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September 30, 2014

Homework Help: Physics

Posted by Ishani on Sunday, June 25, 2006 at 5:13am.

Using the first law of thermodynamics explain why the specific latent heat of vaporisation of water is greater than the specific latent heat of fusion of ice.


The first law of thermodynamics (conservation of energy) has nothing to do with why vaporization requires more energy than melting. I have no idea what you instructor had in mind when assigning this question, and would be interested in hearing his or her version of the answer.

You have to consider the intramolecular binding process itself. Vaporizing requires that the molecules separate from each other by "climbing out" of a potential-energy "well" created by van der Waals intramolecular dipole-dipole binding forces. This requires a certain amount of enery that is typically on the order of an electron volt per molecule.

When a substance melts, the intramolecular distance does not change much, and the vearious molecules still remain bound to each other, but the orientation of the molecules changes with the added thermal energy so that they are not held as tightly and are not locked into a particular crystalline orientation.

Basically, it takes more energy to break away from adjacent molecules than to wiggle to a different, less ordered, stacking pattern.


which one of the following temperatures is equal to 5*c?

A. 0K
B. 41K
c. 278K
d. 465 K

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