JEE Advanced · Chemistry · 6. Thermodynamics (C)
Match the transformation in Column I with appropriate options in Column II.
| Column I | Column II |
| A. \(CO _2(s) \rightarrow CO _2(g)\) | p. Phase transition |
| B. \(CaCO _3(s) \rightarrow CaO (s)\) \(+~ CO _2(g)\) | q. Allotropic change |
| C. \(2 H \rightarrow H _2(g)\) | r. \(\Delta\text{H}\) is positive |
| D. \(P _{\text {(white, solid) }} \rightarrow P _{\text {(red, solid) }}\) | s. \(\Delta\text{S}\) is positive |
| t. \(\Delta\text{S}\) is negative |
- A (A) p,q,r,s, (B) r,s, (C) t, (D) p,q
- B (A) p,s, (B) p,s, (C) s, (D) p,q,s
- C (A) p,s, (B) q,r, (C) s, (D) p,q
- D (A) p,r,s, (B) r,s, (C) t, (D) p,q,t
Answer & Solution
Correct Answer
(D) (A) p,r,s, (B) r,s, (C) t, (D) p,q,t
Step-by-step Solution
Detailed explanation
(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, \(\Delta S < 0\).
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, \(\Delta S < 0\).
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