In chemistry, Q is the reaction quotient. It is used to determine which direction a reaction will proceed by comparing it to the equilibrium constant, Kc. At equilibrium, the forward reaction and the reverse reaction rates are equivalent. If Kc is greater than Q, then the reaction proceeds in the forward direction (to the right), creating more products. If Kc is less than Q, then the reaction proceeds in the reverse direction (to the left), forming more reactants. If Kc = Q, the reaction is at equilibrium.
Write the equation for the reaction. A hypothetical reaction is: aA(aq) + bB(s) " cC(aq) + dD(g), where A and B species are the reactants, C and D the products and a, b, c and d coefficients, which in this case all equal 1. For example, in the reaction 2 NaOH + H2SO4 ' 2 H2O + Na2SO4, the coefficient for the species NaOH is 2, and the coefficient for H2SO4 is 1. The abbreviation "aq" stands for "aqueous solution," "s" is for "solid" and "g" means "gas."
Note the physical states of all species. If a species is aqueous (aq) or gas (g), the concentration should be expressed in mols/liter (molarity, M). Liquids and solids are not used in the computation of the reaction quotient.
Write the reaction quotient formula. It is Q = concentration of products/concentration of reactants, where the concentration is raised to the power of the coefficient. In this article's example, Q = [C][D]/[A], and all the coefficients equal 1; so all concentrations are raised to the power of 1. Species B is left out of the equation because it is a solid.
Plug the concentrations into the formula. For example, Kc = 20, [A] = 0.5 M, [C] = 2 M, and [D] = 3 M, where M = molarity. Using the formula, Q = [C][D]/[A], Q = (2)(3)/(0.5) = 12.
Determine which direction the reaction will proceed given the concentrations of the reactants and products by using Q. According to an application of Le Chatelier's Principle, the law of mass action, if you increase the concentration of any of the reactants, more products will form, and if you increase the concentration of the products, more reactants will form. Because Kc > Q as 20 > 12 in this article's example, the reaction will proceed forward (to the right), creating more products until Kc = Q, at which time the reaction will be back in equilibrium.