Question

Which of the following conditions are not suitable for a spontaneous reaction?

• A. $\Delta H<0$ and $\Delta S>0$ at low temperature
• B. $\Delta H>0$ and $\Delta S<0$
• C. $\Delta H<0$ and $\Delta S = 0$ at low temperature
• D. $\Delta H>0$ and $\Delta S>0$ at high temperature

Hint:

Use the Gibbs free energy formula $\Delta G = \Delta H - T \Delta S$ to determine spontaneity.
$\Delta H>0$ and $\Delta S<0$ always gives a non-spontaneous reaction.

Answer 1

The Correct Option is B

The spontaneity of a process is governed by the change in Gibbs free energy: \[ \Delta G = \Delta H - T\Delta S \] For a reaction to be spontaneous, $\Delta G$ must be negative. - If $\Delta H<0$ and $\Delta S>0$, $\Delta G$ is always negative — suitable. - If $\Delta H>0$ and $\Delta S<0$, both terms make $\Delta G$ positive — not suitable. - If $\Delta H<0$ and $\Delta S = 0$, $\Delta G = \Delta H$, which is negative — suitable. - If $\Delta H>0$ and $\Delta S>0$, spontaneity depends on temperature being high — still suitable at high $T$. Hence, only (B) is not suitable.