5.2.2
Enthalpy and
Entropy
Section 5.2.2 covers entropy as a measure of the dispersal of energy, the entropy of solids, liquids and gases and of reactions that change the number of gas molecules, calculating the entropy change of a system from standard entropies, feasibility in terms of TΔS and ΔH, the Gibbs’ equation ΔG = ΔH − TΔS with calculations, feasibility when ΔG is negative, and the limits of ΔG predictions in terms of kinetics.
Revision Notes
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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, units and worked examples.
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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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Enthalpy and Entropy Map
OCR A Level Chemistry A · 5.2.2 Enthalpy and entropy
The whole section on one A4 page: entropy, state and temperature, the sign of ΔS, ΔS and ΔG, temperature and feasibility, and the limits of ΔG.
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5.2.2
- a
explanation that entropy is a measure of the dispersal of energy in a system which is greater, the more disordered a system
Revise - b (i)
explanation of the difference in magnitude of the entropy of a system of solids, liquids and gases
Revise - b (ii)
explanation of the difference in magnitude of the entropy of a system for a reaction in which there is a change in the number of gaseous molecules
Revise - c
calculation of the entropy change of a system, ΔS, and related quantities for a reaction given the entropies of the reactants and products
Revise - d
explanation that the feasibility of a process depends upon the entropy change and temperature in the system, TΔS, and the enthalpy change of the system, ΔH
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