Question

2. With the aid of a chemical potential versus temperature phase diagram, explain why graphite sublimes rather than melts when it is heated from 300 K to 3825 K at standard pressure. Also draw a pT phase diagram that is compatible with this observation.

Answer 1

Solution:-

Sublimation is the process where a solid undergoes a phase transition to being a gas. It’s less familiar to human interaction with ‘normal’ environments, where the expectation is that solids, when heated, will first melt into a liquid, and then later boil into gas vapor.

In the case of graphite sublimating, what’s implied in there in all likelihood is ‘at, or near Standard pressure. See the phase diagram for graphite below. Graphite structure is a sheet of hexagons of covalently bonded carbon atoms. They’re pretty strong bonds. To dissociate those bonds, you need to get the temperature up to the point where kT is about the same as the bond strength. At 1 atmosphere, that’s in the neighborhood of 4000 degrees. When those bonds finally break, whether the un-bonded stuff turns to liquid or gas depends on how much the local gas environment will be tending to run into those carbon atoms and give them a chance to re-form bonds or other attractions that make them clump together and behave like a liquid. If the pressure is low, then the un-bonded atoms will have more of a tendency to fly off and fill the local high-temperature region. From the phase diagram, one can see that from about 10 to 1000 atmospheres, the graphite will ‘melt’ into liquid first as the temperature rises. Above 1000 atmospheres, the graphite will tend to turn to diamond first at lower temperatures, but then turn into liquid first at higher temps.

10 DIAMOND direct Gr->Di Gr->hex Di 10 LIQUID alytic Gr->Di direct 10 kbar) GRAPHITE 10 VAP OF 10 T (10 K)