Chemistry / Unit 11 Chem 2086 worked example, 60 to 90 seconds
Predicting the Sign of ΔS
The idea
Predicting the sign of ΔS for a process means checking, in order, whether the number of gas-phase particles changes, whether a phase change occurs, whether mixing or dissolution occurs, and whether temperature changes, since each factor that increases the number of accessible microstates predicts ΔS positive and each that decreases it predicts ΔS negative.
In practice
For 2 NO(g) + O2(g) → 2 NO2(g), gas moles fall from three to two, so fewer microstates are accessible afterward and ΔS is predicted negative, consistent with a net loss of gas-phase particles. For NaCl(s) → Na+(aq) + Cl−(aq), dissolution increases the number of mobile, independently arranged particles, so ΔS is predicted positive.
Where people slip
Checking only the phase-change factor while a reaction also changes the number of gas moles risks predicting the wrong sign, since the gas-mole factor usually dominates when both are present.
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