Reaction Quotient - AP® Chemistry Definition

What is the reaction quotient?

The reaction quotient, denoted as Q, is a dimensionless number that provides a snapshot of a chemical reaction's progress at a particular point in time. It is calculated using the concentrations of reactants and products in a manner similar to the equilibrium constant, K, except the values are not necessarily at equilibrium.

For a given reaction at a specific temperature, the reaction quotient is determined by inserting the current concentrations or partial pressures into the expression for the equilibrium constant. Comparing Q to K allows students to predict the direction in which the reaction will proceed to attain equilibrium; if Q < K, the reaction shifts to the right (towards products), if Q > K, it shifts to the left (towards reactants), and if Q = K, the system is at equilibrium.

Understanding the reaction quotient is crucial for analysing chemical dynamics and predicting reaction behaviour in various conditions.