University11 min

Gibbs energy and spontaneity

Compute ΔG = ΔH − TΔS without falling into the units trap, read the four sign cases and find the temperature at which a reaction tips over.

The essentials

1

The definition, with its units

G=H−TSG = H - TS, hence at constant TT and PP: ΔG=ΔH−T ΔS\Delta G = \Delta H - T\,\Delta S TT in kelvins. Tables give ΔH\Delta H in kJ/mol\text{kJ/mol} and ΔS\Delta S in J/(mol⋅K)\text{J/(mol·K)}: bring both to the same energy unit before subtracting.
2

The spontaneity criterion

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3

The four sign cases

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4

The inversion temperature

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5

Standard or not: two quantities

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