Physical Chemistry
Edexcel AS Chemistry — 1.10 Equilibrium I — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 42 of 42
Lesson summary
Equilibrium expressions: derive Kc from a balanced equation, omit pure solids, and use equilibrium concentrations with the correct powers. For aA + bB ⇌ cC + dD, write Kc = [C]ᶜ[D]ᵈ ÷ ([A]ᵃ[B]ᵇ).
- not included
- Kc = [Fe³⁺] ÷ ([Ag⁺][Fe²⁺])
- Kc = [CO₂]
- N₂ + 3H₂ ⇌ 2NH₃
- [NH₃]² ÷ ([N₂][H₂]³)
- 2SO₂ + O₂ ⇌ 2SO₃
Part 1 of 4General Kc expression
1AA + bB ⇌ cC + dD
For aA + bB ⇌ cC + dD, write Kc = [C]ᶜ[D]ᵈ ÷ ([A]ᵃ[B]ᵇ). Products go in the numerator and reactants in the denominator; each power comes from the balanced coefficient.
2Equilibrium concentration
Square brackets mean equilibrium concentration, usually in mol dm⁻³. Write an expression from the balanced reaction before substituting any numerical values.
Part 2 of 4Omit pure solids
1Not included
Pure solids have effectively constant activity and are not included in the Kc expression. For Ag⁺(aq) + Fe²⁺(aq) ⇌ Ag(s) + Fe³⁺(aq), omit Ag(s).
2Kc = [Fe³⁺] ÷ ([Ag⁺][Fe²⁺])
The expression is Kc = [Fe³⁺] ÷ ([Ag⁺][Fe²⁺]). For CaCO₃(s) ⇌ CaO(s) + CO₂(g), both solids are omitted, leaving Kc = [CO₂].
Part 3 of 4Gas-equilibrium examples
N₂ + 3H₂ ⇌ 2NH₃
For N₂ + 3H₂ ⇌ 2NH₃, Kc = [NH₃]² ÷ ([N₂][H₂]³). The 2 and 3 are exponents, not multipliers in front of concentration.
What the symbols mean: Kc = equilibrium constant in terms of concentration · [ ] = concentration at equilibrium (mol dm−3) · each power is the number in front of that substance in the balanced equation
2SO₂ + O₂ ⇌ 2SO₃
For 2SO₂ + O₂ ⇌ 2SO₃, Kc = [SO₃]² ÷ ([SO₂]²[O₂]). Check that all species and coefficients match the balanced equation.
Part 4 of 4Interpret the constant
A larger Kc generally means the equilibrium mixture is more product-favoured; a smaller Kc means more reactant-favoured, for the reaction as written.
Temperature
At a fixed temperature, the value of Kc is constant for that reaction. Temperature can change its value; changes in concentration shift the mixture until the same Kc is restored.
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3 & IGCSE —
Kc
- : Kc! (ΔH < 0) Kc. (ΔH > 0) Kc.
- : ( ).
- : Kc .