Energy Released When H + H -> H2

particular

13th April 2011 - 11:11 PM

Hello,
Can anyone enlighten me as to how much energy is released when atomic hydrogen combines to form molecular hydrogen? I'm interested in the numbers (and units of course) for gaseous, non-ionised, hydrogen atoms pairing up to form molecular H2.
It would be a bonus if you could point me in the direction of a good web resource that explains the derivation of the answer.
Thanks.

rpenner

14th April 2011 - 01:08 AM

Isn't this just the negative of the standard enthalpy of forming two moles of atomic hydrogen?

http://www.webelements.com/hydrogen/thermochemistry.html

Or about -436 kJ mol-1 H₂

http://www.webelements.com/hydrogen/thermochemistry.html
http://www.codata.org/resources/databases/key1.html

You might want to brush up on the various measures of energy to make sure this is the correct definition:
http://en.wikipedia.org/wiki/Enthalpy (H)
http://en.wikipedia.org/wiki/Gibbs_free_energy (G)
http://en.wikipedia.org/wiki/Thermodynamics#Potentials (also, U, A, ... )

particular

14th April 2011 - 09:08 PM

Thanks, you've provided what I was looking for.

Your links are especially appreciated because I think I must have drifted off during the explanation of moles when I was at school, and the word 'enthalpy' was never mentioned - it was all 'heat of reaction' in those days. I reckon I've got the idea now though.

I'm an electronics engineer but sometimes chemistry becomes part of the problem I'm trying to solve.

Thanks!

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