A-Level Chemistry OCR Notes

5.2.2 Enthalpy and entropy

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Entropy

Entropy is a measure of dispersal of energy through a system, which increases as disorder increases

The terms feasible or spontaneous are used to describe reactions that can take place on their own.
It is a fundamental observation that in any spontaneous process, the total entropy of the universe will always increase

Reactions that produce gases result in an increase in entropy
Entropy increases during changes in state that give the particle access to a greater number of configurations

solid < liquid < gas (in order of increasing entropy)

The sign of an entropy change can be predicted when the reactants and products have different numbers of moles of gas molecules. If the products have fewer moles, then entropy decreases

Fewer moles of gas → Fewer particles → Fewer configurations → Lower entropy

Gibbs Free Energy

Whether a reaction will happen spontaneously depends on temperature, enthalpy change and entropy changes
The Gibbs Free-Energy Equation:

∆G = ∆H - T∆S
∆G = change in free energy of the system (kJ mol^-1)
∆H = change in enthalpy of the system (kJ mol^-1)
T = temperature of the system (K)
∆S = change in entropy of the system (J K^-1 mol^-1)

∆G allows us to predict whether a reaction is feasible
Reactions are only feasible if ∆G is zero or negative

∆G depends on temperature – some reactions may be feasible at one temperature and not another
To calculate the temperature at which a reaction becomes feasible, the Gibbs Free-Energy equation has to be rearranged

Kinetics also influences whether a reaction can occur. If the activation energy is too high or the rate of reaction is very slow, the reaction may not happen even if the reaction is thermodynamically favourable.

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