Reversible reactions and chemical equilibrium

Dynamic equilibrium state and reaction quotient

A reaction that does not proceed to completion

Some reactions do not proceed completely in one direction: they are reversible. They are denoted by a double arrow:

a A + b B <=> c C + d D

This means that the forward reaction (to the right) and the reverse reaction (to the left) take place simultaneously.

The state of dynamic equilibrium

When the rates of the forward and reverse reactions become equal, the concentrations of all the reactants cease to vary macroscopically: this is the state of equilibrium. Note that ‘equilibrium’ does not mean ‘stagnation’: both reactions continue to occur constantly, at the same rate, which gives the illusion of a static system. This is why we refer to dynamic equilibrium.

The reaction quotient Qr

At any given moment, the reaction quotient Qr can be calculated from the current concentrations, using the same expression as for the equilibrium constant K, but evaluated at any point in time (not necessarily at equilibrium):

Qr = [C]^c * [D]^d / ([A]^a * [B]^b)

The equilibrium constant K

When the system reaches equilibrium, the reaction quotient takes on a specific value, denoted by K, which depends only on temperature. Comparing Qr with K allows us to predict the direction of change in a system that is not at equilibrium:

if Qr < K, the reaction proceeds in the forward direction (to the right);
if Qr > K, the reaction proceeds in the reverse direction (to the left);
if Qr = K, the system is at equilibrium, with no macroscopic change.

Common mistake