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))

log10(Q)

log_q = log10((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 C [C](M)0.1
coeff_cCoefficient of C (c)1
conc_dConcentration of D [D](M)0.2
coeff_dCoefficient of D (d)1
conc_aConcentration of A [A](M)0.5
coeff_aCoefficient of A (a)1
conc_bConcentration of B [B](M)0.3
coeff_bCoefficient of B (b)1

How It Works

Reaction Quotient (Q)

The reaction quotient has the same mathematical form as the equilibrium constant but is evaluated at non-equilibrium concentrations.

Formula

Q = [C]^c [D]^d / ([A]^a [B]^b)

Compare Q to the equilibrium constant K:

  • If Q < K: reaction proceeds forward (toward products)
  • If Q > K: reaction proceeds in reverse (toward reactants)
  • If Q = K: system is at equilibrium

    • Worked Example

    For A + B -> C + D with [A]=0.5, [B]=0.3, [C]=0.1, [D]=0.2 M.

    conc_c = 0.1coeff_c = 1conc_d = 0.2coeff_d = 1conc_a = 0.5coeff_a = 1conc_b = 0.3coeff_b = 1
    1. 01Q = [C][D] / ([A][B])
    2. 02Q = (0.1 × 0.2) / (0.5 × 0.3)
    3. 03Q = 0.02 / 0.15 = 0.1333

    Frequently Asked Questions

    What is the difference between Q and K?

    K is the equilibrium constant, evaluated only at equilibrium. Q uses the same formula but can be calculated at any point during the reaction. Q tells you which direction the reaction must shift to reach equilibrium.

    What about pure solids and liquids?

    Pure solids and pure liquids have an activity of 1 and are omitted from the Q expression. Only aqueous species and gases are included.

    How does Q relate to Gibbs free energy?

    delta_G = delta_G° + RT*ln(Q). At equilibrium, delta_G = 0 and Q = K, giving delta_G° = -RT*ln(K). The sign of delta_G tells the reaction direction.

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