The Reaction Quotient (College Board AP® Chemistry): Study Guide

The reaction quotient

• The reaction quotient, Q, is the ratio of products and reactants in a system at any point in the reaction

• The equilibrium constant (K) is the ratio of products to reactants in a system at equilibrium

• Q and K are defined by an expression that relates the products’ and reactants’ concentrations (c) or partial pressures (P)

• Consider the generic equation:

aA + bB ⇌ cC + dD

Where a, b, c, and d represent stoichiometric coefficients and A, B, C, and D represent chemical species

• The expressions for Q_c and K_c are:

 ${\mathrm{Q}}_{\mathrm{c}}=\frac{{\left[\mathrm{C}\right]}^{\mathrm{c}}{\left[\mathrm{D}\right]}^{\mathrm{d}}}{{\left[\mathrm{A}\right]}^{\mathrm{a}}{\left[\mathrm{B}\right]}^{\mathrm{b}}}$       ${\mathrm{K}}_{\mathrm{c}}=\frac{{\left[\mathrm{C}\right]}^{\mathrm{c}}{\left[\mathrm{D}\right]}^{\mathrm{d}}}{{\left[\mathrm{A}\right]}^{\mathrm{a}}{\left[\mathrm{B}\right]}^{\mathrm{b}}}$ 

• [A] indicates the concentration of species A in mol/L

• For reactions involving gases, the reaction quotient and equilibrium constant can instead be written in terms of partial pressures, Q_p and K_p:

$Q_p=\frac{{\left(P_C\right)}^c{\left(P_D\right)}^d}{{\left(P_A\right)}^a{\left(P_B\right)}^b}$       $K_p=\frac{{\left(P_C\right)}^c{\left(P_D\right)}^d}{{\left(P_A\right)}^a{\left(P_B\right)}^b}$  

• P_A indicates the partial pressure of species A in atm

• Solids and pure liquids are omitted from all of these expressions as their concentrations and partial pressures do not depend on the amount of substance present

• As a reversible reaction proceeds towards equilibrium, the value of Q approaches K such that at equilibrium Q = K

• To calculate Q, substitute the concentrations or partial pressures at any moment; to calculate K, substitute the equilibrium values

  • Q and K have no units

• The value of K does not depend on the starting amounts, but it changes when the temperature changes