Reaction Quotient

\[Q=\frac{[C]^c[D]^d}{[A]^a[B]^b}\]

Variables

Qreaction quotient

Acurrent concentration of reactant A (mol/L)

Bcurrent concentration of reactant B (mol/L)

Ccurrent concentration of product C (mol/L)

Dcurrent concentration of product D (mol/L)

astoichiometric coefficient of A

bstoichiometric coefficient of B

cstoichiometric coefficient of C

dstoichiometric coefficient of D

Description

What is this formula?

The reaction quotient is calculated using the same mathematical expression as the equilibrium constant, but it uses the current concentrations or activities of reactants and products rather than equilibrium values.

When to use it

Use this formula to determine whether a reaction system will proceed forward, backward, or is already at equilibrium.

Example

Reaction:

N2 + 3H2 ⇌ 2NH3

Given:

[N2] = 1.00 mol/L

[H2] = 0.50 mol/L

[NH3] = 0.20 mol/L

Formula:

Q = [NH3]^2 / ([N2][H2]^3)

Substitution:

Q = (0.20)^2 / (1.00×0.50^3)

Result:

Q = 0.32

If K = 20:

Q < K

The reaction will proceed toward the products until equilibrium is reached.

Applications

Chemical equilibrium prediction

Reaction engineering

Industrial chemistry

Process monitoring

Thermodynamics

Note

The reaction quotient and equilibrium constant have identical mathematical forms but different meanings. Q is calculated from the current state of the system, while K is calculated at equilibrium.

Interpretation:

Q < K → Reaction proceeds toward products

Q = K → System is at equilibrium

Q > K → Reaction proceeds toward reactants

This comparison is one of the most important tools in equilibrium analysis.