C2: Reversible Reactions, Exothermic and Endothermic Reactions (including the Haber Process)

Here are a set of revision cards for students taking AQA Additional Science. These cards are based on Endothermic and Exothermic reactions, Reversible reactions and also the Haber Process from the C2 (Chemistry) section. I hope these help you to revise! Please rate and comment on how to improve :) Also, I have a study group called AQA Additional Science where we discuss topics such as these ones and many more. Feel free to become a member, the more the merrier!

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Exothermic Reactions

When chemical reactions occur energy is transferred to or from the surroundings

An exothermic reaction is one that transfers energy, usually heat, to the surroundings

Examples of exothermic reactions are combustion, many oxidation reactions and neutraisation

During an exothermic reaction, you say that the material giving out the heat is getting hotter too

If the temperature is raised, the yield decreases

If the temperature is lowered, the yield increases

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Endothermic Reactions

An endothermic reaction is one that takes in energy, often heat, from the surroundings

Endothermic reactions include thermal decompositions and some dissolving reactions

During an endothermic reaction, you say the material taking in heat is getting colder

If the temperature is rised, the yield increases

If the temperature is lowered, the yield decreases

In gaseous reactions, an increase in pressure will favour the reaction that produces the least number of molecules

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Reversible Reactions

In some chemical reactions, the products of the reaction can react to produce the original reactants. This is called a reversible reaction.

A + B = C + D

This is the way a reversible reaction is presented, however instead of the = sign a two headed arrow is used which points in both directions.

In a closed system, equilibrium is reached when the reactions occur at exactly the same rate in each direction

If a reversible reaction is exothermic in one direction, it is endothermic in the other direction

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Reversible Reactions Continued...

A common example of a reversible reaction:

hydrated copper sulfate = anhydrous copper sulfate + water

The reverse of this reaction can be used to test for water

The relative amounts of all the reacting subbstances at equilibrium depend on the conditions of reaction

It is important for sustainable development as well as economic reasons to minimise energy requirements and energy wasted in industrial processes.

Non-vigorous conditions mean less energy is used and less is released in to the environment.

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Varying Conditions on reversible reactions


Increasing the temperature increases the rate of reaction equally in both directions. So the product is formed faster but also breaks down faster.


Increasing the pressure favours the smaller volume. High pressure favours a reaction where less molecules are beig made from more molecules.

An increase in pressure would favour the production of ammonia as four molecules are being changed in to two molecules, so here the pressure would increase the yield.

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The Haber Process

Although reversible reactions may not go to completion, they can still be used in industrial processes e.g. the Haber Process

The raw materials for the Haber Process are nitrogen and hydrogen. Nitrogen is obtained from the air and hydrogen is obtained from natural gas or other sources

The purified gases are passed over a catalyst of iron at a high temperature (450C) and a high pressure (about 200-250 atmospheres)

Some of the nitrogen and hydrogen reacts to form ammonia. The reaction is reversible so ammonia breaks down again into nitrogen and hydrogen

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The Haber Process Continued...

On cooling, the ammonia liquefies and is removed.

The remaining hydrogen and nitrogen is recycled.

nitrogen + hydrogen = ammonia

Dont forget that instead of the equals sign, there will be a two headed arrow pointing in both directions

The reaction conditions are chosen to produce a reasonable yield of ammonia quickly

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A Compromise Solution - Haber Process

The formation of ammonia is exothermic so a low temperature increases the yield but the reaction is very slow. A high temperature makes the reaciton faster but produces lower yield.

The volume of ammonia produced is less that the total volume of the reactants so a high pressure favours the reaction but is expensive. A lower pressure is more affordable but produces a low yield.

So a compromise is reached:

200atmos. pressure is used

450C is used

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Claire Burgess

thank you, I'm finding the Haber process hard to get my head round but it's all clear now! thanks :D


I agree, very good. It was good. 

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