Thermodynamics And Thermochemistry

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 $\Delta S > 0$

(d) $\Delta H > 0$ and $\Delta S > 0$

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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