A chemistry GCSE question
CO(g) + 2 H2 ⇌ CH3OH(g)
The reaction is carried out at a temperature of 250oC and a pressure of 100 atmospheres.
The forward reaction is exothermic.
Explain what happens to the yield of methanol if a temperature higher than 250oC is used.
[2 marks}AQA June 2018
An answer you might be pleased to see:
The yield of methanol would decrease because a higher temperature favours the endothermic reaction, which is forwards.
The following would probably also get 2 marks:
This is a reversible reaction so the production of carbon monoxide and hydrogen is endothermic, meaning there will be more energy for this reaction to take place, so less CH3OH will be made since the CO and H2 will have a higher yield so there will be less methanol.
Science GCSE encourages ‘good enough’ answers. Time spent developing the quality of written explanation beyond a certain point does not translate into better exam performance.
This could be addressed using comparative judgement. If all of the answers were compared and judged and given a scaled score from 0 to 2, you could reward clarity of explanation. It would be worth spending time teaching a skill which is meaningful and important – the ability to communicate scientific concepts accurately and succinctly.
If GCSEs become obsolete and we are given control of our curriculum, this gets my vote.