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Date	May 2017	Marks available	1	Reference code	17M.3.sl.TZ2.14
Level	SL	Paper	3	Time zone	TZ2
Command term	Construct	Question number	14	Adapted from	N/A

Question

The combustion of fossil fuels produces large amounts of CO₂, a greenhouse gas.

The diagram below illustrates a range of wavelengths in the electromagnetic spectrum.

Synthesis gas, or syngas, mainly composed of CO(g) and H₂(g), is an alternative form of fuel. It can be produced by coal or biomass gasification, passing steam over the source material in a low oxygen environment.

Identify which region, A or B, corresponds to each type of radiation by completing the table.

[1]

Oceans can act as a carbon sink, removing some CO₂(g) from the atmosphere.

CO₂(g) $\rightleftharpoons$ CO₂(aq)

Aqueous carbon dioxide, CO₂(aq), quickly reacts with ocean water in a new equilibrium reaction. Construct the equilibrium equation for this reaction including state symbols.

[1]

b.i.

Describe how large amounts of CO₂ could reduce the pH of the ocean using an equation to support your answer.

[2]

b.ii.

Suggest an equation for the production of syngas from coal.

[1]

c.i.

The Fischer-Tropsch process, an indirect coal liquefaction method, converts CO(g) and H₂(g) to larger molecular weight hydrocarbons and steam.

Deduce the equation for the production of octane by this process.

[1]

c.ii.

Suggest a reason why syngas may be considered a viable alternative to crude oil.

[1]

c.iii.

Markscheme

Accept “B” alone for incoming radiation from sun.

All three correct answers necessary for mark.

[1 mark]

CO₂(aq) + H₂O(l) $\rightleftharpoons$ H₂CO₃(aq)

State symbols AND equilibrium arrow required for mark.

Accept

CO₂(aq) + H₂O(l) $\rightleftharpoons$ H⁺(aq) + HCO₃^–(aq).

CO₂(aq) + H₂O(l) $\rightleftharpoons$ 2H⁺(aq) + CO₃^2–(aq).

[1 mark]

b.i.

CO₂(aq) + H₂O(l) $\rightleftharpoons$ 2H⁺(aq) + CO₃^2–(aq)
OR
CO₂(aq) + H₂O(l) $\rightleftharpoons$ H⁺(aq) + HCO₃^–(aq)
OR
H₂CO₃(aq) + H₂O(l) $\rightleftharpoons$ H₃O⁺(aq) + HCO₃^–(aq)
OR
H₂CO₃(aq) $\rightleftharpoons$ H⁺(aq) + HCO₃^–(aq)
OR
H₂CO₃(aq) + 2H₂O(l) $\rightleftharpoons$ 2H₃O⁺(aq) + CO₃^2–(aq)
OR
H₂CO₃(aq) $\rightleftharpoons$ 2H⁺(aq) + CO₃^2–(aq)

equilibrium shifts to the right causing increase in [H₃O⁺]/[H⁺] «thereby decreasing pH»

Equilibrium sign needed in (b) (ii) but penalize missing equilibrium sign once only in b (i) and (ii).

Do not accept “CO₂(aq) + H₂O(l) $\rightleftharpoons$ H₂CO₃(aq)” unless equation was not given in b (i).

[2 marks]

b.ii.

C(s) + H₂O(g) → CO(g) + H₂(g)
OR
3C(s) + H₂O(g) + O₂(g) → 3CO(g) + H₂(g)
OR
4C(s) + 2H₂O(g) + O₂(g) → 4CO(g) + 2H₂(g)
OR
5C(s) + H₂O(g) + 2O₂(g) → 5CO(g) + H₂(g)

Accept other correctly balanced equations which produce both CO AND H₂.

[1 mark]

c.i.

8CO(g) + 17H₂(g) → C₈H₁₈(l) + 8H₂O(g)

[1 mark]

c.ii.

coal more plentiful than crude oil
OR
syngas can be produced from biomass/renewable source
OR
syngas can undergo liquefaction to form octanes/no need to transport crude
OR
syngas can be produced by gasification underground, using carbon
OR
capture/storage «to not release CO₂ to the atmosphere»
OR
coal gasification produces other usable products/slag