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The Haber Process

Chemistry (Year 12) - Chemical Synthesis

Melanie Gamble

The Haber process is a reversible chemical reaction where nitrogen and hydrogen gas react to produce ammonia:

From the reaction above there are two key things we need to identify:

  1. Is that the reaction is exothermic (the change in enthalpy is negative).

  2. Is that there are 4 moles of reactant gas particles for every 2 moles of product gas particles.

For large-scale ammonia production, it occurs in two reaction chambers:

  • In the first chamber: Nitrogen and hydrogen gas are pumped in and passed over several layers of mesh made of an iron oxide catalyst. The gases react to produce ammonia. During this process a moderate temperature of 350 - 550°C and a very high pressure of 150 - 350 atm are used.

  • In the second chamber: The ammonia and leftover reactants (hydrogen and nitrogen gas) move through a cooling circuit. As ammonia has the highest boiling point it will condense first and drops to the bottom of the chamber in liquid form. This liquified ammonia is then collected and then used in fertilisers, explosives, and cleaning products.

The leftover nitrogen and hydrogen gas are then recycled, and the process is repeated to achieve an even higher overall yield.

The process is summarised below:

The Haber Process is great to use as an example of how manipulating the reaction conditions can influence reaction rate and yield.

What happens when we manipulate the temperature?

The reaction is exothermic, therefore an increase in temperature will increase the rate of reaction but decrease the yield as the reverse reaction is favoured. This explains why a moderate temperature is used, as if it is too high, the yield will suffer and if too low, the reaction rate will be affected.

What happens when we manipulate the pressure?

There are 4 moles of reactant gas particles for every 2 moles of product gas particles. Therefore, applying high pressures will increase both the reaction rate and the yield of the reaction, as the forward reaction is favoured. Therefore, a very high pressure is used to maximise the reaction rate and yield. Another point that should be considered here is the cost of the apparatus that can withstand such pressures.

The Iron Oxide Catalyst

An iron oxide catalyst is used in this process to further increase the reaction rate by providing an alternate pathway with a lower activation energy. The iron oxide catalyst is in the form of a mesh with multiple layers to increase the surface area of the catalyst to further increase the reaction rate.

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