Kaustikos

Archerize It
10+ Year Member
Jan 18, 2008
12,205
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Always Bespin
Yes, I made a new word.:D
I was curious if anyone could help with identifying whether or not certain reactions were endothermic/exothermic in terms of enthalpy for change of state.
I always thought that going from gas to solid was exoothermic and the reverse tended to be endothermic. But there was this curveball thrown at me asking about the enthalpy of FORMATION of these states. Anyone have any insight into that?
ie; enthalpy of formation of water vapor from it's constituents. (Not "what happens when you go from liquid to gas?")
I'm just trying to understand how these reactions shoudl be looked at
Here's the question:
The standard enthalpy of formation for liquid water is:
H2(g) + fiO2(g) [Don't ask what the fi means, assuming it's unimportant]---> H20 (l) H = -285.8 kj/mol

Which of the following could be the standard enthalpy of formation for water vapor?
A)-480
B)-285
C)-241
D)+224.6

Answer: C
The way I reasoned it was that you were forming water vapor from gas. So that meant combination of molecules (exothermic, yes) but how do you know it's more/less exothermic than the previous? Assume you're keeping it in the gas state so it's less exothermic because formation of a liquid from a gas would release more energy than just the formation of the gas itself?
 

tncekm

MS-1
10+ Year Member
Jul 18, 2006
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Medical Student
edit: nm, guess my answer wasnt' the right answer?