Predicting the Sign of ΔS

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.

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.