Reaction Quotient

The reaction quotient compares product and reactant activities in the mixture at its current composition. The relation is useful for Predict whether a reaction mixture is driven toward products or reactants as it approaches equilibrium.. For N₂ + 3 H₂ ⇌ 2 NH₃, let a(NH₃)=0.20, a(N₂)=0.50 and a(H₂)=0.40. Then Q=0.20²/(0.50·0.40³)=1.25. Check: Q is dimensionless. The final check tests a balance, direction or magnitude relevant to this formula. First map each quantity to the actual experiment, then substitute values. The result describes the modelled relationship; it does not automatically establish every mechanism in the real sample.

The symbols mean: Q: instantaneous reaction quotient (dimensionless); a_i: dimensionless activity of species i (dimensionless); ν_i: stoichiometric coefficient (dimensionless). Follow Q and activities are dimensionless relative to their standard states. Concentrations, activities, volumes and times must use compatible scales. A logarithm needs a dimensionless argument, and a balance must include all contributing species. State temperature and standard state whenever they are part of the definition; otherwise the result may refer to a different chemical condition. An amount is not a mass without molar mass, and activity is not an unnormalised concentration. Say whether inputs were measured or taken from a reference.

Use activities relative to defined standard states. Concentrations or partial pressures are approximations for sufficiently ideal systems. Q describes the current mixture; it equals the equilibrium constant only at equilibrium. Check this assumption for the actual sample, temperature and phase. Nonideality, mass transport or side reactions can make a precise calculation systematically wrong. Compare the balanced reaction and the activity or concentration scale with the model, then state the assumption that most affects the result. Near model boundaries, check activities and phase state rather than extending a dilute-solution approximation. Different conditions require a fresh model check.

Recalculate the example: For N₂ + 3 H₂ ⇌ 2 NH₃, let a(NH₃)=0.20, a(N₂)=0.50 and a(H₂)=0.40. Then Q=0.20²/(0.50·0.40³)=1.25. Check: Q is dimensionless. Check Q and activities are dimensionless relative to their standard states and substitute the value back into the relation. Also ask whether its sign, balance or magnitude is chemically plausible. If a check fails, inspect units, coefficients and reference values before blaming rounding. Substitution checks the algebra; comparing assumptions with the experiment checks whether the model is appropriate.

Reaction Quotient is related to Massenwirkungsgesetz (Gleichgewichtskonstante), Gibbs-Energie (Freie Enthalpie), Nernst-Gleichung, Relationship between Kp and Kc, Chemical Potential of a Component, Standard Gibbs Energy and Equilibrium Constant, Reaction Gibbs Energy and Reaction Quotient, but answers a distinct question: Predict whether a reaction mixture is driven toward products or reactants as it approaches equilibrium.. Do not confuse it with a relation for a neighboring quantity or a different standard state. Name the target quantity and sample conditions first, then choose the equation containing that quantity. Similar-looking equations may use different stoichiometric bases or validity ranges. The linked entries may supply inputs for another step, but they do not replace this distinction. This keeps the interpretation auditable.