reaction quotient

Meaning: Q is calculated from the current amounts in a reaction mixture, using the same expression as K: products raised to their coefficients over reactants raised to their coefficients. Pure solids and pure liquids are left out. Comparing Q with K shows which way the net reaction must go to reach equilibrium. If Q < K, there is too little product, so the reaction shifts toward products. If Q > K, it shifts toward reactants. If Q = K, there is no net change. This is the quantitative basis of Le Chatelier's principle: adding reactant lowers Q below K, so the reaction moves forward.

Equation: For aA + bB ⇌ cC + dD, Q = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ), where brackets are concentrations in mol/L (gases may use partial pressures in Pa or atm). Q is treated as unitless. Related: ΔG = ΔG° + RT ln Q = RT ln(Q/K), where R = 8.314 J/(mol·K), T is in K, and ΔG is in J/mol.

On the MCAT: A passage gives K and a set of concentrations, and you compute Q to predict the direction of shift, or decide whether a concentration will rise or fall. Another version changes a volume or adds a species, and you ask whether Q is now above or below K. Q < K also means ΔG is negative.

Common confusion: Q is not K. K depends only on the reaction and temperature, while Q changes as the reaction proceeds. Q < K means forward, not reverse.