What Is the Reaction Quotient (Q)?
The reaction quotient, denoted as Q, is a value that measures the relative amounts of products and reactants present in a reversible chemical reaction at any given moment. It provides a snapshot of the reaction's status, indicating how far it is from equilibrium. Q is calculated using the same mathematical expression as the equilibrium constant (K), but with concentrations that are not necessarily at equilibrium.
Section 2: How to Calculate the Reaction Quotient
To calculate the reaction quotient, you use the concentrations of the products and reactants at a specific point in time. For a general reversible reaction, aA + bB ⇌ cC + dD, the expression for the reaction quotient (Qc) is: Qc = ([C]^c * [D]^d) / ([A]^a * [B]^b). In this formula, [A], [B], [C], and [D] represent the molar concentrations of the substances, and a, b, c, and d are their stoichiometric coefficients from the balanced chemical equation.
Section 3: A Practical Example
Consider the synthesis of ammonia (the Haber process): N2(g) + 3H2(g) ⇌ 2NH3(g). At a certain moment in a reactor, the concentrations are [NH3] = 0.50 M, [N2] = 0.20 M, and [H2] = 0.30 M. The reaction quotient Q would be calculated as: Q = [NH3]^2 / ([N2] * [H2]^3) = (0.50)^2 / (0.20 * (0.30)^3) = 0.25 / (0.20 * 0.027) = 0.25 / 0.0054 ≈ 46.3.
Section 4: Importance and Application of Q
The primary importance of the reaction quotient is its ability to predict the direction a reaction will shift to reach equilibrium. By comparing the value of Q to the known equilibrium constant (K) for that reaction at the same temperature: If Q < K, the ratio of products to reactants is too small, so the reaction will shift to the right (produce more products). If Q > K, the ratio is too large, and the reaction will shift to the left (produce more reactants). If Q = K, the system is already at equilibrium and no net change will occur.