Second Law And Entropy
Mediumchemistry
Melting of ice at 273 K and 1 atm is a spontaneous process. Which of the following correctly describes the thermodynamic parameters at the melting point?
Select the correct option:
Solution
Incorrect! Answer:
ΔH > 0, ΔS > 0, ΔG = 0
- Nature of Melting: Ice → Water is endothermic (ΔH > 0) because energy is needed to overcome hydrogen bonds in the crystal lattice.
- Entropy Change: The liquid state is more disordered than the solid, so ΔS > 0.
- At the Melting Point: The solid and liquid phases coexist in equilibrium at 273 K and 1 atm. At equilibrium, ΔG=0 (neither phase is thermodynamically favoured over the other).
- Consistency Check with Gibbs Equation: ΔG=ΔH−TΔS=0⇒Tmp=ΔSΔH. Both ΔH and ΔS are positive, confirming T_mp is a positive, finite value.
- Why Not the Others?
- ΔH < 0 would describe an exothermic transition (e.g., freezing, not melting).
- ΔS < 0 would imply ordering, which contradicts the solid-to-liquid transition.
- ΔG > 0 would mean the process is non-spontaneous, contradicting equilibrium at the exact melting point.
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About This Question
- Subject
- chemistry
- Chapter
- chemical thermodynamics
- Topic
- second law and entropy
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
ΔH > 0, ΔS > 0, ΔG = 0
- Nature of Melting: Ice → Water is endothermic (ΔH > 0) because energy is needed to overcome hydrogen bonds in the crystal lattice.
- Entropy Change: The liquid state is more disordered than the solid, so ΔS > 0.
- At the Melting Point: The solid and liquid phases coexist in equilibrium at 273 K and 1 atm. At equilibrium, ΔG=0 (neither phase is thermodynamically favoured over the other).
- Consistency Check with Gibbs Equation: ΔG=ΔH−TΔS=0⇒Tmp=ΔSΔH. Both ΔH and ΔS are positive, confirming T_mp is a positive, finite value.
- Why Not the Others?
- ΔH < 0 would describe an exothermic transition (e.g., freezing, not melting).
- ΔS < 0 would imply ordering, which contradicts the solid-to-liquid transition.
- ΔG > 0 would mean the process is non-spontaneous, contradicting equilibrium at the exact melting point.
This medium difficulty chemistry question is from the chapter chemical thermodynamics, covering the topic of second law and entropy. It appeared in the 2025 exam.
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