Equilibrium Constant help?

Question

Okay, I basically just need to know how I'm supposed to do this type of problem.

Instructions: Write the equilibrium constant expression for the following reactions, then calculate the value Q and predict how the reaction will proceed with the given information.

Sample Problem:
At 740 degrees celcius, Keq = 0.0060 for the decomposition reaction: CaCO3 (s) (double arrow) CaO (s) + CO2 (g), where [CO2] = 0.0004 M.

I know how to do the constant expression, but how do I calculate for Q? (I don't even know what Q is...) If you could just show me how I should start this, I would be grateful.

Answer 1

Q has exactly the same form as the equilibrium constant, but it is calculated using non-equilibrium concentrations.

So, for this reaction the expression for K is just:

Keq=[CO2] because all the other reactants are solids.

Since Q is less than Keq, the reaction will proceed to the right until the equilibrium constant is reached. If Q is greater than Keq, the reaction will proceed to the left to eventually make Q equal K.

Hope this helps...