University11 min

Second law and entropy

Predict the sign of an entropy change, compute it (tables or reversible Q over T) and settle spontaneity with the system + surroundings balance.

The essentials

1

Clausius definition

dS=δQrevT\mathrm{d}S = \frac{\delta Q_{\text{rev}}}{T} with TT in kelvins; SS is a state function, in J/K\text{J/K}. Case to know: a phase change at its equilibrium temperature is reversible, so ΔS=ΔHphase changeT\Delta S = \dfrac{\Delta H_{\text{phase change}}}{T}.
2

Second law

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3

Boltzmann formula

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4

Third law and absolute entropies

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5

The bridge to Gibbs energy

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