![]() ![]() Since we are talking about relative enthalpies of fusion and vaporization for a given system, we don't have to worry about how this changes for different molecules. What will change is the "steepness," width, and depth of the valley (or "potential energy well"), and the slope of the infinitely long "hill" to the right of the well. Now, the potential energy function between any two types of molecules will be different, but it will always have the same basic shape. The result is that unless there is enough kinetic energy for the molecules to move apart, they tend to stick together. If you try to pull them away, again you have to climb a hill (although it isn't as tall or steep). If it were any closer than just touching, it would have to climb up another very steep hill. ![]() In other words, the white molecule "wants" to roll down the valley until it sits next to the gray molecule. Since this is a potential energy curve, you can imagine the system as if it were the surface of the earth, and gravity was the potential. Here the y-axis represents electrostatic potential energy, the x-axis is radial separation (distance between the centers), and the spheres are "molecules." distance between two molecules, it will look something like this: If you make a graph of potential energy vs. There is an attractive force between all molecules at long(ish) distances, and a repelling force at short distances. Enthalpies of phase changes are fundamentally connected to the electrostatic potential energies between molecules. ![]()
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