Question

Match the following processes with entropy change:

$\begin{array}{cccc}& \text{Reaction}& & \text{Entropy change}\\ i)& \text{A liquid vaporises}& a)& \Delta S=0\\ ii)& \text{Reaction is non spontaneous at all temperatures and}\Delta H\text{is positive}& b)& \Delta S=positive\\ iii)& \text{Reversible expansion of an ideal gas}& c)& \Delta S=negative\end{array}$

Answer 1

i) A liquid vapourises to gaseous state, therefore randomness increase $\therefore \Delta S=positive$

ii) Reaction is nonspontaneous at all temperature and $\Delta H>0$ (positive value)
For nonspontaneous reactions, $\Delta G>0$ $\Delta G=\Delta H^o–T\Delta S$
For $\Delta G$ to be positive at all temperature,
If $\Delta H=positivevalue$
then $(-T\Delta S^o)$ should also be positive and
therefore, $\Delta SShouldbenegative$. as temperature $\left(T\right)$ is always positive.

iii) For Reversible expansion of an ideal gas, there is no change in entropy i.e.

$\therefore \Delta S=0$

as per $2^{nd}$ law of thermodynamics.

So, match the following will be as -

$\begin{array}{cccc}& \text{Reaction}& & \text{Entropy change}\\ i)& \text{A liquid vaporises}& b)& \Delta S=positive\\ ii)& \text{Reaction is non spontaneous at all temperatures and}\Delta H\text{is positive}& c)& \Delta S=negative\\ iii)& \text{Reversible expansion of an ideal gas}& a)& \Delta S=0\end{array}$