Question

consider the following changes at constant temperature & pressure

H2O(s) --yields-- H20(l); Delta H1
H2O(l) --yields-- H2O(g); Delta H2
H2O(g) --yields-- H2O(s); Delta H3

Using Hess's law, the sum of the deltas is
a. equal to 0
b. sometimes greater than zero, sometimes less than zero
c. less than 0
d. cannot be determined without the numerical values of delta H
e. greater than 0

I picked e because delta 1 looks to be an endothermic reaction, delta 2 also endothermic, and delta 3 is exothermic reaction. But the answer is equal to 0. Why?

Answer 1

dHsub = dHfusion + dHvaporization
So you add dHfusion to dHvap and you end up with dHsub which means that if you complete the cycle you end you with zero.