The Haber process

In a nutshell

The Haber process produces ammonia. It is a reversible reaction so its yield is affected by pressure, concentration and temperature as these factors affect the position of an equilibrium. The Haber process conditions are optimised to improve rate, yield and expense.

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The Haber process

Ammonia ($$NH_3$$​) is produced by the Haber process:

​$$nitrogen + hydrogen \rightleftharpoons ammonia​\newline \ \newline N_2 (g) + 3H_2 (g)\rightleftharpoons 2NH_3 (g)$$

PROCEDURE 

Ammonia has a variety of uses, particularly the production of fertilisers.

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Factors affecting the Haber process

• Rate and yield are two important factors considered during the Haber process
  
• Rate of reaction refers to how quickly a product is produced 
  
• Yield refers to how much product is produced 
• The Haber process is a reversible reaction so a dynamic equilibrium is established
• Shift in equilibrium is related to yield
• When the equilibrium shifts to the right, more ammonia is produced, increasing yield 

Three key factors are considered during the Haber process: temperature, pressure and the use of an iron catalyst, which all affect the rate and yield in different ways. These conditions are optimised to deliver a high yield without compromising rate of production. 

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Temperature  

Rate

As temperature increases, particles gain more kinetic energy, which increases the frequency of collisions. More particles have energy equal to or above the activation energy so more collisions are likely to be successful. This increases the rate of reaction.

Yield

The forward reaction of the Haber process is an exothermic process, the backward reaction is endothermic. When temperature increases, the equilibrium shifts in the endothermic direction to oppose this change. The equilibrium shifts to the left, as the backward reaction is endothermic. This decreases the yield of ammonia.

Optimisation 

Although temperature increases the rate of ammonia production, very high temperatures would incur higher expenses, as more energy would be needed to maintain a higher temperature. Moreover, it would also compromise the yield of ammonia. To optimise yield, rate and cost, a temperature of $$450$$​°C is used. ​

Pressure

Rate

As pressure increases, the particles of gas move closer together, which increases the frequency of collisions. This increases the rate of reaction, which is beneficial for the quick production of ammonia.

Yield

A higher pressure increases the yield of ammonia because the equilibrium shifts to the right. This is because there are fewer moles of gas on the right-hand side.

Optimisation

Although high pressure is beneficial for both rate and yield, a very pressure would incur higher costs, as more energy would be needed to maintain a higher pressure. To optimise yield, rate and cost, a pressure of $$200$$​ atmospheres is used

Catalyst

Rate

Catalysts are substances which speed up the rate of reaction, without being used up themselves, by providing an alternative mechanism or pathway. 

Yield

The use of a catalyst does not affect the yield because catalysts speed up the rate of both the forward and backward reaction by the same amount, so the position of equilibrium is unaffected by the addition of a catalyst. 

Optimisation

An iron catalyst is used in the Haber process so that extremely high pressures and temperatures are not required to provide a high rate of reaction.

1. ​$$50\ atm$$​​ 2. ​$$100\ atm$$​​ 3. ​$$200\ atm$$​​ 4. As pressure increases, the graph gets steeper. Therefore, the rate increases. 5. Amount of product made 6. Time 7. The yield of ammonia also increases as pressure increases.