Question

Which of the options given below is / are correct?

• A. $\Delta H=negative$, $\Delta S=positive$ ; reaction is spontaneous at all temperatures
• B. $\Delta H=positive$, $\Delta S=negative$ ; reaction is non-spontaneous regardless of the temperature
• C. $\Delta H=positive$, $\Delta S=positive$ ; reaction is spontaneous only at high temperatures
• D. $\Delta H=negative$, $\Delta S=negative$ ; reaction is spontaneous only at low temperatures

Answer 1

Answer: (A), (B), (C), (D)

The correct options are
A $\Delta H=negative$, $\Delta S=positive$ ; reaction is spontaneous at all temperatures
B $\Delta H=positive$, $\Delta S=negative$ ; reaction is non-spontaneous regardless of the temperature
C $\Delta H=positive$, $\Delta S=positive$ ; reaction is spontaneous only at high temperatures
D $\Delta H=negative$, $\Delta S=negative$ ; reaction is spontaneous only at low temperatures

Sign of $\Delta G$ determines whether a reaction is spontaneous or not. According to equation
$\Delta G=\Delta H-T\Delta S$
when $\Delta G$ = negative, reaction is spontaneous
$\Delta G$ = positive, reaction is non spontaneous
$\Delta G$ = 0, reaction is at equilibrium
Here in the options i, iii and iv, $\Delta G$ is found to be negative. So, these are spontaneous.
In the options ii, $\Delta G$ would be positive. So, it will be non-spontaneous.
Hence all the options are correct.