(corrected the subject line, of course the produced energy is equal to _lost_ enthalpy -dH, not to enthalpy change dH which is _gained_ enthalpy)
BTW there is another way to recover the ~572kJ found below: let's consider the reverse reaction, and pretend we don't know how much energy it absorbs: 2H2O(l) + E -> 2H2(g) + O2(g) (dissociation of water, can be done e.g. by electrolysis, as all of us here know quite well :) We know the electrolysis voltage threshold is 1.48 V, and we know that two electrons are transferred for each molecule of H2O dissociated. So for the 2 moles of H2O in the reaction, the total charge q transferred is 2*(2*NA)*e where e is the electron charge 1.6*10^(-19) C and NA is the number of molecules per mole (Avogadro's number 6.02*10^23). So the total energy needed to dissociate 2 moles of water is: E= V*q = 1.48*2*(2*NA)*e = 1.48 * 2 * (2 * 6.02 * (10^23)) * 1.6 * (10^-19) = 570 214.4 =~ 570 kJ =~572 kJ QED hence the "=" sign which is being increasingly used instead of an arrow in chemical equations, e.g. in the CHEMIX software: 2H2(g) + O2(g) = 2H2O(l) + 571.6kJ <=> 2H2O(l) + 571.6kJ = 2H2(g) + O2(g) Hope this makes sense. Michel ----- Original Message ----- From: Michel Jullian To: [email protected] Sent: Sunday, June 04, 2006 12:38 PM Subject: Produced energy is enthalpy change dH (was Re: Free Radical Chain Reactions) Hi Robin, the NIST webbook, which I find much handier indeed than those CRC handbooks, doesn't solve the controversy which is "is the energy produced in a reaction equal to -dG or to -dH? (d=delta)" BTW I just found that I was wrong in thinking that my trial version of CHEMIX had expired, it seems it only counts down the days in which you actually use the software, nice ! :) So here is CHEMIX's thermochemistry answer for the reaction we discussed (copy-paste): 2H2(g) + O2(g) = 2H2O(l) + 571.6kJ which solves the controversy (produced energy is equal to -dH=572kJ/mol, not to -dG=474kJ/mol), doesn't it Fred? :))) Michel ----- Original Message ----- From: Robin van Spaandonk To: [email protected] Sent: Sunday, June 04, 2006 5:13 AM Subject: Re: Free Radical Chain Reactions In reply to Michel Jullian's message of Sat, 3 Jun 2006 20:58:38 +0200: Hi, [snip] >The spreadsheet did the dH algebra like it's own mentor told it, Fred. > >You could try downloading the trial version of CHEMIX (Google it) ans see >what their thermochemistry section gives for this reaction? Mine has >expired, and was in Norwegian or something for some reason (must have >missed the language option) [snip] You may both find this of use :) http://webbook.nist.gov/chemistry/form-ser.html.en-us.en Regards, Robin van Spaandonk http://users.bigpond.net.au/rvanspaa/ Competition provides the motivation, Cooperation provides the means.
