Question

How to calculate thermochemistry equations for phase changes?

Answer 1

The mass of the substance, along with the values for latent heat of fusion and vaporisation need to be given in order to determine the final heat during change of phase.

Complete answer:
In order to answer our question, we need to know about latent heat. Now, what is latent heat? For example, suppose water is present in a liquid state and we heat the water. A point will come when the water will reach its boiling point, but liquid water does not instantly change to the vapour form. Here, latent heat comes to play. After reaching the boiling point, the liquid water gains the latent heat of evaporation to overcome the forces of liquid molecules and transform into gas. Same is the case when ice is melted to form liquid water. At the melting point of ice, some extra heat is required for phase change. This heat is called the latent heat of fusion.
Latent heat of fusion is the heat required to melt 1 g of solid into liquid form, once it reaches melting point, whereas latent heat of vaporization represents the heat required to convert 1 g of liquid into it vapour state, and the values for water are $\Delta {{H}_{fus}}=334J\,{{g}^{-1}},\Delta {{H}_{vap}}=2257J\,{{g}^{-1}}$. These are calculated by using the formula $q=m\Delta H$, where q is heat and m is mass and in our case, we have taken mass as 1g.
So, if we know the mass of any substance, along with the value of heat of fusion, or the heat of evaporation, then by substituting the values in the respective equation, we can successfully determine the heat required for the phase changes.

Note:
Latent heat cannot be seen but can be observed very easily. Liquid water at its boiling point will not cause that much skin burn as steam water at 100 degree Celsius. That is why steam engines were more powerful.