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*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

3.2.3 Chemical equilibrium

Chemical equilibriumThe equilibrium constant, Kc

The Equilibrium Constant, Kc

Specification Reference Topic 3.2.3 (a)–(g)

Quick Notes

  • The equilibrium constant (Kc) is the ratio of product and reactant concentrations at equilibrium.
  • For the reaction: OCR (A) A-Level Chemistry general reaction aA + bB ⇌ cC + dD used for equilibrium constant expression.Kc expression:
    OCR (A) A-Level Chemistry equation showing Kc expression as products over reactants with powers from balanced equation.
    where a, b, c, and d are the balancing numbers from the chemical equation and [ ] = concentration in mol dm−3
  • Kc only depends on temperature and does not change with concentration, pressure, or catalysts.
  • Calculated from equilibrium concentrations.
  • Kc interpretation:
    • Kc > 1 means equilibrium favours products.
    • Kc < 1 means equilibrium favours reactants.

Full Notes

Kc is an equilibrium constant and shows the position of equilibrium for reactions in homogeneous equilibrium.

A homogenous equilibrium means all reactants and products are in the same phase.

Kc is calculated using the concentrations of reactants and products in the mixture at equilibrium.

General Formula for Kc

For a reaction:

OCR (A) A-Level Chemistry general reaction aA + bB ⇌ cC + dD used for equilibrium constant expression.

Kc =

OCR (A) A-Level Chemistry equilibrium constant Kc expression showing concentrations of C and D over A and B.

where [A], [B], [C], [D] are equilibrium concentrations in mol dm−3 and a, b, c, and d are the balancing numbers from the equation.

If...

Kc values are only valid for a specific temperature - Kc changes with temperature.

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Matt’s exam tip

Solids aren’t ever included in Kc expressions and if water is a solvent, it also isn’t included (even if it is also a reactant or product).

How to Calculate Kc

Worked Example: Esterification Reaction

CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O

Equilibrium concentrations:
[CH3COOH] = 0.20 mol dm⁻³
[C2H5OH] = 0.20 mol dm⁻³
[CH3COOC2H5] = 0.40 mol dm⁻³
[H2O] = 0.40 mol dm⁻³

  1. Kc = [CH3COOC2H5][H2O] ÷ [CH3COOH][C2H5OH]
  2. = (0.40 × 0.40) ÷ (0.20 × 0.20)
  3. = 4.0

Since Kc > 1, equilibrium favours the products. This means in the equilibrium mixture there is a higher concentration of products (ester and water) compared to reactants.


Photo of Matt
Matt’s exam tip

Remember concentrations in Kc are equilibrium values. Always check if you need to calculate equilibrium concentrations from given data first.

Summary