3.2.3 - Chemical equilibrium

Question 1

N2(g) + 3H2(g) ⇌ 2NH3(g) ΔH = −92 kJ mol−1 Equilibrium 24.1
Explain how le Chatelier’s principle can be used to predict the conditions of temperature and pressure for a maximum equilibrium yield of ammonia. (4)

Answer 1

Pressure:
- Right-hand side has fewer (gaseous) moles / 4 (gaseous) moles form 2 (gaseous) moles
- High pressure

Temperature:
- (Forward) reaction is exothermic/ΔH is negative / (Forward) reaction gives out heat
- Low temperature

Question 2

State and explain how these conditions (high pressure + low temperature) could be changed to achieve a compromise between equilibrium yield, rate and other operational factors. (2)

Answer 2

Rate:
- Low temperature/pressure gives a slow rate/slower reaction so high temperatures / higher pressure needed to increase rate /
frequency of collisions

Industrial and operational factors:
- High pressure provides a safety risk /Higher temperatures increase energy costs / reduce yield / shift equilibrium to left
- (High) pressure is expensive (to generate) / uses a lot of energy

Question 3

State le Chatelier’s principle. (1)

Answer 3

• The (position of a dynamic) equilibrium shifts to minimise (the effect of) any change

Question 4

State one other feature of this dynamic equilibrium. (1)

Answer 4

• Rate of the forward reaction is equal to the rate
  of the reverse reaction
• Concentrations do not change