Predict the algebraic sign of the entropy change for the...

Predict the algebraic sign of the entropy change for the following reactions. (a) $\mathrm{I}_{2}(s) \longrightarrow \mathrm{I}_{2}(g)$ (b) $\mathrm{Br}_{2}(g)+3 \mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{BrCl}_{3}(g)$ (c) $\mathrm{NH}_{3}(g)+\mathrm{HCl}(g) \longrightarrow \mathrm{NH}_{4} \mathrm{Cl}(s)$ (d) $\mathrm{CaO}(s)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{Ca}(\mathrm{OH})_{2}(s)$

Predict the algebraic sign of the entropy change for the following reactions.
(a) $\mathrm{I}_{2}(s) \longrightarrow \mathrm{I}_{2}(g)$
(b) $\mathrm{Br}_{2}(g)+3 \mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{BrCl}_{3}(g)$
(c) $\mathrm{NH}_{3}(g)+\mathrm{HCl}(g) \longrightarrow \mathrm{NH}_{4} \mathrm{Cl}(s)$
(d) $\mathrm{CaO}(s)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{Ca}(\mathrm{OH})_{2}(s)$

Key Concepts

Reaction Processes

Reaction processes, especially those involving the formation of products from reactants with different phase states or numbers of species, play a significant role in determining entropy change. Reactions that lead to more ordered states (for example, the formation of a solid from gases) tend to show a decrease in entropy, while those that increase the disorder (such as vaporization) display an increase in entropy.

Mole Count and Gaseous Species

The number of moles of gaseous species is a critical factor when assessing entropy change in chemical reactions. An increase in the number of gas molecules in a reaction mixture typically results in higher entropy, reflecting greater dispersal of energy and matter, whereas a decrease in gas moles usually causes a reduction in entropy.

Phase Transition

Phase transitions greatly influence entropy. Generally, gases have far higher entropy than liquids or solids because their molecules are more dispersed and move more randomly. Thus, when a substance is transformed from a solid or liquid to a gas, the entropy usually increases, while the reverse process leads to a decrease in entropy.

Entropy Change

Entropy change, symbolized as ?S, is a measure of the system's disorder or randomness. Positive entropy change means that the system becomes more disordered, while a negative change implies increased order. This concept is central to understanding the second law of thermodynamics and explains the natural tendency of systems to evolve towards greater disorder.

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