Why is the latent heat of vaporization of water greater than the latent heat of fusion of water?

Asked by: Fidel

Answer

The latent heat of fusion and vaporization both involve the heat required to change the state of a substance without a change in temperature. In the case of the latent heat of fusion it is the heat required to change a substance from a solid (ice) to a liquid (water) or vice versa while the latent heat of vaporization from a liquid (water) to a gas (steam) or vice versa.

In solids, the molecules are very close together and the attraction between the molecules are great. This causes a substance to have a structure in which the molecules have little freedom to move, as you would see in the case of ice. In the case of a liquid, the molecules are closely spaced, though not as closely spaced as a solid, they have more freedom to move and the intermolecular forces are weaker that that of a solid. Thus a liquid can flow, unlike a solid. Now in a gas, the molecules are sufficiently far apart that there are little to no attractive forces. Because of this a gas can easily be compressed and take the shape of the container.

Now as you heat a solid turning it into a liquid, you increase the kinetic energy of its molecules, moving them further apart until the forces of attraction are reduced to allow it to flow freely. Keep in mind the forces of attraction still exists. Now as you heat a liquid, turning it into a gas, the kinetic energy of the molecules are increased to a point where there are no forces of attraction between the molecules.

The energy required to completely separate the molecules, moving from liquid to gas, is much greater that if you were just to reduce their separation, solid to liquid. Hence the reason why the latent heat of vaporization is greater that the latent heat of fusion.
Answered by: David Latchman, B.Ss. Physics, University of the West Indies

The above answer is essentially correct, however the folowing should be further clarified:

The difference in enthalpies comes from the fact that a liquid molecule is stabilized by interactions with other nearby molecules (therefore a small heat of fusion) and a gas has very little intermolecular stabilization (hence a large heat of vaporization). The confusion comes from the idea that temperature is a measure of the average kinetic energy of a system, and since the temperature remains constant, the average kinetic energy must also remain constant.

The extra energy required to cause a phase transition is actually potential energy. It is the energy required to overcome the bonds of nearest neighbors to the point that a phase transition can occur. So it really isn't a change in kinetic energy.
Answered by: Joe Larsen, Ph.D. Chemistry, Rockwell Science Center, Los Angeles, CA