Reaction Quotient (Q) Calculator
The reaction quotient Q is calculated the same way as the equilibrium constant K, but using the actual concentrations at any moment in time rather than equilibrium concentrations. Comparing Q to the equilibrium constant K tells you which direction the reaction will proceed to reach equilibrium: if Q is less than K, the reaction goes forward; if Q exceeds K, it goes in reverse. This calculator handles a simple two-reactant, two-product reaction: aA + bB going to cC + dD with adjustable stoichiometric exponents.
Reaction quotient formula
Q = [C]^c * [D]^d / ([A]^a * [B]^b) If Q < K: reaction proceeds forward (products favored) If Q > K: reaction proceeds in reverse (reactants favored) If Q = K: at equilibrium
Concentrations are in mol/L. Exponents equal the stoichiometric coefficients of each species in the balanced equation. Pure solids and liquids are omitted (activity = 1).
Practical applications of Q
- Predicting reaction direction without knowing equilibrium concentrations.
- Checking whether a precipitate will form when two solutions are mixed (Q vs Ksp).
- Industrial process control: adjusting concentrations to drive reactions toward products.
- Le Chatelier's principle: Q changing due to added reactant or removed product drives the reaction forward.
Frequently asked questions
What is the reaction quotient Q?
The reaction quotient (Q) has the same mathematical form as the equilibrium constant (K), but uses current concentrations rather than equilibrium concentrations. Q = [C]^c[D]^d / ([A]^a[B]^b) for the reaction aA + bB = cC + dD.
How does Q compare to K predict reaction direction?
If Q < K, the reaction proceeds forward (to produce more products). If Q > K, the reaction proceeds in reverse (toward reactants). If Q = K, the reaction is at equilibrium.
What concentration units does Q use?
For reactions in solution, Q uses molar concentrations (mol/L). For gas-phase reactions, Q may be expressed in terms of partial pressures (Qp) or concentrations (Qc). This calculator uses molar concentrations.
Are pure solids or liquids included in Q?
No. The concentrations of pure solids and pure liquids (including water as a solvent) are not included in Q or K because their activities are defined as 1. Only dissolved species and gases appear in Q.
What stoichiometric coefficients are used?
The coefficients are the exponents in the Q expression. For 2NO(g) + O2(g) = 2NO2(g), Q = [NO2]^2 / ([NO]^2 * [O2]^1). Each concentration is raised to the power of its coefficient.
Official sources
- IUPAC Gold Book: Reaction quotient definition.
- NIST: NIST thermochemical data.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.