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.
Edexcel iGCSE Chemistry - 19th May 2016 - Equilibrium
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