AQA AS CHEM2 - Equilibrium

Notes on Le Chatelier's principles, reversible reactions and dynamic equilibria

NOTE: does not contain notes on carbon nuetral fuels

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  • Created on: 13-02-13 11:15
Preview of AQA AS CHEM2 - Equilibrium

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Chapter 7 ­ Equilibrium
Reversible reactions
Reversible reactions can go backwards and forwards. For a chemical reaction it means that the
reactants will react to form products but then the products would react to form the reactants. It is
very difficult to fully change the reactants into products.
Copper sulfate reaction ­ heat blue hydrated copper sulfate to form white copper sulphate powder
and water.
CuSO4.5H 2O (s) CuSO4(s) + 5H 2O(l)
When water is added to the white copper sulfate crystals they react to form the blue hydrated
copper sulphate.
Dynamic Equilibrium
In a reversible reaction, when the concentration of reactants and products are equal, then the
reaction is at equilibrium. This means the reactants are still making products and the products are still
making reactants, so the reaction is called a dynamic equilibrium.
The speed at which the reactants react is called forward rate
The speed at which the products react is called backward rate.
At dynamic equilibrium the forward rate is equal to the backward rate.
Le Chatelier's Principle
The principle says: When the conditions of equilibrium are changed then the position of equilibrium
will shift to minimize the change.
Effect of change in amounts of chemicals:
We can use the example of storing hydrogen as fuel:
C O (g) + 2H 2 CH 3 OH (l)
When the hydrogen is taken away more hydrogen will be made as the methanol will react again. This
is because:
At equilibrium the amounts of all three elements remain constant
When some hydrogen is removed the position of the equilibrium will shift to resist the
More hydrogen is made because some methanol reacts to make carbon monoxide and
Effect of changing a concentration:

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The position of the equilibrium will change if the concentration of one reactants or products is
changed.…read more

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If the temperature is decreased the opposite of the above will take place.
Effects of catalysts
Catalysts have no effect on the position of the equilibrium but will still have effect on the rate of the
reaction.…read more


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