(A) \(\mathrm{CO}_2(\mathrm{~s}) \longrightarrow \mathrm{CO}_2(\mathrm{~g})\)
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) \(\mathrm{CaCO}_3(\mathrm{~s}) \longrightarrow \mathrm{CaO}(\mathrm{s})+\mathrm{CO}_2(\mathrm{~g})\)
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) \(2 \mathrm{H} \longrightarrow \mathrm{H}_2(\mathrm{~g})\)
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 \(\mathrm{P}_{\text {(white, solid) }} \rightarrow \mathrm{P}_{\text {(red, solid) }}\) is a phase transition as well as allotropic change.
Also, red phosphorus is more ordered than white phosphorus, \(\Delta S < 0\).