### 9.15

Posted:

**Sat Feb 10, 2018 8:06 pm**For 9.15 part a, the answer book gives the heat of fusion as -6.01 kJ/mol. How do we know that the heat of fusion should be negative?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=131&t=27331

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Posted: **Sat Feb 10, 2018 8:06 pm**

For 9.15 part a, the answer book gives the heat of fusion as -6.01 kJ/mol. How do we know that the heat of fusion should be negative?

Posted: **Sat Feb 10, 2018 8:36 pm**

The book gives the heat of fusion of water as 6.01 kj.mol^-1, meaning thats how much energy is required to go from solid to liquid water. However, the question is asking you to calculate the entropy for the freezing of water; therefore, you would use negative heat of fusion, because that's how much energy the system will release.

Posted: **Sat Feb 10, 2018 9:26 pm**

Heat is always required to melt a substance, so ∆H is positive.

Heat is released when a substance freezes, so ∆H is negative.

∆H_{freezing} = ∆H_{fusion}, and since you are given ∆H_{fusion} from Table 8.3, you just reverse the sign of ∆H_{fusion} to get ∆H_{freezing} = -6.01 kJ/mol

Heat is released when a substance freezes, so ∆H is negative.

∆H

Posted: **Sun Feb 11, 2018 2:36 pm**

How do you get -6.01 kJ/mol or 4.35 kJ/mol from appendix 2A?