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Edexcel International A Level Chemistry

Topic 13
Chemical
Equilibria

Topic 13 turns equilibrium into numbers. It writes expressions for Kc and Kp for homogeneous and heterogeneous systems, calculates both constants with their units from experimental data, shows that only temperature changes an equilibrium constant while concentration, pressure and a catalyst do not, explains the shift in position with temperature through the change in the constant, links the constant to the total entropy change through ΔStotal = R ln K, and uses the size of the constant to predict how far a reaction goes. It is examined in Unit 4.

Exam Unit
Unit 4
WCH14/01 – IAL
Specification Points
13.1 – 13.9
9 points covered
Sub-topics
6 lessons
Revision notes available
Exam Board
Edexcel IAL
Pearson International

Revision Notes

6 revision notes, in the order the specification teaches them.

A flask of mixed gases beside a flask with a solid and a gas
1

Kc Expressions: Homogeneous and Heterogeneous Systems

Kc expressions: homogeneous and heterogeneous equilibria, writing the expression from the equation, leaving out solids and pure liquids, and the units of Kc.

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Sealed tubes in a warm bath beside a burette
2

Calculating Kc from Experimental Data

Calculating Kc: initial, change and equilibrium tables, equilibrium concentrations, the value and units of Kc, and the experiments that measure equilibrium amounts.

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A gas mixture beside a bar split into matching shares
3

Partial Pressures and Kp Expressions

Partial pressures and Kp: mole fraction, partial pressure from mole fraction and total pressure, and writing Kp expressions for homogeneous and heterogeneous systems.

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A gas mixture in a cylinder with a pressure gauge
4

Calculating Kp

Calculating Kp: equilibrium moles, mole fractions and partial pressures, the value and units of Kp in atm, and finding a pressure from Kp.

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A sealed gas flask in ice beside one in hot water
5

What Changes an Equilibrium Constant?

Equilibrium constants and conditions: the effect of temperature on Kc and Kp, and why concentration, pressure and catalysts do not change the constant.

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A level balance with forward and reverse arrows
6

Equilibrium Constants, Entropy and the Extent of Reaction

Equilibrium constants and entropy: ΔStotal = R ln K, the effect of temperature on ΔStotal and K, and using the size of K to predict the extent of a reaction.

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Summary Map

Download, print and revise the key facts of this topic at a glance.

Summary maps

Download, print and revise the key facts of this topic at a glance.

Illustration of a summary map: compounds joined by reaction arrows
Summary map · PDF

Equilibrium Constant Kc Map

Edexcel International A Level Chemistry · Unit 4 · Topic 13A Equilibrium Constant Kc

The whole section on one A4 page: writing Kc, the expression and its equation, heterogeneous systems, units of Kc, ICE tables, and measuring Kc.

Download the map6 key ideas + a data table · A4 portrait · print ready
Illustration of a summary map: compounds joined by reaction arrows
Summary map · PDF

Kp and the Equilibrium Constant Map

Edexcel International A Level Chemistry · Unit 4 · Topic 13B Kp and the Equilibrium Constant

The whole section on one A4 page: mole fractions and partial pressures, writing Kp, calculating Kp, why only temperature changes K, the ratio against K, and entropy and K.

Download the map6 key ideas + a data table · A4 portrait · print ready

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Specification

Every specification point in this topic, in the exam board's own words. Rate how confident you are with each one.

Official Pearson Edexcel IAL documentsSpecification (opens the Pearson Edexcel IAL website in a new tab)Data booklet (opens the Pearson Edexcel IAL website in a new tab)

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13.1

  1. 13.1

    be able to deduce an expression for Kc , for homogeneous and heterogeneous systems, in terms of equilibrium concentrations

    Revise

13.2

  1. 13.2

    be able to deduce an expression for Kp for homogeneous and heterogeneous systems, in terms of equilibrium partial pressures in atm

    Revise

13.3

  1. a

    be able to calculate a value for Kc with units where appropriate from experimental data

    Revise
  2. b

    be able to calculate a value for Kp with units where appropriate from experimental data

    Revise

13.4

  1. a

    understand how temperature affects equilibrium composition

    Revise
  2. b

    understand how pressure affects equilibrium composition

    Revise
  3. c

    understand how the presence of a catalyst affects equilibrium composition

    Revise

13.5

  1. a

    understand that equilibrium constant is not affected by changes in concentration

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  2. b

    understand that equilibrium constant is not affected by pressure

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  3. c

    understand that equilibrium constant is not affected by addition of a catalyst

    Revise

13.6

  1. a

    know the effect of changing temperature on Kc for exothermic reactions

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  2. b

    know the effect of changing temperature on Kc for endothermic reactions

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  3. c

    know the effect of changing temperature on Kp for exothermic reactions

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  4. d

    know the effect of changing temperature on Kp for endothermic reactions

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13.7

  1. 13.7

    understand that the effect of temperature on the position of equilibrium is explained using a change in the value of the equilibrium constant

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13.8

  1. i

    understand the effect of a change in temperature on: the value of ΔStotal

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  2. ii

    understand the effect of a change in temperature on: the magnitude of the equilibrium constant, since ΔStotal = R lnK

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13.9

  1. 13.9

    be able to apply knowledge of equilibrium constant values to predict the extent to which a reaction takes place

    Revise