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Edexcel iGCSE Chemistry - 19th May 2016 - Equilibrium

To help people with one of the more difficult concepts within this specification, I will try to explain equilibrium as simply as possible.

When you have a reversible reaction, there should always be a delta H value next to it in the exam (depending on what they ask you about it).

For example we have the equation to make sulfur trioxide:

2SO2(g) + O2(g) ⇌ 2SO3(g) +[delta]H = +96kJ/mol (FOR EXAMPLE)

REMEMBER if the delta H value is POSITIVE the FORWARD reaction is ENDOTHERMIC. If the delta H value is NEGATIVE the FORWARD reaction is EXOTHERMIC.

So for this example we have a positive delta H value hence the forward reaction is ENDOTHERMIC.

An increase in TEMPERATURE favours the ENDOTHERMIC reaction and so there will be more products/reactants being made in the ENDOTHERMIC reaction. In this case it will make more sulfur trioxide.

A decrease in TEMPERATURE favours the EXOTHERMIC reaction and so there will be more products/reactants being made in the EXOTHERMIC reaction. This means that more sulfur dioxide and oxygen will be made.

To see how pressure affects the amount of product/reactant made we do not need to use the delta H value. All we need to do is count how many molecules are on each side of the reaction.

2SO2(g) + O2(g) ⇌ 2SO3(g)

There are 3 moles of reactants and 2 moles of products.

An INCREASE in pressure means the position of equilibrium will shift to the side with FEWER molecules. In this case more sulfur trioxide will be made.
A DECREASE in pressure means the position of equilibrium will shift to the side with MORE molecules. In this case more sulfur dioxide and oxygen will be made.

Hope this helps.

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