Thermodynamics

For the given changes: (A) $\mathrm{CO}_2$ (s) $\xrightarrow{\Delta}$ $\mathrm{CO}_2$ (g) Phase transition, ΔH = +ve and ΔS = + ve (B) $\mathrm{CaCO}_3$ (s) $\xrightarrow{\Delta}$ $\mathrm{CaO}(s) + \mathrm{CO}_2 (g)$ No phase transition, ΔH= +ve and ΔS = +ve (C) 2H → $\mathrm{H}_2 (g)$ Two particles give one gaseous particle, so ΔS = −ve (D) $\mathrm{P}_{(\text{white , solid})}$ $\xrightarrow{\Delta}$ $\mathrm{P}_{(\text{red , solid})}$ This is an allotropic change. ΔH is positive, as white phosphorus converts to red phosphorus on heating. ΔS is negative, as red phosphorus is a polymeric solid, less reactive than white phosphorous.