Chemistry - Thermodynamics - II Question with Solution | TestHub

(A) ${\mathrm{CO}}_2(\mathrm{s})⟶{\mathrm{CO}}_2(\mathrm{g})$ It is just a phase transition (sublimation) as no chemical change has occurred. Sublimation is always endothermic. Product is gas, more disordered, hence $\Delta s$ is positive. (B) ${\mathrm{CaCO}}_3(\mathrm{s})⟶\mathrm{CaO}(\mathrm{s})+{\mathrm{CO}}_2(\mathrm{g})$ It is a chemical decomposition, not a phase change. Thermal decomposition occur at the expense of energy, hence endothermic. Product contain a gaseous species, hence, $\Delta S>0$. (C) $2\mathrm{H}⟶{\mathrm{H}}_2(\mathrm{g})$ A new $\mathrm{H}-\mathrm{H}$ covalent bond is being formed, hence, $\Delta H<0$. Also, product is less disordered than reactant, $\Delta S<0$. (D) Allotropes are considered as different phase, hence ${\mathrm{P}}_{\text{(white, solid) }}\to {\mathrm{P}}_{\text{(red, solid) }}$ is a phase transition as well as allotropic change. Also, red phosphorus is more ordered than white phosphorus, $\Delta S<0$.