The product of the concentrations of the products , divided by the product of the concentrations of the reactants , for a chemical reaction which is not necessarily at equilibrium . For example, the reaction quotient for A + B = C + D is equal to (C)(D) / ((A)(B)), where the parentheses indicate concentrations. Each concentration is raised to a power equal to its stoichiometric coefficient in the expression. The reaction quotient for A + 2B = 3C is equal to (C)3/((A)(B)2). For gas phase reactions, partial pressures can be used in the reaction quotient in place of concentrations.