Q = [products]^n / [reactants]^m — compare to K
The reaction quotient Q has the same form as K but uses current (non-equilibrium) concentrations. Comparing Q to K predicts the direction of reaction: Q < K → forward reaction (too few products), Q > K → reverse reaction (too many products), Q = K → equilibrium. This is a quantitative expression of Le Chatelier's principle. As a reaction proceeds, Q changes until it equals K. The relationship ΔG = ΔG° + RT ln(Q) shows that ΔG = 0 when Q = K (equilibrium). For gases, Qp uses partial pressures. Q is used in: predicting precipitation (Q vs Ksp), electrochemistry (Nernst equation uses Q), and monitoring reaction progress in industrial processes.