Section 10 - Equilibrium and Yield

Question 1

Describe how temperature is taken into account when trying to maximise the yield of ethanol from ethene (3)

Answer 1

1) Lowering the temperature would favour the forward reaction and so it should increase the yield of ethanol.
2) But lowering the temperature also means that fewer of the particles in the reaction mixture will have enough energy to react. The particles will also be moving more slowly, so there will be fewer collisions. So lowering the temperature will slow down the rate of both the forward and reverse reactions.
3) A low temperature would make the forward reaction too slow to be useful. So a compromise temperature of 300 degrees is used.

Question 2

Describe how pressure is taken into account when trying to maximise the yield of ethanol from ethene (3)

Answer 2

1) Increasing the pressure would favour the forward reaction and increase the rate of reaction (as the particles will be closer together so will collide more frequently). This would increase the yield of ethanol.
2) But producing high pressures uses a lot of energy and costs a lot of money. You’d need some pretty strong equipment to stand up to the high pressures too - and that would be expensive to buy and maintain.
3) To make the reaction economic, a moderately high pressure of 60 atm is used.

Question 3

Describe how concentration is taken into account when trying to maximise the yield of ethanol from ethene

Answer 3

1) Ethanol is removed from the reaction vessel as it is produced.
2) This reduces the concentration of products so the equilibrium shifts to favour the forwards reaction. This improves the yield of ethanol.

Question 4

Describe how catalysts are taken into account when trying to maximise the yield of ethanol from ethene

Answer 4

1) Using a solid phosphoric acid (V) catalyst increases the rate of both the forward and the backward reactions.
2) The catalyst has no effect on the position of the equilibrium - it just means the equilibrium is reached faster and the temperature and pressure at which the reaction can happen, at a reasonable rate, are reduced.