Question

The process with negative entropy change is:
A) dissolution of iodine in water
B) synthesis of ammonia from $${\text{N}}_{\text{2}}$$ and $${\text{H}}_{\text{2}}$$
C) dissociation of $$\text{CaS}{\text{O}}_4\left(s\right)$$to$$\text{CaO}\left(s\right)$$ and $$\text{S}{\text{O}}_3\left(g\right)$$
D) sublimation of dry ice

Answer 1

Hint:Entropy is one of the thermodynamic properties which measures the randomness or disorder in the system.All spontaneous processes lead to an increase in their entropy that is any system gives positive entropy change then it is spontaneous while entropy change is negative then such system is non-spontaneous.

Complete answer:
The negative entropy change indicates the entropy of the system decreases that is randomness in the system decreases.
There, here we have to find out the system where randomness is decreased.
Now, in option(A) dissolution of the iodine is given. In this process, the solid iodine is converted into the solution that increases the randomness in the system. Hence, the entropy change for the reaction is positive.
Therefore, option(A) is incorrect.
Now, in option(B) synthesis of ammonia from $${\text{N}}_{\text{2}}$$ and $${\text{H}}_{\text{2}}$$.
The reaction of the ammonia synthesis is as follows:
$${\text{N}}_{\text{2}}\left(g\right)+3{\text{H}}_{\text{2}}\left(g\right)\to 2\text{N}{\text{H}}_{\text{3}}\left(g\right)$$
In this reaction the four moles of gaseous reactants are converted into two moles of the gaseous products indicates the randomness in the system decreases. Hence, the entropy change for the reaction is negative.
Now, in option(C) dissociation of $$\text{CaS}{\text{O}}_4\left(s\right)$$to$$\text{CaO}\left(s\right)$$and$$\text{S}{\text{O}}_3\left(g\right)$$.
The reaction of dissociation is as follows:
$$\text{CaS}{\text{O}}_4\left(s\right)\to \text{CaO}\left(s\right)+\text{S}{\text{O}}_3\left(g\right)$$
Here, in this reaction, the one mole of solid reactant is converted into one mole of solid and one mole of gaseous products indicates an increase in randomness of the system. Hence, the entropy change for the reaction is positive.
Therefore, option(C) is incorrect.
Now, in option(D) sublimation of ice is given. In this process, the solid ice is converted into water vapors that are the randomness in the system. Hence, the entropy change for the system is positive.
Therefore, option(D) is also incorrect.
Therefore, option(B) is correct.

Note:When any system is converted from the solid to a liquid state, or from the liquid to gaseous state the randomness of the system increases, hence such a system gives positive entropy change. If the reverse of this is taking place gives the negative entropy change for the system.
In the case of the reaction if the numbers of moles of the products are higher than the moles of reactants then such reaction also gives the positive entropy change.
In this way, the randomness of the system determines the entropy of the system.