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.
Revision Notes
7 revision notes, in the order the specification teaches them.
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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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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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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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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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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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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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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Lattice Energy Map
Edexcel A Level Chemistry · Topic 13A Lattice Energy
The whole section on one A4 page: atomisation and electron affinity, lattice energy, Born–Haber cycles, covalent character, solution and hydration, and charge and radius.
Entropy and Free Energy Map
Edexcel A Level Chemistry · Topic 13B Entropy and Free Energy
The whole section on one A4 page: entropy, state and temperature, the natural direction of change, calculating ΔS, temperature and feasibility, and stability.
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13
- A.1
be able to define lattice energy as the energy change when one mole of an ionic solid is formed from its gaseous ions
Revise - A.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
Revise - A.9
be able to define the terms ‘enthalpy change of solution, ΔsolH’, and ‘enthalpy change of hydration, ΔhydH’
Revise - A.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
Revise - A.11ii
understand the effect of ionic charge and ionic radius on the values of: enthalpy change of hydration
Revise - B.12
understand that, since some endothermic reactions can occur at room temperature, enthalpy changes alone do not control whether reactions occur
Revise - B.13 b
know that the natural direction of change is increasing total entropy / positive entropy change
Revise - B.14iii
understand why entropy changes occur during: 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.
Revise - B.15 a
understand that total entropy change is the entropy change in the system plus the entropy change in the surroundings
Revise - B.16
be able to calculate the entropy change for the system, ΔSsystem , in a reaction, given the entropies of the reactants and products
Revise - B.17
be able to calculate the entropy change in the surroundings, and hence ΔStotal , using the expression: ΔSsurroundings=−ΔH\T
Revise - B.19i
be able to use the equation ΔG = ΔH − TΔSsystem to: predict whether a reaction is feasible
Revise - B.19ii
be able to use the equation ΔG = ΔH − TΔSsystem to: determine the temperature at which a reaction is feasible
Revise - B.20 a
be able to use ΔG = −RT ln K to show that feasible reactions have large equilibrium constants
Revise - B.20 b
be able to use ΔG = −RT ln K to show that reactions with large equilibrium constants are feasible
Revise - B.22
know that reactions that are thermodynamically feasible may be inhibited by kinetic factors
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