Homework 9.19

[JukaKim_1D]

Why did the solution manual heat the water to 100C then cool it back down to 85C?

[Trixie Le 3K]

The water is heated up to 100 C because that is the boiling point of water where water turns from liquid to gas. There is a change because the question asks for the standard enthalpy of vaporization of water at 85 C, which implies that it was a liquid that was heated up to 100C (accompanied by entropy change) along with the entropy of vaporization at 100C and then it implies that now the water is in gas form that needs to be cooled back down to 85C since the gas will condensate once it reaches 85 C (which is accompanied by the entropy change). Then, use the respective standard molar entropies of water at the different phases for the right accompanied steps of heating, heat of fusion, and cooling of water.