Michel Jullian wrote: > > 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? :))) > I'm not sure, Michel. The heat of formation from the elements dH is 2 x 498,000 for 2 H2 molecules from 4 H atoms and 498,000 for an O2 molecule from 2 O atoms. Then you have to break 3 x 498,000 = 1.49E6 kJ to form 2 H2Omolecules from 4 H atoms + 2 O atoms ---> H-O-H + H-O-H (4 x 498 kJ) - (3 x 498 kJ) = 498 kJ
Which implies that 571.6 kJ - 498 kJ = 73.6 kJ is the heat given up by the two H2O molecules when cooled from a hot gas to liquid H2O in the calorimeter???. Whereas -dG = for 2 H2O (liquid) is - 477 kJ going by my late edition CRC tables. And things are a lot simpler-quicker if you take H2 and O2 as 0.00 and pull the values off the Ellingham Diagrams (stored in my documents) for about any temperature from 0.0 to 2,000 C http://www.chem.mtu.edu/skkawatr/Ellingham.pdf 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. >
