Reaction quotient Q and equilibrium direction
The reaction quotient has the same mass-action form as an equilibrium constant, but it uses the mixture’s current activities rather than equilibrium values.
If Q is below K, net forward reaction is favored; if Q is above K, net reverse reaction is favored until equilibrium is restored.
How to calculate reaction quotient
- Enter product terms: Add every gaseous or dissolved product activity and coefficient.
- Enter reactant terms: Add every gaseous or dissolved reactant activity and coefficient.
- Optionally enter K: Use the matching Kc, Kp, or activity-based constant.
- Calculate: Generate Q and, when K is supplied, the predicted direction.
Formula and variables
Multiply each dimensionless activity raised to its stoichiometric coefficient on each side.
Q = ∏a(products)^ν / ∏a(reactants)^ν- a — Activity
- Dimensionless effective amount of a species
- ν — Stoichiometric coefficient
- Positive power from the balanced equation
- K — Equilibrium constant
- Q at equilibrium for matching conditions
Ammonia reaction quotient
For N₂ + 3H₂ ⇌ 2NH₃, use activities 1, 1, and 0.1.
- a(NH₃)
- 0.1
- a(N₂), a(H₂)
- 1, 1
- Q = 0.1²/(1×1³)
Result: Q = 0.01.
Compare 0.01 with the equilibrium constant at the same temperature.
Understanding your results
Q must match K convention
Compare quantities built from the same activity, concentration, or partial-pressure convention.
- Q < K favors net forward change.
- Q > K favors net reverse change.
- Q = K indicates equilibrium.
Assumptions
- Activities and K are dimensionless and use matching standard states.
- Coefficients come from the balanced reaction.
Limitations
- Does not derive activities from concentration or pressure.
- Does not include pure solid or pure liquid terms.
- Does not calculate equilibrium composition.
Common mistakes
- Including pure solids in Q.
- Reversing numerator and denominator.
- Using unmatched Qc and Kp values.
- Forgetting coefficient exponents.
Practical use cases
Equilibrium direction
Predict the net change needed to approach equilibrium.
Frequently asked questions
What is the difference between Q and K?
Q describes the current mixture; K is the value of Q at equilibrium at a specified temperature.
Are pure solids included?
No. Their activity is conventionally one and they are omitted from the expression.
Sources and review
- Free Energy and Equilibrium — OpenStax Chemistry 2e. Accessed 2026-07-13.
Reviewed 2026-07-13.