Ammonia is manufactured industrially by the Haber process, using the typical conditions of 450 °C, 200 atmospheres (20 000 kPa) and an iron catalyst: N2(g) + 3H2(g) 2NH3(g) The forward reaction is exothermic. A newly-appointed plant manager wants to reduce operating costs and ma…

Answer: ((a)) No. There are 4 moles of gas on the reactant side (1 + 3) but only 2 moles of gas on the product side, so lowering the pressure shifts the position of equilibrium towards the side with more gas molecules, i.e. away from ammonia, decreasing the equilibrium yield ((b)) No. Raising the temperature would increase the rate of reaction, but since the forward reaction is exothermic, a higher temperature shifts the position of equilibrium towards the reactants (favouring the endothermic, reverse direction), decreasing the equilibrium yield of ammonia ((c)) Any valid reason other than yield, e.g. a much higher temperature would need more fuel/energy to maintain, increasing running costs, or would increase safety risks when handling such a highly exothermic reaction at more extreme conditions. Part (a): Evaluating Suggestion 1 (lower pressure) Count the moles of gas molecules on each side of the equation: N2(g)1 mol + 3H2(g)3 mol 2NH3(g)2 mol There are 1 + 3 = 4 moles of gas on the reactant side, but only 2 moles of gas on the p…

IGCSE Chemistry 0620: Evaluating Proposed Changes to the Haber Process

Ammonia is manufactured industrially by the Haber process, using the typical conditions of 450 °C, 200 atmospheres (20 000 kPa) and an iron catalyst:

$\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$

The forward reaction is exothermic.

A newly-appointed plant manager wants to reduce operating costs and makes two suggestions.

Suggestion 1: lower the pressure from 200 atmospheres to 50 atmospheres, since high-pressure equipment is expensive to build and maintain.

Suggestion 2: raise the temperature from 450 °C to 650 °C, since a hotter reaction mixture reacts faster, so ammonia could be produced more quickly.

(a) State whether Suggestion 1 would give a good yield of ammonia, and explain your answer by referring to the effect of pressure on the position of equilibrium. [3]

(b) State whether Suggestion 2 is a good idea, and explain your answer by referring to the effect of temperature on both the rate of reaction and the equilibrium yield of ammonia. [3]

(c) State one reason, other than yield, why 450 °C is chosen as a compromise rather than the manager's proposed 650 °C. [1]

Part (a): Evaluating Suggestion 1 (lower pressure)

Count the moles of gas molecules on each side of the equation:

$\underbrace{\text{N}_2(g)}_{1\text{ mol}} + \underbrace{3\text{H}_2(g)}_{3\text{ mol}} \rightleftharpoons \underbrace{2\text{NH}_3(g)}_{2\text{ mol}}$

There are $1 + 3 = 4$ moles of gas on the reactant side, but only $2$ moles of gas on the product side, fewer moles on the ammonia side.

Lowering the pressure shifts the position of equilibrium towards the side with more gas molecules, since that side can partly relieve the drop in pressure. That is the reactant side, so lowering the pressure to 50 atmospheres would shift the equilibrium away from ammonia.

Suggestion 1 would not give a good yield of ammonia, although it would reduce equipment costs, it would also reduce the equilibrium yield.

Part (b): Evaluating Suggestion 2 (raise temperature)

Raising the temperature does increase the rate of reaction, since particles have more kinetic energy and collide more frequently, with a greater proportion of collisions having energy $\geq$ the activation energy.

However, the forward reaction (formation of ammonia) is exothermic. By Le Chatelier’s principle, increasing the temperature always shifts the position of equilibrium in the endothermic direction. Here, that is the reverse reaction (breakdown of ammonia back into nitrogen and hydrogen).

So raising the temperature to 650 °C would increase the rate, but it would decrease the equilibrium yield of ammonia.

Suggestion 2 is not a good idea on its own. Faster production of a much lower-yielding equilibrium mixture is not necessarily more efficient overall.

Part (c): Another reason for the 450 °C compromise

Besides its effect on yield, operating at a much higher temperature such as 650 °C would require significantly more energy (fuel) to heat and maintain the reactor, increasing running costs. (Increased safety risk from operating a highly exothermic reaction at more extreme conditions is also an acceptable reason.)

Final answers

(a) No. Lower pressure shifts equilibrium towards the reactants (more gas moles), reducing the yield of ammonia.
(b) No. Higher temperature increases the rate but, since the forward reaction is exothermic, decreases the equilibrium yield of ammonia.
(c) A much higher temperature increases energy/fuel costs (or safety risk).