Reaction Quotient Calculator — Q vs K Chemistry

Formula

How this is calculated

The reaction quotient Q has the same mathematical form as the equilibrium constant K — products raised to their stoichiometric coefficients divided by reactants raised to theirs — but it is evaluated using the actual (non-equilibrium) concentrations at a given moment, whereas K is defined for the equilibrium state. Comparing Q to K tells you which way the reaction must shift to reach equilibrium: if Q < K there are too few products (or too many reactants) and the reaction proceeds forward; if Q > K there are too many products and the reaction reverses; if Q = K the system is already at equilibrium and no net change occurs. This is the quantitative basis of Le Chatelier's principle.

For the generic reaction aA + bB ⇌ cC + dD, Q = [C]^c · [D]^d / ([A]^a · [B]^b), where concentrations are in mol/L and exponents are the stoichiometric coefficients. A coefficient of 0 effectively removes a species (X^0 = 1). For gas-phase reactions, partial pressures in atm can be substituted for concentrations to give Q_p; the relationship between Q_c and Q_p involves the gas constant and temperature.

Limitations: this calculator assumes aqueous solution equilibria where the activity of pure water is 1 and solid species are omitted. For heterogeneous equilibria, leave out solid and liquid species by setting their coefficient to 0. The formula assumes ideal solution behaviour — activity coefficients are assumed to be 1, which holds best at dilute concentrations.