Homework Problem 9.17

[Samira 2B]

Why do we have to account for the entropy of the cooling process when finding the standard entropy of vaporization of water?

[Johann Park 2B]

I believe you are asking about Problem 9.19? If so, we need to find the standard entropy of the vaporization of water at 85^oC because that is what the problem is asking of us. We have ∆S_TOT = ∆S₁ + ∆S₂ + ∆S₃.

∆S₁ = Heat Capacity of liquid water x ln T₂/T₁ or 75.3 x ln 373/358
∆S₂ = Standard entropy at 100^oC or 109.0
∆S₃ = Heat Capacity of water vapor x ln T₂/T₁ or 33.6 x ln 358/373

We must account for the rise in temperature to its actual boiling point, the entropy of vaporization at 100 degrees, and then the cooling which is necessary to answer the problem at 85 degrees.