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Edexcel International A Level Chemistry

Topic 12
Entropy and
Energetics

Topic 12 explains why reactions happen. It builds entropy as the measure of disorder and the dispersal of energy, calculates the entropy changes of the system, the surroundings and the total, uses them to decide feasibility and the temperature at which a reaction becomes feasible, separates thermodynamic from kinetic stability, then turns to ionic solids: lattice energy and Born–Haber cycles, covalent character and polarisation, enthalpies of solution and hydration, and predicting solubility. It is examined in Unit 4.

Exam Unit
Unit 4
WCH14/01 – IAL
Specification Points
12.1 – 12.19
19 points covered
Topic Parts
8 pages
Revision notes available
Exam Board
Edexcel IAL
Pearson International

Revision Notes

Disorder and dispersal
Entropy
S rises: solid, liquid, gas
1
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
2
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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ΔStotal > 0
Feasibility and
Temperature
T = ΔH ÷ ΔS
3
Available

Feasibility and Temperature

Feasibility: ΔStotal = ΔSsystem + ΔSsurroundings, 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
4
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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Hess’s law for ionic solids
Born–Haber
Cycles
lattice energy, ΔatH, electron affinity
5
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
6
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
7
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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ΔsolH and ΔS together
Predicting
Solubility
Group 2 hydroxides and sulfates
8
Available

Predicting Solubility

Predicting solubility from enthalpy and entropy changes of solution, with the Group 2 hydroxide and sulfate trends explained.

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Specification Coverage

Topic 12 Entropy and Energetics – Edexcel International A Level Chemistry

The following specification points outline the knowledge and skills students are expected to demonstrate for Topic 12. Wording follows the Pearson Edexcel International A Level Chemistry specification.

12.1 – 12.11 Entropy, feasibility and stability

12.1
understand that, since endothermic reactions can occur spontaneously at room temperature, enthalpy changes alone do not control whether reactions occur
12.2
a
understand entropy as a measure of disorder of a system in terms of random dispersal of molecules
b
understand entropy as a measure of disorder of a system in terms of random dispersal of energy quanta between molecules
12.3
a
understand that entropy of a substance increases with temperature
b
understand that entropy increases as solid changes to liquid
c
understand that entropy increases as liquid changes to gas
d
understand that perfect crystals at zero kelvin have zero entropy
12.4
a
be able to interpret the natural direction of change as the direction of increasing total entropy
b
be able to interpret gases spreading spontaneously through a room as an example of increasing total entropy
12.5
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
12.6
a
understand that total entropy change of any reaction is the sum of entropy change of system and entropy change of surroundings
b
know the expression ΔStotal = ΔSsystem + ΔSsurroundings
12.7
be able to calculate the entropy change of the system for a reaction, ΔSsystem, given the entropies of the reactants and products
12.8
a
be able to calculate entropy change in the surroundings using ΔSsurroundings = −ΔH/T
b
be able to calculate ΔStotal using ΔSsurroundings = −ΔH/T
12.9
understand that the feasibility of a reaction depends on:
i
the balance between ΔSsystem and ΔSsurroundings, so that even endothermic reactions can occur spontaneously at room temperature
ii
temperature, as higher temperatures decrease the magnitude of ΔSsurroundings so its contribution to ΔStotal is less Students should be able to calculate the temperature at which a reaction is feasible. Students may also use ΔG = ΔH – TΔSsystem in answers, although this approach is not a requirement of the specification.
12.10
understand that reactions can occur as long as ΔStotal is positive even if one of the other entropy changes is negative
12.11
a
understand thermodynamic stability
b
understand kinetic stability
c
distinguish between thermodynamic stability and kinetic stability

12.12 – 12.19 Lattice energy, Born–Haber cycles, solution and hydration

12.12
be able to define the terms:
i
standard enthalpy change of atomisation, ΔatH
ii
electron affinity
iii
lattice energy (as the exothermic process for the formation of one mole of an ionic solid from its gaseous ions)
12.13
a
be able to construct Born-Haber cycles
b
be able to carry out calculations related to Born-Haber cycles
12.14
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
12.15
understand that polarisation of anions by cations leads to some covalency in an ionic bond, based on evidence from the Born-Haber cycle
12.16
a
be able to define the term ‘enthalpy change of solution, ΔsolH’
b
be able to define the term ‘enthalpy change of hydration, ΔhydH of an ion’
12.17
a
be able to use energy cycles to calculate enthalpy change of solution of an ionic compound using enthalpy change of hydration and lattice energy
b
be able to use energy level diagrams to calculate enthalpy change of solution of an ionic compound using enthalpy change of hydration and lattice energy
12.18
a
understand the effect of ionic charge on enthalpy change of hydration
b
understand the effect of ionic radius on enthalpy change of hydration
c
understand the effect of ionic charge on lattice energy
d
understand the effect of ionic radius on lattice energy
12.19
a
be able to use entropy changes of solution to predict solubility of ionic compounds
b
be able to use enthalpy changes of solution to predict solubility of ionic compounds
c
be able to discuss trends in solubility of ionic compounds covered in Unit 2