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4.4.1Unit 4. Rates, Equilibria and Further Organic Chemistry

Equilibrium Constant, Kc — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 29 of 56 Unit quiz

📚 IAL Edexcel Linear (Chemistry) Hub / Topical Revision
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R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
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Section 1 of 5Writing the Expression

For aA + bB ⇌ cC + dD at equilibrium, concentrations are linked by a fixed ratio at a given temperature.

General form
  • General form Kc = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ, with products on the top and reactants on the bottom of the fraction.
Powers
  • Powers Each concentration is raised to the power of its coefficient in the balanced equation, so the equation must be balanced.
Square brackets
  • Square brackets Square brackets mean concentration in mol dm⁻³ measured when the system has reached equilibrium.
Heterogeneous systems
  • Heterogeneous systems Solids and pure liquids are left out because their concentrations stay constant, so only gases and aqueous species appear.

Section 2 of 5Units and Size of Kc

Units and Size of Kc
Finding units
  • Finding units Substitute mol dm⁻³ into the expression and cancel; for N₂O₄ ⇌ 2NO₂ the units are mol dm⁻³.
No units
  • No units If the powers on top and bottom are equal, as in H₂ + I₂ ⇌ 2HI, Kc has no units.
Large Kc
  • Large Kc A value much greater than 1 means the equilibrium mixture contains mostly products.
Small Kc
  • Small Kc A value much smaller than 1 means the mixture contains mostly reactants at equilibrium.

Section 3 of 5Calculating Kc

Use a consistent method for any equilibrium calculation.

1
Write the balanced equation and the Kc expression, including state symbols so you can see which species to include.
2
Convert amounts in moles to concentrations by dividing by the volume in dm³, unless moles of reactants and products cancel the volume.
3
Use an initial, change and equilibrium table, applying the reaction ratio to the change row.
4
Substitute equilibrium concentrations into the expression and calculate, giving units and an appropriate number of significant figures.

Section 4 of 5Worked Example

Question

  • At equilibrium, N₂O₄(g) ⇌ 2NO₂(g) has [N₂O₄] = 0.0400 mol dm⁻³ and [NO₂] = 0.0120 mol dm⁻³.
  • Calculate Kc.

  1. Write the expressionKc = [NO₂]² / [N₂O₄].
    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
  2. SubstituteKc = (0.0120)² / 0.0400 = 1.44 × 10⁻⁴ / 0.0400.
  3. EvaluateKc = 3.6 × 10⁻³, with units mol dm⁻³ because mol² dm⁻⁶ divided by mol dm⁻³ gives mol dm⁻³.

Answer Kc = 3.6 × 10⁻³ mol dm⁻³.

Section 5 of 5Exam Checklist & Tips

4.4.1 – Key Points for the Exam

  • Kc Equilibrium constant calculated from equilibrium concentrations.
  • Dynamic equilibrium State in which forward and reverse rates are equal and concentrations are constant.
  • Homogeneous Equilibrium in which all species are in the same phase.
  • Equilibrium concentration Concentration measured after the system stops changing.

Common Mistakes

  • Including solids in the Kc expression.
  • Using initial concentrations instead of equilibrium concentrations.
  • Forgetting to raise the concentration to the power of its coefficient.
  • Giving no units when the powers do not cancel.

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3 & IGCSE —

Kc

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