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Edexcel A Level Chemistry

Topic 13
Energetics II

Topic 13 has two parts. 13A Lattice energy covers the definitions, Born–Haber cycles and their calculations, lattice energy as a measure of ionic bond strength, the perfect ionic model against experimental values, polarisation and covalent character, and enthalpies of solution and hydration with their energy cycles. 13B Entropy covers entropy and the direction of change, ΔSsystem, ΔSsurroundings and ΔStotal, feasibility from ΔG = ΔH − TΔSsystem, ΔG = −RT ln K, and why a feasible reaction may still not occur.

Exam Paper
Paper 1
9CH0/01 (and Paper 3)
Specification Points
13A.1 – 13B.22
2 parts covered
Topic Parts
7 pages
Revision notes available
Exam Board
Edexcel
Pearson 9CH0

Revision Notes

Hess’s law for ionic solids
Born–Haber
Cycles
lattice energy, ΔatH, electron affinity
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
theoretical against experimental
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
ΔsolH = −LE + ΣΔhydH
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
S rises: solid, liquid, gas
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
system, surroundings, total
5
Available

Calculating Entropy Changes

Entropy calculations: ΔS from standard entropies, ΔS of the surroundings from −ΔH/T and the total entropy change.

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ΔG = ΔH − TΔS
Feasibility and
Temperature
T = ΔH ÷ ΔS
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
ΔG, Eₐ, diamond and petrol
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 13 Energetics II – Edexcel A Level Chemistry

The following specification points outline the knowledge and skills students are expected to demonstrate. Wording follows the Edexcel A Level Chemistry (9CH0) specification.

13A Energetics II: Lattice energy

13A.1
be able to define lattice energy as the energy change when one mole of an ionic solid is formed from its gaseous ions
13A.2
be able to define the terms:
i
enthalpy change of atomisation, ΔatH
ii
electron affinity
13A.3
be able to construct Born-Haber cycles and carry out related calculations
13A.4
know that lattice energy provides a measure of ionic bond strength
13A.5
understand that a comparison of the experimental lattice energy value (from a Born-Haber cycle) with the theoretical value (obtained from electrostatic theory) in a particular compound indicates the degree of covalent bonding
13A.6
understand the meaning of polarisation as applied to ions
13A.7
know that the polarising power of a cation depends on its radius and charge
a
know that the polarising power of a cation depends on its radius
b
know that the polarising power of a cation depends on its charge
13A.8
know that the polarisability of an anion depends on its radius and charge
a
know that the polarisability of an anion depends on its radius
b
know that the polarisability of an anion depends on its charge
13A.9
be able to define the terms ‘enthalpy change of solution, ΔsolH’, and ‘enthalpy change of hydration, ΔhydH’
13A.10
be able to use energy cycles and energy level diagrams to carry out calculations involving enthalpy change of solution, enthalpy change of hydration and lattice energy
13A.11
understand the effect of ionic charge and ionic radius on the values of:
i
lattice energy
ii
enthalpy change of hydration

13B Energetics II: Entropy

13B.12
understand that, since some endothermic reactions can occur at room temperature, enthalpy changes alone do not control whether reactions occur
13B.13
a
know that entropy is a measure of disorder of a system
b
know that the natural direction of change is increasing total entropy / positive entropy change
13B.14
understand why entropy changes occur during:
i
changes of state
ii
dissolving of a solid ionic lattice
iii
reactions in which there is a change in the number of moles from reactants to products Students should be able to discuss typical reactions in terms of disorder and enthalpy change, including: o dissolving ammonium nitrate crystals in water o reacting ethanoic acid with ammonium carbonate o burning magnesium ribbon in air o mixing solid barium hydroxide, Ba(OH)₂.8H₂O, with solid ammonium chloride.
13B.15
a
understand that total entropy change is the entropy change in the system plus the entropy change in the surroundings
b
know the expression ΔStotal = ΔSsystem + ΔSsurroundings
13B.16
be able to calculate the entropy change for the system, ΔSsystem , in a reaction, given the entropies of the reactants and products
13B.17
be able to calculate the entropy change in the surroundings, and hence ΔStotal , using the expression: ΔSsurroundings=−ΔH\T
13B.18
a
know that the balance between entropy change and enthalpy change determines feasibility
b
know that feasibility is represented by ΔG = ΔH − TΔSsystem
13B.19
be able to use the equation ΔG = ΔH − TΔSsystem to:
i
predict whether a reaction is feasible
ii
determine the temperature at which a reaction is feasible
13B.20
a
be able to use ΔG = −RT ln K to show that feasible reactions have large equilibrium constants
b
be able to use ΔG = −RT ln K to show that reactions with large equilibrium constants are feasible
13B.21
understand why a reaction for which the ΔG value is negative may not occur in practice
13B.22
know that reactions that are thermodynamically feasible may be inhibited by kinetic factors