Question

Free energy, $G=H-T\Delta S$, is a state function that indicates whether a reaction is spontaneous or non-spontaneous. If you think of $TS$ as the part of the system`s energy that is disordered already, then $(H-TS)$ is the part of the system`s energy that is still ordered and therefore free to cause spontaneous change by becoming disordered.
Also, $\Delta G=\Delta H-T\Delta S$
From the second law of thermodynamics, a reaction is spontaneous if ${\Delta }_{total}S$ is positive, non-spontaneous if ${\Delta }_{total}S$ is negative, and at equilibrium if ${\Delta }_{total}S$ is zero.
Since $-T\Delta S=\Delta G$ and since $\Delta G$ and $\Delta S$ have opposite signs, we can restate the Thermodynamic criterion for the spontaneity of a reaction carried out at a constant temperature and pressure.
If $\Delta G<0$, the reaction is spontaneous
If $\Delta G>0$, the reaction is non-spontaneous
If $\Delta G=0$, the reaction is at equilibrium
Read the above paragraph carefully and answer the following questions based on the above comprehension.

Which of the following is correct?

• A. $\Delta H=-ve;\Delta s=+ve$
• B. $\Delta G=-ve;\Delta S=-ve$
• C. $\Delta H=+ve;\Delta S=-ve$
• D. None of these

Answer 1

Answer: (A)

$\Delta H=-ve;\Delta s=+ve$

$\begin{array}{ccc}△H& △S& △G\\ (-)& (+)& (-)\\ (+)& (-)& (+)\end{array}$

$△G=△H-T△S$

So when $△H=(-)ve$ and $△S=(+)ve$, reaction will be always spontaneous.