In general the energy needed differs from one liquid to another depending on the magnitude of the intermolecular forces. We can thus expect liquids with strong intermolecular forces to have larger enthalpies of vaporization. The list of enthalpies of vaporization given in the table bears this out. Two other features of the table deserve mention. One is the fact that the enthalpy of vaporization of a substance is always higher than its enthalpy of fusion.
When a solid melts, the molecules are not separated from each other to nearly the same extent as when a liquid boils. Second, there is a close correlation between the enthalpy of vaporization and the boiling point measured on the thermodynamic scale of temperature. Periodic trends in boiling point closely follow periodic trends in heat of vaporization.
If we divide the one by the other, we find that the result is often in the range of 75 to 90 J K —1 mol —1. To a first approximation therefore the enthalpy of vaporization of a liquid is proportional to the thermodynamic temperature at which the liquid boils. An equivalent rule does not hold for fusion. The energy required to melt a solid and the temperature at which this occurs depend on the structure of the crystal as well as on the magnitude of the intermolecular forces. 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. First let's think about what happens when you add heat to a system of molecules positive enthalpy change.
Heat is a transfer of thermal energy between a hot substance and a cold one. It is defined by a change in temperature, which means that when you add heat to something, its temperature increases this might be common sense, but in thermodynamics it is important to be very specific.
The main thing we need to know about this is:. In other words, as the temperature increases, the average kinetic energy the speed of the molecules increases. Let's go back to the potential energy diagram between two molecules. You know that energy is conserved, and so ignoring losses due to friction there won't be any for molecules the potential energy that can be gained by a particle is equal to the kinetic energy it started with.
In other words, if the particle is at the bottom of the well and has no kinetic energy, it is not going anywhere:.
If it literally has no kinetic energy, we are at absolute zero, and this is an ideal crystal a solid. Real substances in the real world always have some thermal energy, so the molecules are always sort of "wiggling" around at the bottom of their potential energy wells, even in a solid material.
This means you need enough energy to let the molecules climb up the well at least a little bit, so that they can slide around each other. If we draw a "liquid" line approximating how much energy that would take, it might look something like this:.
The red line shows the average kinetic energy needed for the particles to pull apart just a little - enough that they can "slide" around each other - but not so much that there is any significant space between them.
The height of this line compared to the bottom of the well times Avogadro's number is the enthalpy of fusion. As the kinetic energy increases, eventually there is enough that the molecules can actually fly apart their radial separation can approach infinity. That line might look something like this:. I have drawn the line a little bit shy of the "zero" point - where the average molecule would get to infinite distance - because kinetic energies follow a statistical distribution , which means that some are higher than average, some are lower, and right around this point is where enough molecules would be able to vaporize that we would call it a phase transition.
Depending on the particular substance, the line might be higher or lower. In any case, the height of this line compared to the bottom of the well times Avogadro's number is the enthalpy of vaporization. As you can see, it's a lot higher up. The reason is that for melting, the molecules just need enough energy to "slide" around each other, while for vaporization, they need enough energy to completely escape the well.
This means that the enthalpy of vaporization is always going to be higher than the enthalpy of fusion. Ice is less dense than water, that's why ice floats on water. The lower density of ice means that the average distance between water molecules in ice is greater than the average distance between water molecules in the liquid state. Because of the greater distance between water molecules in the solid compared to the liquid, molecule - molecule interactions such as van der Waals and dipole interactions, as well as hydrogen bonding will be less in the solid than the liquid.
So while we need to put energy into ice to disrupt the lattice structure and break the attractive interactions, this energy is offset to some degree by the even stronger attractive forces that exist in the liquid, since the attractive forces are actually greater in the liquid than the solid. In the gas phase the molecules are far enough apart that attractive forces between molecules are minimal.
Therefor, when we go from liquid to gas we must put in a lot of energy to break all of the strong attractive forces that exist in the liquid without any offset because of the lack of significant attractive forces in the gas. The molar heat of vaporization is greater than that of molar heat of fusion due to the larger amount of energy required to break the strong attractive forces that exist between molecules of liquids than that of the attractive forces in molecules of gases.
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