The Reaction Quotient (HL) | HL IB Chemistry Revision Notes 2025

The Reaction Quotient

What is the reaction quotient?

The reaction quotient, Q, is calculated using the same equation as the equilibrium constant expression, but with non-equilibrium concentrations of reactants and products
The expression for Q is therefore the same as K

For more information on the equilibrium constant expression, see our revision notes on The Equilibrium Law

It is a useful concept because the size of Q can tell us how far a reaction is from equilibrium and in which direction the reaction proceeds
For example,
- If Q = K then the reaction is at equilibrium, no net reaction occurs
- If Q < K the reaction proceeds to the right in favour of the products
- If Q > K the reaction proceeds to the left in favour of the reactants
Using concentration values of the substances present, we can work out if a reaction is at equilibrium or not, as the following example shows:

Worked example

The equilibrium constant for the reaction below is 5.1 x 10^-2 at 298 K.

COI₂ (g) ⇌ CO (g) + I₂ (g)

Deduce whether the following reaction mixture concentrations represent a reaction at equilibrium and for those not at equilibrium indicate the direction is proceeding.

Reaction mixture	[COI₂ (g)]	[CO (g)]	[I₂ (g)]
1	0.012	0.050	0.050
2	0.020	0.032	0.032
3	0.150	0.025	0.025

Answer:

The reaction quotient expression is:

$Q = \frac{[CO (g)] [I_{2} (g)]}{[{COI}_{2} (g)]}$

For reaction mixture 1:

$Q = \frac{0.050 \times 0.050}{0.012} = 0.21$

- In this mixture Q >> K, so Q has to decrease to reach K
- This means the reaction must be moving to the left, in order to reach equilibrium, so the reactants are favoured
For reaction mixture 2:

$Q = \frac{0.032 \times 0.032}{0.020} = 0.051$

- In this mixture, the value of Q = K, so the reaction is at equilibrium

For reaction mixture 3:

$Q = \frac{0.025 \times 0.025}{0.150} = 0.0042$

- In this mixture Q < K, so Q has to increase to reach K
- This means the reaction must be moving to the right, in order to reach equilibrium, so the products are favoured

Exam Tip

The value of Q is not a fixed value as it can be measured at any time but the value of K is constant at a given temperature.

The Reaction Quotient (HL) (HL IB Chemistry)

Revision Note