Calculating enthalpy change with phase changes

[Adam Kramer 1A]

Can someone remind me what Lavelle was saying about having to add enthalpy if the phase changes? How do we calculate the change of enthalpy this way?

[Kaylee Sepulveda 4G]

ΔH_sub=ΔH_fus+ΔH^vap

If we know the enthalpies of certain phase changes, we can calculate an unknown value using what is given to us.

Some helpful diagrams:

[Ritu Gaikwad 2E]

When Dr. Lavelle was talking about phase changes, he was saying that in the standard state all substances should be a gaseous phase so if some of your reactants are in a liquid or solid phase you have to factor in an extra heat of vaporization or an extra enthalpy value in order to account for the phase change because that would change the value of your reaction.

[Althea Zhao 1B]

You need to calculate a reaction's enthalpy when all reactants and products are in the gas phase. If there are solids and/or liquids present, you need to add in the enthalpy it takes to convert them to gases.