1. Enthalpy and Phase Changes
When a substance goes through a phase change all the energy is used to change phase…..there is no temperature change (increase or decrease).
Therefore we can not use the specific heat formula which needs a temperature change

2. Heats of Fusion and Vaporization
Heat of Fusion (Hf) is the energy required to melt a solid and change it to a liquid (or the reverse change a liquid to a solid….just a different sign….energy flow)
Heat of Vaporization (Hv) is the energy required to change a liquid into a gas (or the reverse, change a gas into a liquid….just a different sign for q)

5. Graphical Representation of Enthalpy