Reaction Quotient

Chemistry: IB Diploma Higher Level, topic 5.7.1

The Reaction Quotient

The relative concentrations of substances in a reversible reaction can be worked out using the reaction quotient (Q).

The reaction quotient tells you the relative concentrations of substances in a reversible reaction at a given time.
The formula for the reaction quotient can be found directly from the stoichiometry of the balanced equation.

Reversible reactions can be generalized to the form
$mA+nB\;\rightleftharpoons\;xC\;+\;yD$
The reaction quotient ( $Q_c$ ) for the generalized reversible reaction above is:
$Q_c=$ $\large\frac{[C]^x [D]^y}{[A]^m [B]^n}$
The 'c' in $Q_c$ tells us that molar concentrations should be used in the expression.

A reversible reaction will always try to reach equilibrium.
As a reversible reaction moves towards equilibrium, the reaction quotient ( $Q_c$ ) will tend towards the equilibrium constant ( $K_c$ ).
A reversible reaction is at equilibrium when $Q_c$ = $K_c$ .

If the reactants and products are gaseous, a reaction quotient can also be found using partial pressures.
If a reversible reaction with gaseous reactants and products is generalized to the form
$mA\;nB\;\rightleftharpoons\;xC\;+\;yD$
The reaction quotient ( $Q_p$ ) is:
$Q_p=$ $\large\frac{P^x_C\; P^y_D}{P^m_A\; P^n_B}$
The 'p' in $Q_p$ tells us that partial pressures should be used in the expression.

A reversible reaction will always try to reach equilibrium.
As a reversible reaction moves towards equilibrium, the reaction quotient ( $Q+p$ ) will tend towards the equilibrium constant ( $K_p$ ).
A reversible reaction is at equilibrium when $Q_p$ = $K_p$ .

What is the reaction quotient expression for the following reaction, given the concentrations of the reaction species?
3O_2(g) $\rightleftharpoons$ 2O_3(g)
What is the reaction quotient expression for the following reaction, given the concentrations of the reaction species?
2SO_2(g) + O_2(g) $\rightleftharpoons$ 2SO_3(g)
What is the reaction quotient expression for the following reaction, given the partial pressures of the reaction species?
N_2(g) + 3H_2(g) $\rightleftharpoons$ 2NH_3(g)