Consider a reaction A + B

C + D occuring at constant temperature.

If we decrease [C], the reaction quotient Q decreases. The net rate of reaction increases in the forward direction to restore Q to the constant value of K

_{c} (i.e. more A and B react forward than C and D backward). After the system has once again reached equilibrium, we find that Q is the same as it was initially. This must be the case since the value of K

_{c} has not changed (though all of the concentrations will have decreased a little bit, but this does not affect the ratio).

Le Chatelier's principle implies that if we reduce [C], the position of equilibrium shifts to the right. I don't understand this, since the quotients at the beginning and end are equal. Does the 'position of equilibrium' in this case refer to the position of the reaction (i.e. the position of the reaction shifts to the right if [C] is reduced), or am I missing something here?

The only context where the phrasing 'equilibrium shifts to the left/right' makes sense to me is when there is a change in temperature, because in that case the new value of the quotient at equilibrium

**has** changed to the new value of K

_{c}.

I believe I'm getting confused over what is meant by the position of equilibrium. I had a thought that it could it be equivalent to the value of Q, however this doesn't make that much sense considering that when it is 'shifting', the system isn't in equilibrium!

Thank you very much in advance