Question

In which case change in entropy is negative?
A) Evaporation of water
B) Expansion of a gas at constant temperature
C) Sublimation of solid to gas
D) $$2H_{(g)}\to H_{2(g)}$$

Answer 1

Hint: We need to know that the adiabatic process occurs without the transfer of mass or heat between a thermodynamics system and its surroundings. It is found that this process transfers energy to the surroundings only as work. Adiabatic processes can be reversible or irreversible. The entropy change depends on the heat exchange of the adiabatic system.

Complete step by step answer:
As we know that the entropy change is represented by $$\mathrm{\Delta }S$$
Let us consider the question,
We can write chemical equation for evaporation of water as,
$$H_2{O}_{(l)}\rightleftharpoons H_2{O}_{(V)},\mathrm{\Delta }S⩾0$$
We have to remember that the value of entropy change increases when water is evopartes.
So, in this reaction, the value of entropy change is greater than 0.
Therefore, the option A is incorrect.
For the expansion of a gas at constant temperature, the entropy change is also greater than zero $$\mathrm{\Delta }S⩾0$$.
Therefore, the option B is incorrect.
In the process of sublimation of solid to gas, the value of entropy change is greater than zero$$\mathrm{\Delta }S⩾0$$.
Therefore, the option C is incorrect.
We need to know that when reactants form products then the change in number of moles of gaseous species is denoted by $\mathrm{\Delta }ng$ . Now we can write the given option as,
$$2H_{(g)}\to H_{2(g)},\mathrm{\Delta }S⩽0(\mathrm{\Delta }ng⩽0)$$
For this above given reaction the value of $\mathrm{\Delta }ng$ is less than zero.
In this reaction, the value of entropy change has less than zero.

So, the correct answer is Option D.

Additional information:
There are several conditions for the natural process to require place like:
It is found that the process must be carried out fast so that there is sufficient time available for heat transfer.
System must be perfectly insulated from the encompassing.
It is also found that in the adiabatic process the work done is due to the change in its internal energy.There is no heat transfer take place and the temperature also cannot be valid.

Note: We must remember that the reversible adiabatic process is also called an isentropic process. In this work transfers of the system are found to be frictionless, there is no transfer of matter or of heat and the process is reversible.