2011-07-11, 12:47 AM
Corn Wrote:Put it this way.
The reaction is exothermic if the heat of formation is negative. This means hydrogen and oxygen combining should release energy.
But you really don't see water spontaneously forming in midair, don't you?
If the concentration of H2(g) in the air was high enough for a large amount of H2O to visibly form then it would also be high enough for you to die the second you light a match.
You are right that it releases energy, all exothermic reactions do because it goes from a high potential energy state to a low one. Be careful though, spontaneous reactions are characterized by having a negative dG.
dG = dH - TdS
From this formula you can see that having a positive dH (endothermic reaction) can still yield a spontaneous reaction if TdS > dH