Looking for classes? Ksquare Career Institute, Bengaluru →
Which of the following pairs of a chemical reaction is certain to result in a spontaneous reaction -
A
Endothermic and decreasing disorder
B
Exothermic and increasing disorder
C
Endothermic and increasing disorder
D
Exothermic and decreasing disorder
Detailed Solution
A reaction is spontaneous when $\Delta G = \Delta H - T\Delta S \lt 0$.
An exothermic reaction has $\Delta H \lt 0$, and increasing disorder means $\Delta S \gt 0$, so $-T\Delta S \lt 0$.
Both terms are then negative, so $\Delta G$ is negative at every temperature; the reaction is certain to be spontaneous.
Endothermic with decreasing disorder ($\Delta H \gt 0$, $\Delta S \lt 0$) gives $\Delta G \gt 0$ at all temperatures, so it is never spontaneous.
Endothermic with increasing disorder is spontaneous only at high temperatures, and exothermic with decreasing disorder only at low temperatures.
So the pair certain to result in a spontaneous reaction is exothermic and increasing disorder.
An exothermic reaction has $\Delta H \lt 0$, and increasing disorder means $\Delta S \gt 0$, so $-T\Delta S \lt 0$.
Both terms are then negative, so $\Delta G$ is negative at every temperature; the reaction is certain to be spontaneous.
Endothermic with decreasing disorder ($\Delta H \gt 0$, $\Delta S \lt 0$) gives $\Delta G \gt 0$ at all temperatures, so it is never spontaneous.
Endothermic with increasing disorder is spontaneous only at high temperatures, and exothermic with decreasing disorder only at low temperatures.
So the pair certain to result in a spontaneous reaction is exothermic and increasing disorder.