7.1.8—The quantities present at equilibrium
- Syllabus
- 9701–2028–2029
- Objective
- 7.1.8
- Level
- AS
Equilibrium quantities are linked by stoichiometry and the constant expression
At equilibrium, the concentrations or partial pressures of all species are related by the balanced equation and the value of K. They are not required to be equal.
Use an ICE-style change table or stoichiometric ratios to express unknown equilibrium amounts in terms of one variable, then substitute into K and solve.
For A ⇌ 2B, if A decreases by x, B increases by 2x. The equilibrium values are [A]₀−x and [B]₀+2x, not independent guesses.
Dynamic equilibrium means forward and reverse rates are equal, not concentrations. The composition depends on K and the starting conditions.
ConceptA-Level CAIE Chemistry AS