Heat of solution and vapor pressure

[silversurf]

I am having trouble with the relationship between these two. My book says that negative heat of solution forms stronger bonds and lowers vapor pressure and positive heat of solution forms weaker bonds and raises vapor pressure. Shouldn't vapor pressure be higher for negative heat of solution because you need more energy to break up the stronger bonds? And lower vapor pressure of for positive heat of solution because you are breaking up weaker bonds?

 

[T imothy]

Dear Silversurf,

Let me try to follow you. A positive heat of solution would be when one adds salt to water or acid to water and observes heat being generated. The explanation is that water has more affinity for the added ions than they do for each other and heat, I guess measured as in Gibbs free energy, is due to the strong ionic bonds being formed as polar water molecules orient themselves around both positive and negative ions. This explanation would suggest and predict a lower vapor pressure for water as now more water molecules would be attached in shells around the ions and be less able to escape the surface. I guess I agree with you...

 

[morrobay]

I am having trouble with the relationship between these two. My book says that negative heat of solution forms stronger bonds and lowers vapor pressure and positive heat of solution forms weaker bonds and raises vapor pressure.

This is correct and you both are having problems with definitions: A negative heat of solution =
a negative enthalpy of solution = an exothermic reaction = stronger bonds = lower vapor pressure.