Thermodynamics and Thermochemistry - Result 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 $\Delta S > 0$
Show Answer
Answer:
Correct Answer: 3. (b)
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 |
BETA
