Hong Life CalculatorsUnderstanding the reaction quotient calculator — concept and principle
This calculator finds the reaction quotient Q from the current concentrations or partial pressures in a reaction mixture, compares it with the equilibrium constant K, and tells you which way the reaction will go. It also gives ΔG = RT ln(Q/K), the driving force. It is used in general and physical chemistry for equilibrium problems.
How it is calculated
For aA + bB ⇌ cC + dD: Q = [C]^c [D]^d ÷ ([A]^a [B]^b)
Q < K: net reaction forward; Q > K: reverse; Q = K: at equilibrium
ΔG = RT ln(Q/K), with R = 8.314462618 J/(mol·K)
- Concentrations in mol/L or partial pressures in bar; pure solids and liquids are left out (coefficient 0).
The principle
Q has the same form as the equilibrium constant but uses the amounts present right now. At equilibrium, Q equals K. If there are too few products compared with equilibrium, Q is smaller than K and the reaction makes more products; if there are too many, it runs backwards. Thermodynamically, ΔG = RT ln(Q/K) measures how far the mixture is from equilibrium: negative means the forward reaction is spontaneous.
Worked example
Default inputs: H₂ + I₂ ⇌ 2HI, [H₂] = [I₂] = 0.10, [HI] = 0.50, K = 50, T = 721 K.
- Q = 0.50² ÷ (0.10 × 0.10) = 25; log₁₀ Q = 1.3979.
- Q ÷ K = 25 ÷ 50 = 0.5.
- Q < K, so the net reaction goes forward → toward products.
- ΔG = 8.314 × 721 × ln 0.5 = −4.16 kJ/mol.
| Coefficient a of reactant A | Reaction quotient Q |
|---|---|
| 0.25 | 4.446 |
| 0.5 | 7.906 |
| 0.75 | 14.06 |
| 1 | 25 |
| 1.25 | 44.46 |
| 1.5 | 79.06 |
| 2 | 250 |
How to read the result
The further Q/K is from 1, the larger the driving force. ΔG near zero means the mixture is close to equilibrium. Q only tells you the direction, not how fast the reaction will get there.
Common mistakes
- Including pure solids or liquids in Q.
- Forgetting to raise concentrations to their coefficients.
- Using K from a different temperature.
- Mixing concentrations and partial pressures in one expression.
Frequently asked questions
What is the difference between Q and K?
K uses equilibrium amounts; Q uses current amounts.
What does Q = 0 mean?
No products are present, so the reaction can only go forward.
Why does ΔG depend on Q?
The free energy change depends on how far the mixture is from equilibrium.
Does Q tell the reaction rate?
No, only the direction.
Sources: OpenStax, Chemistry 2e (2019), section 13.2 "Equilibrium Constants" and section 16.4