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Cambridge International AS & A Level Chemistry – Physical Chemistry (A Level)

Topic 23
Chemical
Energetics

Topic 23 covers lattice energy and the enthalpy change of atomisation, first electron affinity and its trends in Groups 16 and 17, Born–Haber cycles with calculations, the effect of ionic charge and radius on lattice energy, enthalpies of hydration and solution with energy cycles, entropy as the number of arrangements of particles and energy, the sign of entropy changes for changes of state, temperature and gas molecules, ΔS from standard entropies, and the Gibbs equation ΔG⦵ = ΔH⦵ − TΔS⦵ with feasibility and the effect of temperature.

Exam Paper
Paper 4
9701 A Level
Learning Outcomes
23.1 – 23.4
4 sections covered
Topic Parts
7 pages
Revision notes available
Exam Board
Cambridge
9701 (2025 onwards)

Revision Notes

Work through Topic 23 Chemical Energetics in a structured sequence.

Hess’s law for ionic solids
Born–Haber
Cycles
1
Available

Lattice Energy and Born–Haber Cycles

Lattice energy and Born–Haber cycles: definitions of atomisation, ionisation energy and electron affinity, constructing the cycle for NaCl and MgCl₂ and calculating the missing value.

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Charge density
Polarisation and
Covalent Character
2
Available

Lattice Energy Trends and Covalent Character

Lattice energy trends: charge and radius, the perfect ionic model against Born–Haber values, polarisation and covalent character in ionic compounds.

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Lattice broken, ions hydrated
Solution and
Hydration
3
Available

Enthalpy of Solution and Hydration

Enthalpy of solution and hydration: definitions, energy cycles with lattice energy, worked calculations and the effect of ionic charge and radius.

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Disorder and dispersal
Entropy
4
Available

Entropy and the Direction of Change

Entropy: disorder and the dispersal of energy, entropy and temperature, changes of state, dissolving, gas moles and the natural direction of change.

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ΣS(products) − ΣS(reactants)
Entropy
Calculations
5
Available

Calculating Entropy Changes

Entropy calculations: ΔS from standard entropies, units and worked examples.

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ΔG = ΔH − TΔS
Feasibility and
Temperature
6
Available

Feasibility, Gibbs Energy and Temperature

Feasibility: ΔG = ΔH − TΔS, the sign of ΔG, the effect of temperature and the temperature at which a reaction becomes feasible.

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Feasible is not fast
Thermodynamic and
Kinetic Stability
7
Available

Thermodynamic and Kinetic Stability

Thermodynamic and kinetic stability: feasible but slow reactions, activation energy and the limits of ΔG and ΔStotal predictions.

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Specification Coverage

Topic 23 Chemical Energetics – Cambridge International A Level Chemistry

The following Cambridge learning outcomes state what candidates should be able to do. Wording is taken from the Cambridge International AS & A Level Chemistry (9701) syllabus.

23.1 Lattice energy and Born–Haber cycles

23.1.1
(a)
define and use the term enthalpy change of atomisation, ΔHat
(b)
define and use the term lattice energy, ΔHlatt (the change from gas phase ions to solid lattice)
23.1.2
(a)
define and use the term first electron affinity, EA
(b)
explain the factors affecting the electron affinities of elements
(c)-i
describe and explain the trends in the electron affinities of the Group 16 elements
(c)-ii
describe and explain the trends in the electron affinities of the Group 17 elements
23.1.3
construct and use Born–Haber cycles for ionic solids (limited to +1 and +2 cations, –1 and –2 anions)
23.1.4
carry out calculations involving Born–Haber cycles
23.1.5
a
explain, in qualitative terms, the effect of ionic charge on the numerical magnitude of a lattice energy
b
explain, in qualitative terms, the effect of ionic radius on the numerical magnitude of a lattice energy

23.2 Enthalpies of solution and hydration

23.2.1
a
define and use the term enthalpy change of hydration, ΔHhyd
b
define and use the term enthalpy change of solution, ΔHsol
23.2.2
construct and use an energy cycle involving enthalpy change of solution, lattice energy and enthalpy change of hydration
23.2.3
carry out calculations involving the energy cycles in 23.2.2
23.2.4
a
explain, in qualitative terms, the effect of ionic charge on the numerical magnitude of an enthalpy change of hydration
b
explain, in qualitative terms, the effect of ionic radius on the numerical magnitude of an enthalpy change of hydration

23.3 Entropy change, ΔS

23.3.1
define the term entropy, S, as the number of possible arrangements of the particles and their energy in a given system
23.3.2
(a)
predict and explain the sign of the entropy changes that occur during a change in state, e.g. melting, boiling and dissolving (and their reverse)
(b)
predict and explain the sign of the entropy changes that occur during a temperature change
(c)
predict and explain the sign of the entropy changes that occur during a reaction in which there is a change in the number of gaseous molecules
23.3.3
calculate the entropy change for a reaction, ΔS, given the standard entropies, S⦵, of the reactants and products, ΔS⦵ = ΣS⦵(products) − ΣS⦵(reactants) (use of ΔS = ΔSsurr + ΔSsys is not required)

23.4 Gibbs free energy change, ΔG

23.4.1
state and use the Gibbs equation ΔG⦵ = ΔH⦵ − TΔS⦵
23.4.2
perform calculations using the equation ΔG⦵ = ΔH⦵ − TΔS⦵
23.4.3
state whether a reaction or process will be feasible by using the sign of ΔG⦵
23.4.4
predict the effect of temperature change on the feasibility of a reaction, given standard enthalpy and entropy changes