Question

For process ${{\text{H}}_{\text{2}}}{\text{O(l)(1000}}{\,^{\text{o}}}{\text{C, 1}}\,{\text{atm)}}\, \rightleftharpoons {{\text{H}}_{\text{2}}}{\text{O(g)(1000}}{\,^{\text{o}}}{\text{C, 1}}\,{\text{atm)}}$

List 1	List 2
(P). $\Delta {\text{U}}$	$(1)$ 0
(Q). ${\Delta S}$(system + surrounding)	(2)$\Delta {\text{H}}$
(R). Work	$(3)$Positive
(S). Heat involved	$(4)$Negative

The correct option is:
A. P-$3$; Q-$1$; R-$4$; S-$2$
B. P-$1$; Q-$2$; R-$3$; S-$4$
C. P-$2$; Q-$1$; R-$3$; S-$2$
D. P-$4$; Q-$3$; R-$2$; S-$1$

Answer 1

Hint: In the given reaction only the phase is changing at constant temperature. According to the phase transition and equilibrium we will decide the entropy change. As the volume is increasing and the temperature is constant so, according to the isothermal process we will decide the work. Then by using the first law of thermodynamics we will decide the internal energy sign.

Complete solution:
The given reaction is as follows:
: For process ${{\text{H}}_{\text{2}}}{\text{O(l)(1000}}{\,^{\text{o}}}{\text{C, 1}}\,{\text{atm)}}\, \rightleftharpoons {{\text{H}}_{\text{2}}}{\text{O(g)(1000}}{\,^{\text{o}}}{\text{C, 1}}\,{\text{atm)}}$
The given reaction is a phase transition reaction. The phase transition occurring at constant pressure.
Entropy is the randomness of the system.
The reaction is in equilibrium, so the entropy of the reactant and product will be the same and the entropy change of the system and surrounding is zero.
The heat is represented by H so, the heat change of the phase transition will be $\delta {\text{H}}$.
During phase transition, liquid is converting into gas. So, the volume of the system is increasing. So, expansion is taking place. The temperature is constant. During isothermal expansion, the system does work, so the work is negative.
According to the first law of thermodynamics, $\delta {\text{U}}\,{\text{ = q}}\, - \,{\text{w}}$ .
The work is negative so, $\delta {\text{U}}\,{\text{ = q}}\, - \,\left( { - {\text{w}}} \right)$
$\delta {\text{U}}\,{\text{ = }}\,{\text{ + ve}}$
The correct option is P-$3$; Q-$1$; R-$4$; S-$2$.
Therefore, option (A) P-$3$; Q-$1$; R-$4$; S-$2$, is correct.

Note: The reaction in which no chemical reaction takes place only the phase of reactant undergoes change is known as phase transition. Phase transition requires heat. Phase transition takes place in multiple steps. In one step temperature changes and in another step phase changes. When the system does work, the sign of work is negative. When work is done on the system, the sign of work is positive.