7.3 Reaction Quotient and Equilibrium Constant

Reaction Quotient

The reaction quotient is a unitless value that indicates the relative value of the amount of reactants and products at some time. The amount of reactants and products can be represented by the partial pressure of them as a gas or their concentration in a solution (or container). The subscript “P” indicates that the partial pressures are being measured while a subscript “C” indicates the concentrations are being measured. The equations below are for the reaction ${aA + bB \rightleftharpoons cC + dD}$, where the uppercase letters are a chemical species and the lowercase letters are their coefficients.

Where ${Q_c}$ is the concentration reaction quotient, ${[X]}$ is the concentration of X, and ${x}$ is the coefficient in the reaction for X.

Where ${Q_p}$ is the pressure reaction quotient, ${P_X}$ is the partial pressure of X, and ${x}$ is the coefficient in the reaction for X.

More generally, this equation is just the products raised to the power of their coefficients over the reactants raised to the power of their coefficients. Note that the reaction quotient does not include substances whose concentration or partial pressures are independent of how much there is, such as solids or pure liquids. So when there is a solid or pure liquid in the reaction, you do not add it to the reaction coefficient equation.