Equilibrium tables, Kc and reaction-direction reasoning

Equilibrium tables, Kc and reaction-direction reasoning

Equilibrium data can be organised in initial-change-equilibrium tables so each quantity is traceable. Use stoichiometric coefficients to relate changes: for A + B ⇌ 2C, a decrease of x in A and B corresponds to an increase of 2x in C. Kc is calculated from equilibrium concentrations and provides a benchmark at fixed temperature. A reaction quotient Qc has the same expression but uses current concentrations; comparing Qc with Kc predicts direction: Qc < Kc means too few products relative to equilibrium, so the forward reaction is favoured; Qc > Kc means the reverse reaction is favoured. Keep this reasoning tied to a defined temperature.

Work it through

For A + B ⇌ 2C, if current values give Qc = 0.50 and Kc = 1.60 at the same temperature, products are relatively underrepresented. The reaction shifts forward until Qc becomes Kc. Do not say Kc increases merely because concentration changes; Kc remains fixed unless temperature changes.

Mastery target

Organise equilibrium quantities, calculate or compare Qc and Kc, and use the comparison to predict reaction direction with a temperature condition.