Question
State whether $\Delta S$ is positive, negative or zero for the following reactions.
i. $2 H _{2(g)}+ O _{2(g)} \rightarrow 2 H _2 O _{( l )}$
ii. $CaCO _{3(s)} \rightarrow CaO _{( s )}+ CO _{2(g)}$

Answer

(i) $2 H _{2(g)}+ O _{2(g)} \rightarrow 2 H _2 O _{( l )}$
Since the system is converted from gaseous state to a liquid state, the disorder is decreased, hence $\Delta S < O$ (negative).
(ii) $CaCO _{3(s)} \rightarrow CaO _{( s )}+ CO _{2(g)}$
Since molecules of solid $CaCO _3$ break giving gaseous $CO _2$, disorder is increased hence $\Delta S > O$ (positive).

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