What Is the Reaction Quotient?
The reaction quotient (Q) is a dimensionless number that expresses the ratio of product concentrations to reactant concentrations at any given moment during a reversible chemical reaction — not just at equilibrium. It is calculated using the same formula as the equilibrium constant (K), but with current, non-equilibrium concentrations or partial pressures.
Reaction Quotient Formula
For a general reversible reaction:
aA + bB ⇌ cC + dD
The reaction quotient is:
Q = [C]c × [D]d / ([A]a × [B]b)
- Uppercase letters (A, B, C, D) — chemical species (reactants or products)
- Lowercase letters (a, b, c, d) — stoichiometric coefficients
- Square brackets [X] — molar concentration (mol/L) for dissolved species, or partial pressure for gases
Note: Pure solids and pure liquids are excluded from the expression because their activities equal 1.
How to Find the Reaction Quotient
- Write the balanced chemical equation.
- Identify all reactants and products (exclude solids and pure liquids).
- Note the stoichiometric coefficient of each species.
- Measure or look up the current concentrations (mol/L) or partial pressures.
- Plug values into the Q formula: multiply product concentrations raised to their coefficients, divide by reactant concentrations raised to their coefficients.
Relationship Between Q and K (Equilibrium Constant)
The equilibrium constant K uses the same formula as Q, but only at equilibrium. By comparing Q to K, you can predict in which direction the reaction will proceed:
| Condition | Meaning | Direction |
|---|---|---|
| Q < K | Too many reactants relative to equilibrium | → Forward (toward products) |
| Q = K | System is already at equilibrium | No net change |
| Q > K | Too many products relative to equilibrium | ← Reverse (toward reactants) |
Worked Example
Consider the reaction: N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g)
Suppose at a given moment:
- [N₂] = 0.50 atm
- [H₂] = 1.0 atm
- [NH₃] = 0.10 atm
- K = 977 (at 25 °C)
Q = (0.10)² / (0.50 × (1.0)³) = 0.01 / 0.50 = 0.02
Since Q (0.02) < K (977), the reaction will proceed forward to produce more NH₃.
Concentration vs. Partial Pressure
- Dissolved species (liquids): Use molar concentration (mol/L = M).
- Gases: Use partial pressures — atm or psi (US customary) or Pa, kPa, bar (SI/metric).
- This calculator supports both systems so you can work in whichever units you prefer.
Frequently Asked Questions
Can Q be negative?
No. Concentrations and pressures are always positive, so Q ≥ 0. If you get a negative result, check your input values.
What happens if a reactant or product is a solid?
Solids and pure liquids have an activity of 1 and are omitted from the Q (and K) expression entirely.
Is Q the same as K?
Only at equilibrium. At any other moment Q ≠ K, and the system will shift until Q = K.
Can this calculator handle more than two reactants or products?
Yes — this calculator supports up to three reactants and three products. For reactions with more species, calculate the sub-products manually and combine them.