The instantaneous ratio of product activities (or concentrations) to reactant activities raised to their stoichiometric coefficients for a given chemical reaction; a state function of composition that can be compared to the equilibrium constant.

Principle

For a reaction aA + bB ⇌ cC + dD, define Q = (a_C^c a_D^d)/(a_A^a a_B^b) using activities a_i (or concentrations for ideal solutions); Q indicates the direction the reaction will proceed relative to equilibrium constant K.

Demonstration

Given N2 + 3H2 ⇌ 2NH3, if at some moment [NH3]^2/[N2][H2]^3 = 0.1 and K_eq = 0.5 then Q=0.1

Misapplication

Using molar concentrations directly as Q in concentrated or nonideal solutions without correcting with activities, or comparing Q to a K value measured under different temperature or standard states.

Consequence

If QK it proceeds in reverse; at Q=K the system is at chemical equilibrium for that temperature and pressure and composition.

Reversal

The equilibrium constant K is the special case of the reaction quotient evaluated at equilibrium; reversing the reaction inverts the expression for Q (and K).

Boundary

Applies to chemical equilibria where species activities (or appropriately corrected concentrations) are defined; excludes purely kinetic descriptions of reaction rates and systems not well-mixed or lacking defined thermodynamic activities.

Semantic Tension

Sometimes confused with reaction rate or extent of reaction; Q is a compositional comparison to equilibrium, not a rate law or time-dependent progress variable by itself.

Synthesis

The reaction quotient is the composition-dependent ratio of product to reactant activities raised to stoichiometric powers, used to determine the instantaneous thermodynamic direction of a reversible chemical reaction relative to equilibrium.