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*Revision Materials and Past Papers* 1 Atomic Structure 2 Amounts of Substance 3 Bonding 4 Energetics 5 Kinetics 6 Chemical Equilibria & Kc 7 Redox Equations 8 Thermodynamics 9 Rate Equations 10 Kp (Equilibrium Constant) 11 Electrode Potentials & Cells 12 Acids and Bases 13 Periodicity 14 Group 2: Alkaline Earth Metals 15 Group 7: The Halogens 16 Period 3 Elements & Oxides 17 Transition Metals 18 Reactions of Ions in Aqueous Solution 19 Intro to Organic Chemistry 20 Alkanes 21 Halogenoalkanes 22 Alkenes 23 Alcohols 24 Organic Analysis 25 Optical Isomerism 26 Aldehydes & Ketones 27 Carboxylic Acids & Derivatives 28 Aromatic Chemistry 29 Amines 30 Polymers 31 Amino Acids, Proteins & DNA 32 Organic Synthesis 33 NMR Spectroscopy 34 Chromatography RP1–RP12 Required Practicals

1.6 Chemical equilibria, Le Chatelier’s principle and Kc

1.6.1 Equilibrium and Le Chatelier's Principle 1.6.2 Equilibrium constant Kc for homogeneous systems

Equilibrium Constants (Kc)

Specification Reference Physical Chemistry, Kinetics 3.1.5.2

Quick Notes

  • The equilibrium constant (Kc) is the ratio of product and reactant concentrations at equilibrium.
  • Kc expression:
    AQA A-Level Chemistry Kc expression showing concentrations of products over reactants with powers as balancing numbers.
    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.
  • Interpretation:
    • Kc > 1 → equilibrium favours products.
    • Kc < 1 → equilibrium favours reactants.

Full Notes

Definition of Equilibrium Constant (Kc)

Kc shows the position of equilibrium for reactions in homogeneous equilibrium (all reactants and products in the same phase).

General formula for Kc:
For a reaction:

AQA A-Level Chemistry general reversible reaction aA + bB ⇌ cC + dD.

is

AQA A-Level Chemistry Kc expression showing ratio of products to reactants raised to their stoichiometric powers.

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

If…

Photo of Matt
Matt’s exam tip

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

Worked Example: Esterification Reaction

Worked Example

CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O

Given 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

Kc calculation:

Kc = [CH3COOC2H5] [H2O] ÷ [CH3COOH] [C2H5OH]

Kc = (0.40 × 0.40) ÷ (0.20 × 0.20)

Kc = 4.0

Since Kc > 1, equilibrium favours the products (ester and water).


Photo of Matt
Matt’s exam tip

Remember concentrations in the Kc expression are those at equilibrium. Read questions carefully and calculate equilibrium concentrations if needed.

Effect of Changing Conditions on Kc

Temperature
Kc changes with temperature because temperature affects equilibrium position.

Concentration and Pressure
Kc does not change with concentration or pressure because the system shifts to restore Kc.

Catalysts
Catalysts do not change Kc. They speed up how fast equilibrium is reached but do not affect position.

Summary Table: Effect of Factors on Kc

Factor Effect on Kc Explanation
Temperature Changes Kc increases for endothermic reactions, decreases for exothermic reactions
Concentration No change System shifts to maintain Kc
Pressure (gases) No change System shifts to maintain Kc
Catalyst No change Equilibrium reached faster, but position remains the same