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# Gibbs Free-Energy and Entropy Change

Physical

A2

Born-Haber

AQA Content

###### Calculate entropy changes from absolute entropy values

###### Use the relationship âˆ†G = âˆ†H â€“ Tâˆ†S to determine how âˆ†G varies with temperature

###### Use the relationship âˆ†G = âˆ†H â€“ Tâˆ†S to determine the temperature at which a reaction becomes feasible.

Specification Notes

###### âˆ†H, whilst important, is not sufficient to explain feasible change.

###### The concept of increasing disorder (entropy change, âˆ†S).

###### âˆ†S accounts for the above deficiency, illustrated by physical changes and chemical changes.

###### The balance between entropy and enthalpy determines the feasibility of a reaction given by the relationship:

###### âˆ†G = âˆ†H â€“ Tâˆ†S (derivation not required).

###### For a reaction to be feasible, the value of âˆ†G must be zero or negative.

Notes

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