Thanks Robin but we were talking about the liquid: >>2H2(g) + O2(g) = 2H2O(l) + 571.6kJ
and the question was whether produced energy (in the form of heat or whatever e.g. electric power in a fuel cell) was dG for the liquid 474 kJ(and not 483 kJ, twice the dH for the gas you gave), or dH for the liquid 572 kJ, the controversy has finally been solved in favor of the latter cf later posts. Fred must be busy re-doing all his Joe Cell calculations in dH algebra ;) You're right of course about the difference between formation enthalpies of H2O(l) and H2O(g) being the heat of condensation (44kJ/mol) which is physically the energy of the H2O-H2O bonds, but there is no heat derived from the cooling since all these figures are only valid at 25° (and 1 atm). Getting late in this part of the world, good night all. Michel ----- Original Message ----- From: "Robin van Spaandonk" <[EMAIL PROTECTED]> To: <vortex-l@eskimo.com> Sent: Monday, June 05, 2006 12:40 AM Subject: Re: Produced energy is enthalpy change dH (was Re: Free Radical Chain Reactions) > In reply to Michel Jullian's message of Sun, 4 Jun 2006 12:38:24 > +0200: > Hi, > [snip] >>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? :))) > > According to:- > >>http://webbook.nist.gov/chemistry/form-ser.html.en-us.en > > delta Hgas = -241.826 kJ/mole and deltaHliquid = -285.830 kJ/mole. > > IOW both answers are correct, depending on whether the water forms > as a vapor or as a liquid. As I understand it the difference is > the heat of condensation + any heat derived from the cooling of > the liquid. You need to look at the specific circumstances of any > given experiment to determine which answer is "correct". > > In a car engine the water clearly forms as a vapor, and exits the > engine as such. Unless waste heat is recovered from the exhaust, > the energy of condensation is lost. > > Regards, > > Robin van Spaandonk > > http://users.bigpond.net.au/rvanspaa/ > > Competition provides the motivation, > Cooperation provides the means. >
