionic (see C4, J: Ionic theory). Most salts consist of a positive metal ion combined with a negative non-metal ion. The metal ion comes from the parent metal oxide or hydroxide. The non-metal ion comes from the parent acid.



1 Write equations to show
what happens when these
compounds dissolve in
a nitric acid
sulfuric acid
c calcium hydroxide.







dilute sodium
















neutral solution of sodium chloride


O– H









Dilute sodium hydroxide solution neutralises dilute hydrochloric acid, forming a neutral solution of sodium chloride. Water molecules not involved in the reaction are shown in a paler colour.

It is possible to work out the formulae of salts knowing the charges on the ions. Remember that all compounds are overall electrically neutral (see C4, K: Ionic theory and atomic structure). Some nonmetal ions consist of more than one atom. The table below includes some examples. In the formula for magnesium nitrate, Mg(NO3)2, the brackets around the NO3 show that two complete nitrate ions appear in the formula.

Non-metal ions that consist of more than one atom










2 Write down the name of
the salt produced in the
following reactions:
lithium hydroxide with
hydrochloric acid
calcium carbonate with
nitric acid
magnesium oxide with
sulfuric acid.
3 Use the tables of ions in C4
Section K and on this page
to write down the formulae
of these salts:
a potassium nitrate
b magnesium carbonate
c sodium sulfate
d calcium nitrate.
4 Use the tables of ions in C4
Section K and on this page
to write down the charge
on the metal ion in each of
these salts:

C: Salts from acids


D  Purity of chemicals
Find out about
DD purity
DD titrations for testing

Grades of purity
The reactions of acids with metals, oxides, hydroxides, and carbonates can be used to make valuable salts. For uses such as food or medicines, these salts have to be made pure so that they are safe to swallow. Chemicals do not always have to be pure. Calcium carbonate, for example, is used in a blast furnace to extract iron from its ores. The iron industry can use limestone straight from a quarry. Limestone has some impurities but they do not stop it from doing its job in a blast furnace. Suppliers of chemicals offer a range of grades of chemicals. In a school laboratory you might use one of these grades: technical, general laboratory, and analytical. The purest grade is the analytical grade.

Label on a bottle of laboratory grade
calcium carbonate. The term ‘assay’
tells you how pure the chemical is. The
calcium carbonate is 99% pure with the
small amounts of impurities shown.

Key words
DD titration
DD burette
DD end point

Purifying a chemical is done in stages. Each stage takes time and money, and becomes more difficult. So the higher the purity, the more expensive the chemical. Manufacturers therefore buy the grade most suitable for their purpose.

When deciding what grade of chemical to use for a particular purpose, it is important to know:
the amount of impurities
what the impurities are
how they can affect the process
whether they will end up in the product, and whether it matters if they do.

1 Uses of sodium chloride
(salt) include:
i flavouring food
ii melting ice on roads
saline drips in hospitals.
Put these in order of
the grade of sodium
chloride required, with
the purest first.
2 From 'steps involved in a
titration' in which step is:
a a solution made?
b pipette used, and
what is it used for?
c end point reached,
and how does the
technician know?


Testing purity
Medicines contain an active ingredient. Other ingredients are included to make the medicine pleasant to taste and easy to take. This means that the pharmaceutical companies that make medicines need sweeteners, food flavours, and other additives.

The companies buy in many of their ingredients. Technical chemists working for the companies have to make sure that the suppliers are delivering the right grade of chemical.
Citric acid is often added to syrups, such as cough medicines, to control their pH. Technicians can check the purity of the acid using a procedure called a titration, which measures the volume of alkali that it can neutralise. The technician has to know the accurate concentration of the alkali.

C6: Chemical Synthesis

Steps involved in a titration


The technician fills a burette with a
solution of sodium hydroxide. She
knows the concentration of the alkali.



The technician weighs out a sample
of citric acid accurately.

3 A 1.35 g sample of impure
citric acid was dissolved in
water and titrated with
sodium hydroxide solution
of concentration 40 g/dm3.
The average titre was
found to be 20.6 cm3.
Use the following
equation to find the
mass of citric acid in
the sample:

mass of citric acid (g) 5
average titre (cm3) 3
Use the following
equation to calculate
the percentage purity of
the sample:
percentage purity 5

3 The technician dissolves the acid in
pure water. Then she adds a few
drops of phenolphthalein indicator.
The indicator is colourless in the acid


The technician adds alkali from the
burette. She swirls the contents of the
flask as the alkali runs in. Near the end she
adds the alkali drop by drop. At the end
point all the citric acid is just neutralised.
The indicator is now permanently pink.

mass of citric acid

​ _____________​ 3 100 %
mass of sample

4 A technical chemist
measures the purity of
tartaric acid by titration,
and obtains these results:
98.7%, 99.0%. 105.4%,
80.0%, 98.8%, 98.5%
Suggest reasons why
the results are not
exactly the same.
Which two values
should be checked?
Calculate the mean
value of purity after
discarding the two
outlying values.
Suggest the limits
between which the true
value is likely to lie.

The technician will repeat the titration several times. If there are any results that differ greatly from the rest, and there is reason to doubt their accuracy, they will be discarded. The remaining values will be averaged to find the mean.

D: Purit y of chemicals


E  Energy changes in chemical reactions
Find out about
DD reactions that give out
energy and reactions
that take in (absorb)

Exothermic and endothermic reactions
Most chemical reactions need a supply of energy to get them started. Some also need energy to keep them going. That is why Bunsen burners and electric heating mantles are so common in laboratories.

Most reactions give out energy once they are going – combustion (burning), and neutralisation of acids to make salts are common examples. Sulfuric acid is an important chemical in industry. One of the key reactions in making this acid has to be carefully controlled because it gives out a great deal of energy.

Reactions that give out energy to the surroundings (the surroundings get hotter) are called exothermic. There are also reactions that absorb energy from the surroundings (the surroundings get cooler). These are called endothermic.

It is possible to tell whether a chemical reaction is exothermic or endothermic by measuring the temperature of the reactants before the reaction starts, and the temperature of the products after it has finished. If the temperature has risen, the reaction is exothermic. If it has dropped, the reaction is endothermic.

Energy changes in the chemical industry
Scientists working in the chemical industry need to know whether a reaction is exothermic or endothermic when it is part of a synthesis. There are several reasons for this:
It takes fuel, which costs money, to provide the energy input for endothermic reactions.
The energy given out by exothermic reactions can be used elsewhere in the plant, to produce electricity, for example.
A temperature increase makes chemical reactions go faster (see Section F) – a reaction that gives out heat energy will tend to get faster and faster, and may ‘run away’, possibly causing an explosion. Such a ‘runaway’ reaction may have been the cause of a major disaster at a chemical plant in Bhopal, India, in 1984. Between 2000 and 15 000 people may have died and over 25 years later, the health of many people is still being affected by the incident. Proper understanding and control of the energy changes in the reactions concerned might have prevented the disaster.

Both exothermic and endothermic
reactions have practical uses.
An exothermic reaction provides the
energy for welding. A cold
pack absorbs energy from an injured
muscle using an endothermic reaction.


C6: Chemical Synthesis

Energy-level diagrams
All chemical reactions give out or take in energy. Understanding energy changes helps chemists to control reactions.
In an exothermic reaction (where energy is given out), the products must have less energy than the reactants.
In an endothermic reaction (where energy is absorbed), the products must have more energy than the reactants.

Key words
DD exothermic
DD endothermic
DD energy-level diagram

Chemists keep track of the changes in energy in chemical reactions using energy-level diagrams. These show the energies of the reactants and products. Notice that energy is plotted vertically up the diagram. Reactants are on the left and products on the right. These are usually shown using a balanced equation.

An electric heating mantle,
used to heat up reactions
without a naked flame.

An exothermic reaction.


The energy-level diagram for the reaction of magnesium and
hydrochloric acid, which is exothermic. Energy is given out in this reaction, so the products have less energy than the reactants.

An endothermic reaction.

Energy-level diagram for the reaction of citric acid and sodium hydrogencarbonate, which is endothermic. Energy is absorbed in this reaction, so the products have more energy than the reactants.

1 If you have ever had a
plaster cast for a broken
bone, apart from the pain of
the injury, you might
remember the pleasant
feeling of warmth as the wet
plaster begins to set. If you
put a sherbet sweet on your
tongue you will feel your
mouth getting cold. Both of
these are chemical reactions.
Which reaction is
exothermic and which
is endothermic?
bDraw an energy-level
diagram for each reaction.
Use ‘reactants’ and
‘products’ for the names
of the chemicals involved.

E: Energy changes in chemical reac tions


F  Rates of reaction
Find out about
DD measuring rates of
DD factors affecting rates
of reaction
DD catalysts in industry
DD collision theory

Controlling reaction rates
Some chemical reactions seem to happen in an instant. An explosion is an example of a very fast reaction.
Other reactions take time – seconds, minutes, hours, or even years. Rusting is a slow reaction and so is the rotting of food.
It is the chemist’s job to work out the most efficient way to synthesise a chemical. It is important that the chosen reactions happen at a convenient speed. A reaction that occurs too quickly can be hazardous. A reaction that takes several days to complete is not practical because it ties up equipment and people’s time for too long, which costs money.

Measuring rates of reaction
Your pulse rate is the number of times your heart beats every minute. The production rate in a factory is a measure of how many articles are made in a particular time. Similar ideas apply to chemical reactions. Chemists measure the rate of a reaction by finding the quantity of product produced or the quantity of reactant used up in a fixed time. For the reaction

An explosion is an example of a very
fast chemical reaction.

Mg(s) 1 2HCl(aq)

MgCl2(aq) 1 H2(g)

the rate can be measured quite easily by collecting and measuring the hydrogen gas produced.
change in the volume of hydrogen
average rate 5 ​ 
time for the change to happen
In most chemical reactions, the rate changes with time. The graph on the left is a plot of the volume of hydrogen formed against time for the reaction of magnesium with acid. The graph is steepest at the start, showing that the rate of reaction was greatest at that point. As the reaction continues the rate decreases until the reaction finally stops. The steepness of the line is a measure of the rate of reaction.

A plot of the volume of hydrogen
formed against time for a reaction of
magnesium with hydrochloric acid.

Key word
DD rate of reaction


1 For each of these reactions, pick a method from the opposite page that could be used to measure the rate of reaction: (Hint: Look at the states of the reactants and products.)
CaCl2(aq) + CO2(g) + H2O(l)
aCaCO3(s) + 2HCl(aq)
ZnSO4(aq) + H2(g)
b Zn(s) + H2SO4(aq)
2NaCl(aq) + SO2(g) + S(s) + H2O(l)
cNa2S2O3(aq) + 2HCl(aq)

C6: Chemical Synthesis

Methods of measuring rates of reaction
Collecting and measuring a gas product

Record the volume at regular intervals, such as every 30 or
60 seconds. A gas syringe could be used to collect the gas,
instead of a measuring cylinder – see the next page.

Timing how long it takes for a small
amount of solid reactant to disappear

Mix the solid and solution in the flask and start
the timer. Stop it when you can no longer see
any solid.

Measuring the loss of mass as a gas forms

Record the mass at regular intervals such as every
30 or 60 seconds.

Timing how long it takes for a solution to turn cloudy

This is for reactions that produce an insoluble
solid. Mix the solutions in the flask and start the
timer. Stop it when you can no longer see the
cross on the paper through the solution.

Manual timing of reactions is only suitable for reactions that are over in a few minutes (or more). Modern methods using lasers and dataloggers allow chemists to measure the rates of reactions that are over in less than one thousand billion billionths of a second (one femtosecond).

F: Rates of reac tion


Key words
DD concentration
DD surface area
DD catalyst

Factors affecting reaction rates
Powdered Alka-Seltzer reacts faster in water than a tablet. Milk standing in a warm kitchen goes sour more quickly than milk kept in a refrigerator. Changing the conditions alters the rate of these processes and many others.

Factors that affect the rate of chemical reactions are:
the concentration of reactants in solution – the higher the concentration, the faster the reaction
the surface area of solids – powdering a solid increases the surface area in contact with a liquid, solution, or gas, and so speeds up the reaction.
the temperature – typically a 10 °C rise in temperature can roughly double the rate of many reactions
catalysts – these are chemicals that speed up a chemical reaction without being used up in the process.

The factors in action
The apparatus in the diagram was used in an investigation into the effect of changing the conditions on the reaction of zinc metal with sulfuric acid. The graph below shows the results.

Apparatus used to investigate
the factors affecting the rate of
reaction of zinc with sulfuric acid.
The volume of hydrogen formed over time during an investigation of the factors affecting the rate of reaction of zinc with sulfuric acid. The investigator used the same mass of zinc each time. There was more than enough metal to react with all the acid.

Zn(s) 1 H2SO4(aq)

ZnSO4(aq) 1 H2(g)

The red line on the graph plots the volume of hydrogen gas against time using zinc granules and 50 cm3 of dilute sulfuric acid at 20 °C. The reaction gradually slows down and stops because the acid concentration falls to zero. There is more than enough metal to react with all the acid. The zinc is in excess.


C6: Chemical Synthesis

The effect of concentration
Line A on the graph shows the result of using acid that was half as concentrated while leaving all the other conditions the same as in the original set up.
The investigator added 50 cm3 of this more dilute acid. Halving the acid concentration lowers the rate at the start. The final volume of gas is cut by half because there was only half as much acid in the 50 cm3 of solution to start with.

The effect of surface area
Line B on the graph shows the result of using the same excess of zinc metal in fewer larger pieces. All other conditions were the same as in the original setup. Fewer larger lumps of metal have a smaller total surface area so the reaction starts more slowly. The amount of acid is unchanged and the metal is still in excess so that the final volume of hydrogen is the same.

2 When a lump of calcium
carbonate is placed into a
beaker of acid it reacts and
carbon dioxide is given off.
Suggest three ways to
speed up the reaction.
3 How is it possible to
control conditions to speed
up these changes:
a setting of an epoxy
b the cooking of an egg
c conversion of oxides
of nitrogen in car
exhausts to nitrogen.
4 When investigating the
effect of temperature on a
chemical reaction, why is it
important to keep all other
conditions the same?

Breaking up a solid into smaller pieces increases the total surface area. This increases the amount of contact between the solid and the solution, making it possible for the reaction to go faster.

5 The effect of concentration
on the rate of reaction
between zinc and sulfuric
acid was investigated. The
results were plotted on
a graph.

The effect of temperature
Line C on the graph shows the result of carrying out the reaction at 30 °C while leaving all the other conditions the same as in the original set up. This more or less doubles the rate at the start. The quantities of chemicals are the same so the final volume of gas collected is the same as it was originally.

The effect of adding a catalyst
Line D on the graph shows what happens when the investigation is repeated with everything the same as in the original set up but with a catalyst added. The reaction starts more quickly. Catalysts do not change the final amount of product, so the volume of gas at the end is the same as before.

aIs there a correlation?
If so, describe it.
Which result is an
outlier? Suggest a
reason why this result is
different from the
expected value.

F: Rates of reac tion


Catalysts in industry
What is a catalyst?

A catalyst is a chemical that speeds up a chemical reaction. It takes part in the reaction, but is not used up.
Modern catalysts can be highly selective. This is important
when reactants can undergo more than one chemical
reaction to give a mixture of products. With a suitable
catalyst it can be possible to speed up the reaction that
gives the required product, but not speed up other possible
reactions that create unwanted by-products.

Better catalysts
Catalysts are essential in many industrial processes. They
make many processes economically viable. This means
that chemical products can be made at a reasonable cost
and sold at affordable prices.
Research into new catalysts is an important area of
scientific work. This is shown by the industrial
manufacture of ethanoic acid (see Section B) from
methanol and carbon monoxide. This process was first
developed by the company BASF in 1960 using a cobalt
compound as the catalyst at 300 ºC and at a pressure 700
times atmospheric pressure.
About six years later the company Monsanto developed a
process using the same reaction, but a new catalyst system
based on rhodium compounds. This ran under much
milder conditions: 200 ºC and 30–60 times atmospheric

The manufacture of ethanoic acid from methanol and
carbon monoxide uses a catalyst to speed up the
methanol 1 carbon monoxide
CH3OH(g) 1CO(g)


ethanoic acid

In 1986, the petrochemical company BP bought the
technology for making ethanoic acid from Monsanto. It
has since devised a new catalyst based on compounds of
iridium. This process is faster and more efficient. Iridium
is cheaper, and less of the catalyst is needed. Iridium is
even more selective so the amount of ethanoic acid
produced (yield) is greater and there are fewer
by-products. This makes it easier and cheaper to make
pure ethanoic acid and there is less waste.

C6: Chemical Synthesis

Collision theory
Chemists have a theory to explain how the various factors affect reaction rates.

Key word
DD collision theory

The basic idea is that particles, such as molecules, atoms, and ions can only react if they bump into each other. Imagining these particles colliding with each other leads to a theory that can account for the effects of concentration, temperature, and catalysts on reaction rates. According to collision theory, when molecules collide some bonds between atoms can break while new bonds form. This creates new molecules.

Molecules are in constant motion in gases, liquids, and solutions. There are millions upon millions of collisions every second. Most reactions would be explosive if every collision led to a reaction. It turns out that only a very small proportion of all the collisions are successful and actually lead to a reaction. These are the collisions in which the molecules are moving with enough energy to break bonds between atoms.

Any change that increases the number of successful collisions per second has the effect of increasing the rate of reaction. Increasing the concentration of solutions of dissolved chemicals increases the frequency of collisions. The same small proportion of these collisions will be successful, but now there are more of them. This means that there will also be more successful collisions.


Molecules have a greater chance of colliding in a more concentrated solution. More frequent collisions means – faster reactions. Reactions get faster if the reactants are more concentrated.

Breaking up a solid into smaller pieces increases its surface area. Increased surface area means that there are more atoms, molecules, or ions of the solid available to react. This speeds up the reaction by increasing the frequency of successful collisions with particles in liquid, solution, or gas.

Where do cobalt,
rhodium and iridium
appear in the periodic
Why is not surprising
that these three metals
can be used to make
catalysts for the same
7 Suggest reasons why it is
important to develop
industrial processes that:
a at lower
temperatures and
b produce less waste.

F: Rates of reac tion


G  Stages in chemical synthesis
Find out about
DD steps in synthesis
DD making a soluble salt

Making calcium chloride for dialysis
Chemical synthesis is a way of making new compounds. Synthesis puts things together to make something new. It is the opposite of analysis, which takes things apart to see what they are made of.
The kidneys remove toxic chemicals from the blood. In cases of kidney failure, patients are put on dialysis machines that do the job outside the body while they await a transplant. Blood passes out of the body through a tube into the dialysis machine.

Inside the machine, the blood flows past a special membrane. On the other side of the membrane is a solution containing a mixture of salts at the same concentrations as the same salts in the blood. The toxic chemicals pass from the blood through the membrane into the solution and are carried away. It is also possible for useful salts to pass back into the blood.

Kidney dialysis.

One of the salts in the dialysis solution is calcium chloride. It is a particularly important salt as the level of calcium in the blood has to be maintained at a particular level. Just a little bit too much or too little and the patient could become very ill indeed. The calcium chloride therefore has to be very pure, and the quantity added to the solution has to be measured accurately.

The process for making calcium chloride is shown in the flow chart.

The process for making very pure calcium chloride for dialysis.


C6: Chemical Synthesis

Making a sample of magnesium sulfate
Magnesium sulfate is another salt produced by the chemical industry. It has many uses including as a nutrient of plants.
The process of making magnesium sulfate (or any other soluble salt) on a laboratory scale illustrates the stages in a chemical synthesis. In the following method an excess of solid is added to make sure that all the acid is used up. This method is only suitable if the solid added to the acid is either insoluble in water or does not react with water.

Choosing the reaction
Any of the characteristic reactions of acids can all be used to make salts:
acid 1 metal
salt 1 hydrogen
acid 1 metal oxide or hydroxide
salt 1 water
acid 1 metal carbonate
salt 1 carbon dioxide 1 water
Magnesium metal is relatively expensive because it has to be extracted from one of its compounds. So it makes sense to use either magnesium oxide or carbonate as the starting point for making magnesium sulfate from sulfuric acid.

Carrying out a risk assessment
It is always important to minimise exposures to risk. You should take care to identify hazardous chemicals. You should also look for hazards arising from equipment or procedures. This is a risk assessment.

An operator emptying magnesium
sulfate into the tank of a sprayer on
a farm. He is wearing protective
clothing because magnesium
sulfate is harmful. As well as being
a micronutrient needed for healthy
plant growth, the salt is a:
raw material in soaps and
laxative in medicine
efreshing additive in bath water
aw material in the manufacture
of other magnesium compounds
upplement in feed for poultry
and cattle
oagulant in the manufacture of
some plastics.

In this preparation the magnesium compounds are not hazardous. The dilute sulfuric acid is an irritant, which means that you should keep it off your skin and especially protect your eyes. You should always wear eye protection when handling chemicals, for example.

1 Refer to the flow diagram for the process of
making calcium chloride.
Write the word and balanced symbol
equations for the reaction used to make
the salt.
bIdentify steps taken to make the yield of the
pure salt as large as possible.

2 Epsom salts consist of magnesium sulfate.
Magnesium sulfate is soluble in water. Produce a
flow diagram to show how you could remove
impurities that are insoluble in water from a
sample of Epsom salts. Processes you might use
include: crystallisation, dissolving, drying,
evaporation, filtration.

G: Stages in chemical synthesis


3 Write the balanced equation
for the reaction of magnesium
carbonate with sulfuric acid.
4 Why does the mixture of
magnesium carbonate and
sulfuric acid froth up?
5 What is the advantage of:
ausing powdered
magnesium carbonate?
warming when most
of the acid has been
used up?
c adding a slight excess of
the solid to the acid?
6 Why is it impossible to use
this method to make a pure
metal sulfate by the reaction
of dilute sulfuric acid with:
a lithium metal?
b sodium hydroxide?
c potassium carbonate?
7 Look at the procedure on
this and the following page
for making magnesium
sulfate, and identify the step
when risks might arise from:
chemicals that react
vigorously and spill over
chemicals that might
spit or splash on heating
c  apparatus that
might cause burns
apparatus that might
crack and form sharp

Key word
DD risk assessment


Working out the quantities to use
Reacting masses can be used to work out the amount of reactants needed to produce a particular amount of product (see Section H). In this procedure the solid is added in excess. This means that the amount of product is determined by the volume and concentration of the sulfuric acid. The concentration of dilute sulfuric acid is 98 g/litre, and a volume of 50 cm3 dilute sulfuric acid is used. This contains 4.9 g of the acid.

Carrying out the reaction in suitable apparatus under
the right conditions
The reaction is fast enough at room temperature, especially if the magnesium carbonate is supplied as a fine powder.
This reaction can be safely carried out in a beaker. Stirring with a glass rod makes sure that the magnesium oxide or carbonate and acid mix well. Stirring also helps to prevent the mixture frothing up and out of the beaker.


Measure the required volume of acid
into a beaker. Add the metal oxide
or carbonate bit by bit until no more
dissolves in the acid. Warm gently
until all of the acid has been used up.
Make sure that there is a slight excess
of solid before moving on to the
next stage.

Separating the product from the reaction mixture
Filtering is a quick and easy way of separating the solution of the product from the excess solid. The mixture filters more quickly if the mixture is warm.

Filter off the excess solid, collecting
the solution of the salt in an
evaporating basin. The residue on
the filter paper is the excess solid.

C6: Chemical Synthesis

Purifying the product
After the mixture has been filtered, the filtrate contains the pure salt dissolved in water. Evaporating much of the water speeds up crystallisation. This is conveniently carried out in an evaporating basin. The concentrated solution can then be left to cool and crystallise. The drying can be completed in a warm oven. The crystals can then be transferred to a desiccator. This is a closed container that contains a solid that absorbs water strongly. 3



Pour the concentrated solution
into a labelled glass Petri dish
and set it aside to cool slowly.

Heat gently to evaporate some of the
water. Evaporate until crystals form
when a droplet of solution picked up
on a glass rod crystallises on cooling.


Crystals of pure magnesium sulfate
seen through a Polaroid filter (360).

Complete the drying in an
oven and then store in a

Measuring the yield and checking the purity of the product
The final step is to transfer the dry crystals to a weighed sample tube and re-weigh it to find the actual yield of crystals. Often it is important to carry out tests to check that the product is pure. The appearance of the crystals can give a clue to the

purity of the product. A microscope can help if the
crystals are small. The crystals of a pure product are
often well-formed and even in shape.


The weighed sample of product
showing the name and formula of
the chemical, the mass of product,
and the date it was made.

8 Identify the impurities
removed during the
purification stages of the
magnesium sulfate
9 Why is it important that the
magnesium carbonate is
added in excess to the
sulfuric acid?

G: Stages in chemical synthesis


H  Chemical quantities
Find out about
DD reacting masses
DD yields from chemical

Key words
DD relative formula mass
DD reacting mass
DD actual yield
DD theoretical yield
DD percentage yield

Chemists wanting to make a certain quantity of product need to work out how much of the starting materials to order. Getting the sums right matters – especially in industry, where a higher yield for a lower price can mean better profits.

The trick is to turn the symbols in the balanced chemical equation into masses in grams or tonnes. This is possible given the relative masses of the atoms in the periodic table.

Reacting masses
Adding up the relative atomic masses for all the atoms in the formula of a compound gives the relative formula mass of chemicals . Given the relative formula masses, it is then possible to work out the masses of reactants and products in a balanced equation. These are the reacting masses.

1 What mass of:
aHCl in hydrochloric acid
reacts with 100 g
calcium carbonate?
b 3 in dilute nitric
acid neutralises 56 g of
potassium hydroxide?
copper(II) oxide (CuO)

would you need to react
with sulfuric acid (H2SO4)
to make 319 g of copper
sulfate (CuSO4)?

Worked example
What are the masses of reactants and products when sulfuric acid reacts with sodium hydroxide? Step 1

2NaOH 1 H2SO4

Step 2

relative formula mass of NaOH 5 23 1 16 1 1 5 40

relative formula mass of H2SO4 5 (2 3 1) 1 32 1 (4 3 16) 5 98

relative formula mass of Na2SO4 5 (2 3 23) 1 32 1 (4 3 16) 5 142

relative formula mass of H2O 5 (2 3 1) 1 16 5 18

Steps 3 & 4

2NaOH  1  H2SO4  

Na2SO4 1 2H2O

  Na2SO4  1  2H2O

2 3 40 5 80 98


142 g

80 g

98 g

2 3 18 5 36
36 g

C6: Chemical Synthesis

The yield of any synthesis is the quantity of product obtained from known amounts of starting materials. The actual yield is the mass of product after it is separated from the mixture, purified, and dried.

Theoretical yield
The theoretical yield is the mass of product expected if the reaction goes exactly as shown in the balanced equation. This is what could be obtained in theory if there are no by-products and no losses while chemicals are transferred from one container to another. The actual yield is always less than the theoretical yield.

Worked example
What is the theoretical yield of ethanoic acid made from 8 tonnes of methanol?
Step 1 Write down the balanced equation
1 carbon monoxide

CH3OH(g) 1CO(g)


ethanoic acid



Step 2 Work out the relative formula masses

2 What is the mass of salt
that forms in solution
hydrochloric acid
neutralises 4 g sodium
12.5 g zinc carbonate,
ZnCO3, reacts with
excess sulfuric acid?
3 A preparation of sodium
sulfate began with 8.0 g of
sodium hydroxide.
Calculate the theoretical
yield of sodium sulfate
from 8.0 g sodium
Calculate the percentage
yield, given that the
actual yield was 12.0 g.

methanol:12 1 4 1 16 5 32

ethanoic acid:

Steps 3 & 4

24 1 4 1 32 5 60

Write down the relative reacting masses
and convert to reacting masses by adding the units

Theoretically, 32 tonnes of methanol should give 60 tonnes of ethanoic acid.
Step 5 Scale to the quantities actually used
If the theoretical yield of ethanoic acid 5 x tonnes, then

mass of ethanoic acid 60 tonnes x tonnes
mass of methanol 5  32 tonnes 5  8 tonnes

So, the yield of ethanoic acid from 8 tonnes of methanol should be

8 tonnes  3 

60 tonnes
5 15 tonnes
32 tonnes

Percentage yield

Worked example
What is the percentage yield if
8 tonnes of methanol produces
14.7 tonnes of ethanoic acid?
From the previous example:
theoretical yield 5 15 tonnes
actual yield 5 14.7 tonnes
actual yield
yield 5 theoretical yield 3 100
14.7 tonnes
3 100
15 tonnes


5 98%

The percentage yield is the percentage of the theoretical yield that is actually obtained. It is always less than 100%.

H: Chemical quantities


Chemists use their knowledge of chemical reactions to plan and carry out the synthesis of new compounds.

You should kn

that the chemical industry provides useful products such as food additives, fertilisers, dyestuffs, paints, pigments, and

how chemists use indicators and pH meters to detect acids and alkalis and to measure pH

that common alkalis include the hydroxides of sodium,
potassium, and calcium

that there are characteristic reactions of acids with metals, metal oxides, metal hydroxides, and metal carbonates that
produce salts

how chemists use ionic theory to explain why acids have similar properties, and how alkalis neutralise acids to form salts

that safety precautions are important when working with
hazardous chemicals such as corrosive acids and alkalis

how to follow the rate of a change by measuring the
disappearance of a reactant or the formation of a product and then to analyse the results graphically

that the concentrations of reactants, the particle size of solid reactants, the temperature, and the presence of catalysts are factors that affect the rates of reaction

how collision theory can explain why changing the
concentration of reactants, or the particle size of solids, affects the rate of a reaction

that reactions are exothermic if they give out energy, and
endothermic if they take in energy

that a chemical synthesis involves a number of stages and a
range of practical techniques, which are important to achieving a good yield of a pure product in a safe way

that a titration is a procedure that can be used to check the purity of chemicals used in synthesis

how to use the balanced equation for a reaction to work out the quantities of chemicals to use in a synthesis, and to calculate the theoretical yield.


1 ch
the reaction

2 risk


3 cal
culate quantities

4 carry out reactio
te the p
5 separa
the pr
6 purify
7 measure y

salt + wate


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s in

io n s re a c t

io n
o lu t





ti o n

ch e m


l in


s tr



rs a

l in

ic a t



id i



7 n e u tral

pH 8–1


4 al

m a t e ri als


ra w




te c hniq



pla nt







n at

ro m


cru d e il

ls f




ic a




li n


es on ions


it i ve s











drugs and


f e r t il i s e r s

d ye s


ic eq


p H m e te r


for m


s alt + c a



e nt









r ox





ic a

of Na, K, and Ca







hy d
















s alt

t h e s is




k in

sy n





oxides and


ne u tra li s a







ro g


r ic

, HC
ric a

g as e

li q u id


st a g


te r


th er m


e n erg y

f co

ct o



rea of solids
rface a
of su
f ct
c at a l ys t s






e xo






io n

solids eg citric acid

e fe

te m p e r a

tr a

o n lain

t ure

C6: Chemical Synthesis

C6: Chemical Synthesis


Ideas about Science
Scientists can never be sure that a measurement
tells them the true value of the quantity
being measured. Data is more reliable if it can
be repeated. If you take several measurements
of the same quantity, for example, in a titration,
the results are likely to vary. This may be because:
the quantity you are measuring is varying, for
example, the purity of separate solid samples of
a product may not be uniform
there are variations in the judgement of when
the indicator colour corresponds to the endpoint, or of the position of a meniscus • there are limitations in the measuring
equipment, such as burettes.
Usually the best estimate of the true value of a
quantity is the mean (or average) of several repeat
measurements. You should:
be able to calculate the mean from a set of
repeat measurements
know that a measurement may be an outlier if
it lies well outside the range of the other values
in a set of repeat measurements
be able to give reasons to explain why an outlier
should be retained as part of the data or
rejected when calculating the mean.
When comparing sets of titration results you
should know that:
a difference between their means is real if their
ranges do not overlap.
To investigate the relationship between a factor
and an outcome, it is important to control all the
other factors that might affect the outcome, such
as temperature and the concentrations of other
reactants. When investigating the rates of
chemical reactions you should be able to:
identify the outcome and factors that may
affect it
suggest how an outcome might alter when a
factor is changed.


In a plan for an investigation of the effect of a
factor on an outcome, you should:
be able to explain why it is necessary to control
all the factors that might affect the outcome
recognize that the fact that other factors are
controlled is a positive design feature, and the
fact that they are not is a design flaw.
If an outcome occurs when a specific factor is
present but does not when it is absent, or if an
outcome variable increases (or decreases) steadily
as an input variable increases, we say that there is
a correlation between the two. In the context of
studying reaction rates you should:
understand that a correlation shows a link
between a factor and an outcome
be able to identify where a correlation exists
when data is presented as text, as a graph, or in
a table
understand that a correlation does not always
mean that the factor causes the outcome.
identify that where there is a machanism to
explain a correlation, scientists are likely to
accept that the factor causes the outcome.

C6: Chemical Synthesis


A student is developing a new sports pack.
She needs to find two chemicals that will cool
an injury when mixed inside the pack.
a Should the reaction be endothermic or
b Which of the energy-level diagrams below
represents the reaction that is likely to
cool injuries more effectively?






a Explain why the chemist repeated the
titration four times.
b  alculate the volume of alkali added in
run 4.
c  he chemist calculates the mean of his
titration results. Why does he do this?
d  uggest the range within which the true
value probably lies.
A teacher adds 3 g of zinc granules to dilute
hydrochloric acid in a flask. She uses a gas
syringe to measure the volume of hydrogen
gas given off.


Time (min)

Volume of gas in







progress of reaction


Run 1 Run 2 Run 3

Run 4












reading (cm3)
Final burette
reading (cm3)
Volume of acid
added (cm3)




progress of reaction

A chemist tested the purity of a sample of
tartaric acid by titrating with dilute sodium
hydroxide. The table shows his results.

Initial burette







progress of reaction

syringe (cm3)


progress of reaction

a Draw a line graph of the results.
b  alculate the average rate of reaction
between three and four minutes, in cm3
per minute.
c  he teacher decides to repeat the
experiment with 3 g of zinc powder. She
keeps all other factors the same. Draw
and label a line on the graph to show
the expected results. Use collision theory
to explain the effect of this change.
d Explain why it was important for the
teacher to keep all other factors the same.

C6: Chemical Synthesis


Cite This Document

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