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Posted by on Wednesday, November 14, 2012 at 9:24pm.

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?

  • chemistry - , Wednesday, November 14, 2012 at 10:01pm

    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.

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