Choosing Conditions to Maximise Yield of ammonia

Question 1

Equilibrium reactions are involved in some stages of the large-scale production of certain chemicals

An understanding of equilibrium and Le Chatelier’s principle is therefore very important in the chemical industry
Haber process

The Haber process involves the synthesis of ………… according to

Answer 1

ammonia according to:
N­2 (g) + 3H2 (g) ⇌ 2NH3 (g) ΔHr = -92 kJ mol-1

Le Chatelier’s principle is used to get the best yield of ammonia

Question 2

An increase in pressure will result in the equilibrium shifting in the direction of the fewest molecules of gas formed to reduce the pressure

In this case, the equilibrium shifts towards the right so the yield of ammonia increases

An increase in pressure will cause

Answer 2

the particles to be closer together and therefore increasing the number of successful collisions leading to an increased reaction rate

Very high pressures are expensive to produce therefore a compromise pressure of 200 atm is chosen

Question 3

Temperature

To get the maximum yield of ammonia the position of equilibrium should be shifted as far as possible to the

Answer 3

right

Question 4

Since the Haber process is an …………… process

exothermic or endothermic

Answer 4

exothermic

Question 5

according to Le Chatelier’s principle the equilibrium will shift to the right if the temperature is ……….

Answer 5

lowered

Question 6

A decrease in temperature will decrease the

Answer 6

energy of the surroundings so the reaction will go in the direction in which energy is released to counteract this

Question 7

Since the reaction is exothermic, the equilibrium shifts to

Answer 7

the right

Question 8

Since the reaction is exothermic, the equilibrium shifts to the right

However, at a low temperature the gases won’t have enough

Answer 8

kinetic energy to collide and react and therefore equilibrium would not be reached therefore a compromise temperature of 400-450oC is used in the Haber process

Question 9

A heat exchanger warms the incoming gas mixture to give molecules more kinetic energy such that

Answer 9

the gas molecules collide more frequently increasing the likelihood of a reaction

Question 10

Removing ammonia by condensing it to a liquid causes the equilibrium position to………… to replace

Answer 10

shift to the right to replace the ammonia causing more ammonia to be formed from hydrogen and nitrogen

Question 11

The recovered ammonia is stored at very ……..temperatures and there is ………..catalyst present with the stored ammonia so the decomposition reaction of ammonia to ……….

Answer 11

low

no

decompose back into hydrogen and nitrogen will be too slow to be a problem

Question 12

Catalysts

In the absence of a catalyst the reaction is so slow that hardly anything happens in a reasonable time!

Adding an ……….catalyst speeds up the rate of reaction

Answer 12

iron