Re: Free Radical Chain Reactions

[Michel Jullian]

My spreadsheet finds at 25° and 1atm:   O2(g) + 2 H2(g) -> 2 H2O(l) + 571.66 kJ/mol (exothermic) Spontaneous at 25°C. Equilibrium at about 1477°C. Molar masses and thermodynamic properties Enthalpy change kJ/mol Entropy change J/K/mol Gibbs Free Energy change kJ/mol Sources: c.f. bottom of spreadsheet -571.66 -326.69 -474.31   Again I agree with your dG value as you can see at bottom right of the above table, but I suggest innumerable calorimeters have measured a thermal energy of 572 kJ/mole (enthalpy) rather than 474 or 475 kJ/mole (Gibbs)   Michel   ----- Original Message ----- From: "Frederick Sparber" <[EMAIL PROTECTED]> To: <vortex-l@eskimo.com> Sent: Saturday, June 03, 2006 7:42 PM Subject: Re: Free Radical Chain Reactions > Michel Jullian wrote: >> >> Fred I really meant "the reaction below", H2 combustion in O2: >> >> 2 H2 + O2 ----> 2 H2O >> > > Gibbs Free Energy from CRC tables. >  > HOH     - 56.687 (liquid) > OH          + 8.18 > HO-OH   - 28.78 > H               +48.58 > H2              0.00 > O               + 55.39 > O2              0.00 > > > 2 H2  (g) + O2 (g) ----->  2 H2O (liquid) > > 2 times -56.687 Kcal/mole or 2 x KJ/mole = - 475 KJ Gibbs Free Energy > for combustion of 2 moles of H2 at STP. > > Measured innumerable times in a bomb calorimeter. >> >> How many joules per mole does this produce, and does this correspond to > the >> enthalpy change or to the Gibbs free energy change of the reaction? >> >> The question is only intended to solve the controversy one way or > another, I >> haven't looked up the answer. Admittedly I have my own opinion of what it >> will be (enthalpy), so in this sense you can call it a trick question. >> >> Michel >> >> ----- Original Message ----- >> From: "Frederick Sparber" <[EMAIL PROTECTED]> >> To: <vortex-l@eskimo.com> >> Sent: Saturday, June 03, 2006 3:53 PM >> Subject: Re: Free Radical Chain Reactions >> >> >> > Michel Jullian wrote: >> >> >> >> BTW Fred, have you given some thought to our enthalpy vs Gibbs >> > controversy? >> >> Which energy can be recovered from the reaction below do you think, the >> >> enthalpy change or the Gibbs free energy change? >> >> >> > Is that a trick question, Michel? >> > >> > The H-H bond is 498 Kjoule/mole the same as the O-O bond and the O-H >> > bond.. >> > Hence overall,  H-H + O-O ---->  H-O-H   + O  nets Zip Gibbs or > Enthalpy. >> > But, O + Fe ---> Fe-O:  Fe-O (390 KJ/mole) minus  Fe-Fe (100 KJ/Mole) >> > equals a Gibbs Free Energy of 390-100 = 290 KJ when you oxidize iron >> > with O radicals.  :-) >> > >> > OTOH,  H-O-H  2 x 498 KJ/Mole + Ni ----> NiO  (382 KJ/mole) + >> > H-H = 498 - 382 =  116 KJ/mole. Easy to Compare Enthalpy >> > with the Ellingham (enthalpy) Diagrams. >> > >> > http://www.chem.mtu.edu/skkawatr/Ellingham.pdf >> > >> > Fred. >> >> >> >> Michel >> >> >> >> ----- Original Message ----- >> >> From: "Frederick Sparber" <[EMAIL PROTECTED]> >> >> To: "vortex-l" <vortex-l@eskimo.com> >> >> Sent: Friday, June 02, 2006 4:52 PM >> >> Subject: Re: Free Radical Chain Reactions >> >> >> >> >> >> > Actually  2 H2 + O2 ----> 2 H2O has about 14  reaction steps. >> >> > >> >> > http://www.cheresources.com/reactionkinetics3.shtml >> >> > >> >> > "Another important consideration is the formation of chain reactions. >> > The >> >> > basic premise of chain reaction mechanisms is also that free radicals >> > play >> >> > a leading role in the destruction of reactant molecules. The chain >> >> > reaction mechanism itself consists of several steps: initiation, >> >> > propagation, branching (not always present), and termination. This > can >> > be >> >> > illustrated, for certain range of temperature and pressure, by some > of >> > the >> >> > reactions in the following Hydrogen oxidation mechanism:" >> >> > >> >> > "To summarize, reaction mechanisms can be assembled from elementary >> >> > reactions using free radicals as the means for decomposition of the >> >> > reactant, and intermediate products. Chain branching reactions, if > they >> >> > occur, take a very important role in the mechanism as they lead to > the >> >> > formation of increasing concentrations of radicals. Reaction time and >> >> > temperature have a bearing on radical concentration, and the type of >> >> > reaction initiating the consumption of the reactant" >> >> >> > >> > >> > >> > > >