The Reaction Quotient Q and ICE Table Calculations

To decide whether a reaction mixture is at equilibrium, and if not, which way it will move, we use the reaction quotient, $Q_c$. The reaction quotient has exactly the same algebraic form as the equilibrium constant, but it is evaluated using the concentrations present at any moment, not necessarily equilibrium concentrations. For $aA + bB \rightleftharpoons cC + dD$, $Q_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$ using current concentrations.

Comparing $Q_c$ to $K_c$ gives a clear prediction. If $Q_c < K_c$, the ratio of products to reactants is too small, so the reaction proceeds forward, producing more product until $Q_c$ rises to equal $K_c$. If $Q_c > K_c$, there is too much product relative to equilibrium, so the reaction proceeds in reverse. If $Q_c = K_c$, the system is already at equilibriu