Chemistry - Energy

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1. “The energy for life comes from the sun.”
Outline the role of the photosynthesis process to explain what is meant by this statement. (3 marks) chlorophyll 6CO2(g) + 6H2O(l) + energy C6 H12 O6 (aq) + 6O2(g)
2830 kJ glucose

Photosynthesis is the process in which plants use the sun’s light energy to convert carbon dioxide and water into carbohydrates such as glucose, sucrose, starch and cellulose. Carbohydrates are high energy compounds because when they react with oxygen, they produce large amounts of energy. Plants make their own food by using the glucose they produces as an energy source for growth, reproduction, repairing damaged tissues... Animals cannot carry out photosynthesis, so they eat the plants to obtain food, hence, energy. For that reason the energy for life comes from the sun.

2.Explain what is meant by thermal pollution and discuss the implications for aquatic life. (3 marks) Thermal pollution is a discharge into a river or lake of a lot of hot water which is enough to significantly (2°C to 5°C) increase the temperature of water. This abnormal increase in temperature is potentially harmful to aquatic organisms in these ways: •Stress to organisms due to less dissolved oxygen.

Increase metabolism which would increase oxygen demand, and this would lead to decreasing oxygen supply. •Fish eggs don’t hatch.
Migration and spawning would be set off at the wrong time due to false temperature cues. •Sudden change in temperature would kill off fish eggs even if the temperature is in the egg’s survival range.

3.Identify the sources of pollution which accompany the combustion of organic compounds and explain how these can be avoided. (6 marks) Carbon dioxide is produced by power plants and motor vehicles by complete combustion when there is plenty of oxygen supply. It is a major pollution of enhanced greenhouse effect. This can be avoided by using filters such as catalytic converters to remove it. C(s) + O2(g)  CO2(g)

2C8H18(l) + 25O2(g)  16CO2 + 18H2O(l)

Sulfur dioxide is produced by the combustion of fossil fuels which contributes to form acid rain which leads to: •Increasing acidity of lake and/or river.
Erosion of marble and limestone buildings.
Accelerating the rate of corrosion.
This can be avoided by lowering combustion temperatures to prevent the formation of sulfur dioxides. S(s) + O2(g)  SO2(g)
2SO2(g) + O2(g)  2SO3(g)
SO3(g) + H2O(l)  H2SO4(aq)
Nitrogen oxide is produced in the combustion chamber of petrol or from diesel engines and combustion of coal in power plants. The combustion of nitrogen in fuels occurs at high temperatures. It causes respiratory problems and contributes to form photochemical smog and acid rain like sulfur dioxide. This can be avoided by lowering the combustion temperature to prevent the formation of nitrogen oxides. N2(g) + O2(g)  2NO(g)

No(g) + O2(g)  NO2(g)
2NO(g) + O2(g)  2NO2(g)

Carbon monoxide or soot is produced by incomplete combustion when there is insufficient oxygen supply. Carbon monoxide is toxic when it combines with haemoglobin in red blood cells in preference to oxygen; this reduces the ability of blood to transport oxygen - suffocation. Soot is carcinogenic. This is can be avoided by ensuring complete combustion. 2C8H18(l) + 17O2(g)  16CO(g) + 18H2O(l)

OR
C8H18(l) + 5O2(g)  CO(g) +7C(s) + 9H2O(g)

Tetraethyl lead (Pb(C2H5)4) is added to petrol to produce leaded petrol. The leaded emitted to the environment is a toxic metal which can enter the food chain.

4.Explain combustion in terms of slow, spontaneous and explosive reactions and explain the conditions under which these occur. (4 marks) Combustion is a self-sustaining chemical reaction in which a substance reacts with oxygen and release energy, usually in the form of heat and light. Therefore, it is an exothermic reaction. Slow combustion occurs at low temperatures and non-rapid rates. It...
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