Without consulting a table of standard molar entropies,...

Without consulting a table of standard molar entropies, predict whether $\Delta S_{\text {system }}^{\circ}$ will be positive or negative for each of these reactions. (a) $2 \mathrm{NH}_{3}(\mathrm{~g}) \longrightarrow \mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g})$ (b) $2 \mathrm{Na}(\mathrm{s})+\mathrm{Cl}_{2}(\mathrm{~g}) \longrightarrow 2 \mathrm{NaCl}(\mathrm{s})$ (c) $\mathrm{H}_{2}(\mathrm{~g})+\mathrm{I}_{2}(\mathrm{~s}) \longrightarrow 2 \mathrm{HI}(\mathrm{g})$

Without consulting a table of standard molar entropies, predict whether $\Delta S_{\text {system }}^{\circ}$ will be positive or negative for each of these reactions.
(a) $2 \mathrm{NH}_{3}(\mathrm{~g}) \longrightarrow \mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g})$
(b) $2 \mathrm{Na}(\mathrm{s})+\mathrm{Cl}_{2}(\mathrm{~g}) \longrightarrow 2 \mathrm{NaCl}(\mathrm{s})$
(c) $\mathrm{H}_{2}(\mathrm{~g})+\mathrm{I}_{2}(\mathrm{~s}) \longrightarrow 2 \mathrm{HI}(\mathrm{g})$

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Key Concepts

Entropy

Entropy is a measure of the disorder or randomness in a system. In chemical reactions, predicting whether the entropy change (?S) of the system will be positive or negative involves evaluating how the reaction affects this disorder, typically by analyzing changes in the number of gas molecules, the types of phases involved, and the arrangement of particles.

Physical States of Matter

The physical state of the reactants and products has a significant impact on entropy. Gases usually possess much higher entropy than liquids and solids because their molecules move more freely and occupy a larger volume. Thus, reactions that produce gases from solids or liquids typically result in an increase in entropy, while reactions that produce solids from gases generally lead to a decrease in entropy.

Mole Count and Molecular Complexity

The number of discrete gaseous molecules in the reaction is crucial when predicting entropy changes. An increase in the number of gas molecules usually means that there are more ways to distribute the energy among the particles, leading to a higher entropy. Conversely, reactions that result in a decrease in the number of gas molecules tend to have a negative entropy change, as the system becomes more ordered.

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