Page 465 - SUBSEC October 2017_Neat

02212020/CAPE/SPEC/MS/2017
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CHEMISTRY
UNIT 2 — PAPER 02
MARK SCHEME

KC UK XS

High pressures and low temperatures [2 marks] 2

(iii) Conditions used in industry:
No, a set of compromise conditions are used
Award marks based on a discussion of the applications of 2
the principles of equilibrium coupled with economic
factors. [2 marks]

(iv) Hydrogen from Methane

CH4 + H2O  CO + 3H2

Accept also 2
CH4 + 2H2O  CO2 + 4H2
CO + H2O  CO2 + H2 [2 marks]

(d) (i) Prevent waste
Use catalysts not the stoichiometric reagents 2
Maximise atom efficiency [2 marks]

(ii) Prevent waste: the raw materials, nitrogen gas and

hydrogen gas are recycled through the reactor. In the

reactor only 15% is converted to ammonia, by continued

recycling of the reactants there is 98% conversion of the

reactants very little is wasted by recycling the reactants

there is very little waste generated in the Haber process.

Use of catalysts: the iron catalyst (aided by the

potassium hydroxide as a promoter overcomes the

inefficiency in the stoichiometric equation. The reaction

is favored by low temperature but the rate of the reaction
1 2
slows down significantly under these conditions. The

catalyst speeds up the reaction thereby overcoming this.

Maximize atom efficiency: the synthesis of the ammonia

requires just the materials (elements) that are in the

raw material. There are no side reactions or loss of

elements during the reaction. 98% of the N2 and H2 end up

in the final NH3 molecule.

(iii) Use renewable feedstocks: the H2 currently is obtained

from natural gas which is a non-renewable resource. The

hydrogen can be obtained from a renewable/alternative
1 2
source. For example, Electrolysis of water using

electricity from hydroelectric or some other renewable

source of energy. Or any other reliable process.

(e)
Nitrogen fixation
1
Electrical discharges (thunderstorms)

Accept also combustion of fuels from aircrafts [1 mark]

TOTAL 30 marks
10 15 5