Phase Changes

[deleted388502]

If melting a solid requires exactly 20 kj per mole, then which of the following statements is true?

A. Sublimation requires less than 20 kj per mole.
B. Vaporization requires less than 20 kj per mole.
C. Freezing releases more than 20 kj per mole.
D. Condensing releases more than 20 kj per mole.

Answer is D. I don't understand the TBR book solution; they talk all about how it takes more energy to break IMFs than to weaken them, so I thought that C would be the correct answer and I'm real confused.

It's page 76 in TBR GenChem if anyone has their book and wants to see the soln.

Thanks!

---

Members don't see this ad.

 

[dudewheresmymd]

Solid --> Liquid = 20 kj/mole therefore,

A) Sublimation: Solid --> Gas must be more than 20kj/mole because you have to add enough heat to vaporize the liquid to the gas
B) Vaporization: Liquid --> Gas must be more 20 kj/mole b/c Heat of Vaporization is > Heat of Fusion
C) Freezing: is the reverse process given in the question stem, therefore exactly 20 kj needed
D) Condensation gas --> liquid therefore releases more than 20 kj/mole (reverse of B so the same amount as in B), therefore is the right answer


You must know that Heat of Vaporization > Heat of Fusion and the definitions of the phase changes to get this question right.