Question

How does the energy required to subline a substance compare to the energy required to melt the substance? How does the energy required to subline a substance compare to the energy required to melt the substance? @Chemistry

Answer 1

It's always greater. That's because of conservation of energy. Sublimation is changing a solid to a gas. You can always imagine doing it in two steps: first melt the solid, then boil the liquid. The change in energy by taking the first path (solid to gas) MUST equal the change in energy by taking the second (solid to liquid to gas), because otherwise you could make a little perpetual motion machine and create energy forever from nothing.

For example, if it took less energy to go solid directly to gas, you could heat a substance to sublime it, using energy E, then allow it to cool through the liquid back to the solid, releasing MORE than energy E, and keep going around and around. One each cycle, you would get out some free energy. Not possible.

So if the energy required to go solid -> gas (sublimation) is equal to the energy required to go solid -> liquid (melting) PLUS liquid -> gas (boiling), then the energy required to sublime must always be greater than the energy to melt.

Now, I've answered very colloquially, because that's how you put the question. To make these statements quite precise we should be careful about conditions under which this all holds true. What we're really talking about here is the latent heat of sublimation and fusion at a given, constant temperature and pressure. Which I think is what you meant.