Tuesday, June 15, 2010


Kc Equilibrium constant:

eg for: N2 + 3H2 --> 2NH3

Kc = [product]/[reactant]

Kc = [NH3]2/[N2][H2]3

This is a ratio of product concentration to reactant concentration at equilibrium.
If Kc>1 then the equuilibrium position favours the product side
If Kc<1 then the equilibrium position favours the reactant side.

* Solids do NOT get included into the Kc expression

Le Chateliers Principle:
States that:
When a system is at equilibrium, that system will shift to minimise any changes made to it.

[When answering questions on equilibrium, you need to name the chemical(s) affected by the change]

Things that can change the position of equilibrium:
1. Temperature:
When heat is added, the equilibrium will shift in the endothermic direction.
When cooled the equilibrium will shift in the exothermic direction.
Kc will change as it is temperature dependant.

2. Concentration:
When adding or removing chemicals from an equilibrium you are changing the concentration. The equilibrium will shift in the direction to either replace any chemicals removed or use up any chemical added.
Kc remains the same.

3. Pressure:
When the pressure is increased (by decreasing the volume of the container) the equilibrium will shift to the side with the least number of moles of gas.
When the pressure is decreased (by increasing the volume of the container) the equilibrium will shift to the side with the most moles of gas.
Kc remains the same.

4. Catalyst:
A catalyst will not affect equilibrium position. It will just increase the rate at which equilibrium is reacted and it will increase the rate of the forward and reverse reactions equally.
A catalyst is not used up in the reaction.

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