HSC · Topic explainer
Writing an equilibrium expression in HSC Chemistry
Write the equilibrium expression from the balanced equation. Put dissolved or gaseous product terms above the reactant terms and raise each term to its stoichiometric coefficient. Pure solids and pure liquids have constant activity and are omitted. Use equilibrium concentrations when substituting into a concentration-based expression.
Worked example
For NH₄HS(s) ⇌ NH₃(g) + H₂S(g), write Kc = [NH₃][H₂S]. Both gases appear with exponent 1. The solid ammonium hydrosulfide has constant activity and contributes no concentration term.
If both equilibrium gas concentrations are 0.030 mol L⁻¹, their product is 0.00090. For the reverse equation, the same gas terms appear in the denominator and the equilibrium constant is the reciprocal.
Official exam practice
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