Thermodynamics and Thermochemistry 1 Question 3
3. A process will be spontaneous at all temperature if
(2019 Main, 10 April I)
(a) $\Delta H>0$ and $\Delta S<0$
(b) $\Delta H<0$ and $\Delta S>0$
(c) $\Delta H<0$ and $\Delta S<0$
(d) $\Delta H>0$ and $\Delta S>0$
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Solution:
- A process will be spontaneous when its free energy (Gibb’s energy) change will be negative, i.e. $\Delta G<0$.
Spontaneity of a process is decided by the value of $\Delta G$, which can be predicted from the Gibb’s equation, $\Delta G=\Delta H-T \Delta S$ for positive/negative signs of $\Delta H$ and $\Delta S$ at any/higher/lower temperature as:
| $\Delta \boldsymbol{H}$ | $\Delta \boldsymbol{S}$ | Comment on temperature $(T)$ |
$\Delta \boldsymbol{G}$ | Comment on the process |
|---|---|---|---|---|
| $<0$ | $>0$ | at any temp. | $<0$ | spontaneous |
| $>0$ | $<0$ | at any temp. | $>0$ | non-spontaneous |
| $<0$ | $<0$ | at lower temp. | $<0$ | spontaneous |
| $>0$ | $>0$ | at higher temp. | $<0$ | spontaneous |
