alleryAt constant temperature and pressure, the criterion for a process to be **spontaneous** is:
A$\Delta G < 0$
B$\Delta G > 0$
C$\Delta S_\text{system} > 0$ only
D$\Delta H < 0$ only
Answer & Solution
Correct answer: A. $\Delta G < 0$
At constant $T$ and $p$, $\Delta G = \Delta H - T\Delta S$.
The second law combined with the first gives that a spontaneous process must have $\Delta G < 0$.
$\Delta G = 0$ corresponds to equilibrium and $\Delta G > 0$ is non-spontaneous (the reverse reaction is spontaneous instead).
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