Homework Problem #75

[Glenda Marshall DIS 3M]

I am very confused by question 7.75. I understand how to use the bond enthalpies to calculate delta H, but why are they subtracting the enthalpy of vaporization?

[Juliana Smith 2G]

For part a) of 75 (and this applies to the rest): When you write the reaction for H2O it's for when H2O is in a gas phase (because H2 and O2 are gases in their most stable form) so it'd be 2H2 (g) + O2 (g) --> 2H2O (g). The equation in the solutions manual is a bit misleading in the way it's written with H2O as a liquid as the product of two gases instead of H2O as a gas as the product of two gases. So when you used bond enthalpies to find the enthalpy of formation of H2O it's really for the H2O in gas phase. And the enthalpy of vaporization is like the heat gap between the liquid and vapor state of water. So, you'd subtract the enthalpy of vaporization from the enthalpy of formation of water in gas form you calculated to get the enthalpy of formation for the liquid phase of water. Hope this makes some sense :)

[Glenda Marshall DIS 3M]

Yeah that helps a lot! Thank you!

[Justin Le 2I]

Just a note:
I think 2H2 (g) + O2 (g) --> 2H2O (g) is better written as H2 (g) + 1/2 O2 (g) --> H2O (g) because enthalpies of vaporization and fusion are given for just one mol of the product.