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AQA A Level Chemistry

3.1.8
Thermodynamics

Section 3.1.8 covers Born–Haber cycles (both definitions of lattice enthalpy, the enthalpy changes of formation, atomisation, ionisation, bond dissociation and electron affinity, constructing and using cycles, the perfect ionic model as evidence for covalent character, hydration and solution cycles) and Gibbs free-energy change and entropy change (why ΔH alone is not enough, entropy and disorder, ΔG = ΔH − TΔS, ΔG ≤ 0 for feasibility, entropy changes from absolute entropies, how ΔG varies with temperature and the temperature at which a reaction becomes feasible).

Exam Paper
Paper 1
7405/1 (and Paper 3)
Specification Points
3.1.8.1 – 3.1.8.2
2 sections covered
Topic Parts
7 pages
Revision notes available
Exam Board
AQA
7405 (2015 onwards)

Revision Notes

Work through 3.1.8 Thermodynamics in a structured sequence.

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

Lattice Enthalpy and Born–Haber Cycles

Lattice enthalpy 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 Enthalpy Trends and Covalent Character

Lattice enthalpy 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 enthalpy, 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

3.1.8 Thermodynamics – AQA A Level Chemistry

The following AQA specification points outline the knowledge and skills students are expected to demonstrate. Wording follows the AQA A Level Chemistry (7405) specification.

3.1.8.1 Born–Haber cycles

3.1.8.1
i-a
know that lattice enthalpy can be defined as enthalpy of lattice dissociation
i-b
know that lattice enthalpy can be defined as enthalpy of lattice formation
ii-a
define enthalpy of formation
ii-b
define ionisation energy
ii-c
define enthalpy of atomisation
ii-d
define bond enthalpy
ii-e
define electron affinity
ii-f
define lattice enthalpy
iii
construct Born-Haber cycles to calculate lattice enthalpies using enthalpy of formation, ionisation energy, enthalpy of atomisation, bond enthalpy and electron affinity
iv
construct Born-Haber cycles to calculate one of the other enthalpy changes when lattice enthalpy is known
v
compare lattice enthalpies from Born-Haber cycles with those from calculations based on a perfect ionic model to provide evidence for covalent character in ionic compounds
vi
define the term enthalpy of hydration
vii
use cycles to calculate enthalpies of solution for ionic compounds from lattice enthalpies and enthalpies of hydration

3.1.8.2 Gibbs free-energy change, ΔG, and entropy change, ΔS

3.1.8.2
i
understand that DeltaH alone is not sufficient to explain feasible change
ii
understand the concept of increasing disorder (entropy change, DeltaS) and how it accounts for feasibility of change
iii-a
know the relationship DeltaG = DeltaH – TDeltaS
iv
know that for a reaction to be feasible, the value of DeltaG must be zero or negative
v
calculate entropy changes from absolute entropy values
vi
use DeltaG = DeltaH – TDeltaS to determine how DeltaG varies with temperature
vii
use DeltaG = DeltaH – TDeltaS to determine the temperature at which a reaction becomes feasible