Weird Chemistry Question on Enthalyp of Formation - still dont get it!!!

Can someone please explain this to me, i asked my teacher but..well..

Ok well the enthalpy of combustion of C3H7OH is -2011, we had to work that out in the previous question by using the formation enthalpies of things like water and carbon dixoide.

Then the next question is how would the enthalpy of combustion differ if the water used was in gaseous form not liquid like before.

Ok well i thought it would be more negative...but my teacher says its less negative.. :s-smilie:

anyone?

Reply 1

EierVonSatan

21

Well is H₂O_(l) ---> H₂O_(g) going to be exothermic or endothermic?

Reply 2

EierVonSatan

21

you need to heat water to get it to boil - so you're putting in energy, so it is endothermic :smile:

so if the product of the combustion is higher in energy - this means the gap between the reactants and products is smaller and so therefore less negative (less exothermic)

Reply 3

shengoc

12

Lou Reed

Can someone please explain this to me, i asked my teacher but..well..

Ok well the enthalpy of combustion of C3H7OH is -2011, we had to work that out in the previous question by using the formation enthalpies of things like water and carbon dixoide.

Then the next question is how would the enthalpy of combustion differ if the water used was in gaseous form not liquid like before.

Ok well i thought it would be more negative...but my teacher says its less negative.. :s-smilie:

anyone?

In addition to what EVS mentioned, after you grasped the idea of relative values of these phase changes, then try to sketch out an energy level diagram, it might help or might not, depending on individuals. "Picture speaks louder than words!"

Weird Chemistry Question on Enthalyp of Formation - still dont get it!!!

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