Reaction Quotient Calculator Formula

Understand the math behind the reaction quotient calculator. Each variable explained with a worked example.

Use the Reaction Quotient Calculator

Formulas Used

Reaction Quotient (Q)

q = (pow(conc_c, coeff_c) * pow(conc_d, coeff_d)) / (pow(conc_a, coeff_a) * pow(conc_b, coeff_b))

Variables

VariableDescriptionDefault
conc_cConcentration of Product C(M)0.1
coeff_cCoefficient of C1
conc_dConcentration of Product D(M)0.1
coeff_dCoefficient of D1
conc_aConcentration of Reactant A(M)0.5
coeff_aCoefficient of A1
conc_bConcentration of Reactant B(M)0.5
coeff_bCoefficient of B1

How It Works

Reaction Quotient

Formula

Q = [C]^c × [D]^d / ([A]^a × [B]^b)

Where:

  • Q = reaction quotient
  • [X] = current concentration of species X
  • a, b, c, d = stoichiometric coefficients

    • Compare Q to K:

  • Q < K → reaction proceeds forward (toward products)
  • Q > K → reaction proceeds backward (toward reactants)
  • Q = K → system is at equilibrium

    • Worked Example

    A + B → C + D. Current: [A]=0.5, [B]=0.5, [C]=0.1, [D]=0.1. All coefficients = 1.

    conc_c = 0.1coeff_c = 1conc_d = 0.1coeff_d = 1conc_a = 0.5coeff_a = 1conc_b = 0.5coeff_b = 1
    1. 01Q = [C]^c[D]^d / [A]^a[B]^b
    2. 02Q = (0.1)¹(0.1)¹ / (0.5)¹(0.5)¹
    3. 03Q = 0.01 / 0.25
    4. 04Q = 0.04

    Frequently Asked Questions

    What is the reaction quotient?

    Q has the same form as the equilibrium expression but uses current (non-equilibrium) concentrations. It tells you which direction the reaction will shift to reach equilibrium.

    How is Q different from K?

    K uses equilibrium concentrations and is a constant at a given temperature. Q uses current concentrations and can have any value.

    What if Q equals K?

    The system is at equilibrium and no net change occurs. Concentrations remain constant (though individual forward and reverse reactions continue).

    Ready to run the numbers?

    Open Reaction Quotient Calculator