Question

State whether the sign of the entropy change expected for each of the following processes will be positive or negative, and explain your predictions. (a) \(\mathrm{PCl}_{3}(l)+\mathrm{Cl}_{2}(g) \longrightarrow \mathrm{PCl}_{5}(s)\) (b) \(2 \mathrm{HgO}(s) \longrightarrow 2 \mathrm{Hg}(l)+\mathrm{O}_{2}(g)\) (c) \(\mathrm{H}_{2}(g) \longrightarrow 2 \mathrm{H}(g)\) (d) \(\mathrm{U}(s)+3 \mathrm{~F}_{2}(g) \longrightarrow \mathrm{UF}_{6}(s)\)

Short answer

The sign of entropy change: (a) Negative, as there is a decrease in disorder. (b) Positive, as there is an increase in disorder. (c) Positive, as there is an increase in disorder. (d) Negative, as there is a decrease in disorder.

Step-by-step solution

Identify the Change in Entropy of Reaction (a)

For this reaction: \(\mathrm{PCl}_{3}(l)+\mathrm{Cl}_{2}(g) \longrightarrow \mathrm{PCl}_{5}(s)\), the system goes from liquid and gaseous states to a solid state, implying a decrease in disorder. Hence, entropy change here would be negative.

Identify the Change in Entropy of Reaction (b)

\(\) For this reaction: \(2 \mathrm{HgO}(s) \longrightarrow 2 \mathrm{Hg}(l)+\mathrm{O}_{2}(g)\), we move from a solid state to both liquid and gaseous states. This implies an increase in disorder. Therefore, entropy change here would be positive.

Identify the Change in Entropy of Reaction (c)

\(\) For this reaction: \(\mathrm{H}_{2}(g) \longrightarrow 2 \mathrm{H}(g)\), the molecule of hydrogen breaks down into two individual atoms. This increase in the number of particles leads to an increase in disorder. Hence, entropy change here would be positive.

Identify the Change in Entropy of Reaction (d)

\(\) For this reaction: \(\mathrm{U}(s)+3 \mathrm{~F}_{2}(g) \longrightarrow \mathrm{UF}_{6}(s)\), we move from solid and gaseous states to a solid state, implying a decrease in disorder. Therefore, the entropy change here would be negative.