For micelle formation, which of the following statements are correct?
A. Micelle formation is an exothermic process.
B. Micelle formation is an endothermic process.
C. The entropy change is positive.
D. The entropy change is negative.

Solution

Chemistry ma’am · 4 steps

Do not watch the soap ions here, watch the water: freeing it is what drives micelle formation.

1. Heat change. Micelles form only above the Kraft temperature and above the critical micelle concentration (CMC). Heat is needed to pull ions out of the ordered solution and break the water structure around them, so micelle formation is endothermic: Δ H > 0.
2. Entropy change. Each separate hydrocarbon tail holds a cage of ordered water molecules around it. When the tails pack together inside the micelle, this water is released and becomes free to move. That gain in disorder of the water is much larger than the order gained by the soap ions, so Δ S > 0.

   Δ G = Δ H - TΔ S < 0 since TΔ S > Δ H

3. Spontaneous anyway. The process absorbs heat and still happens on its own, because the TΔ S term is larger than Δ H. This is also why micelles form only above a certain temperature.
4. So. Statements B and C are correct: micelle formation is endothermic and has a positive entropy change. That is option (c).

The idea to keep. For micelles, the entropy gain comes from the water set free around the hydrophobic tails, not from the soap ions. That large Δ S is what lets an endothermic process happen on its own.

Answer from the NTA final answer key. Solution by PrepWiser.