AS Unit F321: Atoms, bonds and groups

Sample Student Answers
AS Unit F322: Chains, energy and resources
Module 4: Resources
Question 1
Total marks: 15
(a) A geography teacher told his students that man-made atmospheric pollution causes the
greenhouse effect. He said that the main contribution to the greenhouse effect came from
three gases: carbon dioxide, CO2; methane, CH4; and sulfur dioxide, SO2.
(i) Explain what causes atmospheric gases to contribute to the greenhouse effect and state
how accurate his list was.
(ii) Some politicians are advocating the complete removal of all greenhouse gases from the
atmosphere. Even if this target could be technologically achieved, suggest three scientific
reasons why this outcome would have devastating effects upon the Earth.
(iii) One proposal to reduce excess carbon dioxide is to implement carbon capture and storage.
Outline what is meant by carbon capture and storage.
Marks available:
(i) 3 (ii) 3 (iii) 1
Student answer:
(a) (i) The infrared radiation released by the Earth towards space vibrates covalent bonds within
gas molecules. This energy is emitted and some is directed back towards the Earth.
The teacher is incorrect as he omitted to include water as one of the three major
greenhouse gases.
(ii) The greenhouse effect is an equilibrium which keeps the planet’s temperature at levels
which are not too extreme.
No carbon dioxide would mean photosynthesis would not take place. Thus food shortages
would occur as plants would not grow.
Without water life could not be sustained on the planet.
(iii) Any carbon dioxide released (e.g. from coal fired power stations) would be ‘captured’ at
source and stored away safely underground, maybe in empty oil fields.
Examiner comments:
(a) (i) Be careful about blaming the greenhouse effect on every gas you can think of. Water
vapour is a major greenhouse gas rather than sulfur dioxide.
(ii) This is a good answer as some thought is required for this type of question.
(iii) Remember that there are other possible storage sites the student could have mentioned.
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(b) Commercial jet aircraft are powered by the combustion of hydrocarbon fuels, yet one of the
main pollutants they produce is nitrogen monoxide, NO. Explain how NO is formed and outline
why oxides such as NO are pollutants to the upper atmosphere.
Marks available: 2
Student answer:
(b) The high temperatures within the engine cause atmospheric nitrogen to react with
atmospheric oxygen. Oxides of nitrogen are produced and amongst these are nitrogen
monoxide radicals which catalyse the breakdown of ozone.
Examiner comments:
(b) It is important that the student makes it clear that the origin of the nitrogen was not in the fuel.
The question only asked for an outline into the breakdown of ozone therefore no equations are
necessary here.
(c) The internal combustion engine of motor cars produces both carbon monoxide, CO, and
nitrogen dioxide, NO2. These two gases can be combined within a catalytic converter inside
the exhaust to produce two relatively harmless gases.
(i) Write a possible equation for the reaction which occurs within a catalytic converter.
(ii) Explain how a catalytic converter works to reduce the emission of these two harmful gases.
Marks available:
(i) 1 (ii) 3
Student answer:
(c) (i) 4CO(g)
(ii) The reactant gas molecules, CO and NO2, adsorb onto the surface of the catalyst.
This weakens the covalent bonds within the molecules, effectively lowering the activation
energy, allowing the rate of reaction to increase.
The product gas molecules, CO2 and N2, desorb from the surface.
Examiner comments:
(c) (i) Remember to balance your equation.
(ii) Remember, adsorb, not absorb.
Diagrams often help here.
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(d) One recent proposal to utilise waste carbon dioxide from industrial processes is to convert the
excess carbon dioxide to super critical carbon dioxide, scCO2. Give two possible uses of
Marks available: 2
Student answer:
(d) Super critical carbon dioxide is a good solvent.
It can remove caffeine from coffee without removing taste or flavour and is non-toxic.
It can be used for dry-cleaning, replacing the current toxic solvents.
Examiner comments:
(d) Once again only give the number of uses the question requires.
© Pearson Education Ltd 2008
This document may have been altered from the original