Question

What is the formula for `DeltaH_"fus"`?

Answer 1

Enthalpy of fusion (or really, enthalpy of phase transitions) is a constant-pressure heat flow. So you can work it out just from knowing that and the units.

The molar enthalpy of (common) phase transitions (that is, in `"kJ/mol"`) would be:

`\mathbf(DeltabarH_"trs" = q_p/(n_"substance"))`

where:

• `q` is the heat flow through the system at constant pressure and the same temperature.
• `n_"substance"` is the `\mathbf("mol")`s of the substance at hand that is transitioning to a new phase.

If you were looking at water freezing/melting, then `DeltabarH_"fus" ~~ "6.02 kJ/mol"`. That means if you had `"1 mol"` of water, then you would have

`color(blue)(q_p) = n_"substance" xx DeltabarH_"fus"`

`= ("1 mol")("6.02 kJ/mol")`

`=` `color(blue)("6.02 kJ")` of heat absorbed to melt it, or released when freezing it.