Chemistry · A-Level · AQA · AQA A-level Chemistry - (7405) · Specification point · 3.1.8.2 · Physical chemistry · Thermodynamics (A-level only)
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3.1.8.2 Gibbs free-energy change, ∆G, and entropy change, ∆S (A-level only)
| Content | Opportunities for skills development |
|---|---|
∆H, whilst important, is not sufficient to explain feasible change. The concept of increasing disorder (entropy change, ∆S). ∆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: |
(derivation not required).
For a reaction to be feasible, the value of ∆G must be zero or negative.
Students should be able to:
calculate entropy changes from absolute entropy values
use the relationship
∆G = ∆H – T∆S to determine how ∆G varies with temperature
AT a, b and k PS 3.2 Students could be asked to find ∆S for vaporization of water using a kettle. MS 2.2, 2.3 and 2.4 Students rearrange the equation ∆G = ∆H – T∆S to find unknown values. MS 3.3 Students determine ∆S and ∆H from a graph of ∆G versus T. |