Gibbs free energy change for 2Zn(g)+O2(g) = 2ZnO(g)

[endamcglynn]

Hi all,

I am interested in the gas phase reaction 2Zn(g) + O2(g) = 2ZnO(g). I have used the HSC Chemistry 6 package to get Gibbs free energy changes from 273 - 1273 K for this reaction and have compared them with data from the latest IVTANTHERMO codes result for the same reaction (which seems to be the same data source as the Glushko 96 and Landolt data reference in HSC Chemistry 6, though in one location in HSC the latter is listed Landolt 01 and I could only see Landolt 99 in the data references section, but I guess they must be similar) and my own programme using the equations in the JANAF tables (with the IVTANTHERMO values, specifically 170 kJ/mol for the diatomic molecular dissociation energy at 0K, D_0, of ZnO(g)).

The problem is the Gibbs free energy changes from 273 - 1273 K for this reaction are different from IVTANTHERMO and HSC, using the same "input" data on molecular properties. My own programme (labelled JANAF) matches the IVTANTHERMO result well up to ~1000K, but seeing as I don't include anharmonic vibration terms it diverges thereafter and this doesn't worry me unduly. I attach a pdf attachment with a graph of the 3 results and one can see the HSC data are markedly different, both in terms of slopes versus temperature and absolute values at any one temperature.

So my question is - why the difference? For example, can one access the actual molecular parameters such as molecular dissociation energy, rotational and vibrational data etc. for the reaction species in HSC to see what has been used in the calculation?

Any help appreciated, sorry if the question is silly.

sincerely,

Enda McGlynn
Dublin City University

[Eltee]

Since there is a major difference in dG even at 298K, I'd look first at the basic thermodynamic parameters (dHf and So) for the species. Checking my old JANAF Tables (ca 1982), I find that values for Zn(g) and O2(g) are essentially the same as those found in the HSC 6 data base, but that ZnO(g) appears only in the HSC data base, not the JANAF Tables. (Perhaps there are updated values, but I don't have them ready to hand). I suspect the difference is in your value for dHf for ZnO(g). You hint that you obtained that value from a dissociation energy of ZnO(g) of 170 kJ/mol. Using the following: dHf [ZnO(g)] = dHf[Zn(g)] + dHf[O(g)] - D[ZnO(g)], I calculate dHf[ZnO(g)] = 130.4 + 249.2 - 170 = 209.6 kJ/mol. the HSC data base lists dHf[ZnO(g)] = 136.5 kJ/mol (much different). Using the 209 value, at 298K the dG for the reaction is about 170 kJ/mol (pretty much what you plotted as "JANAF") whereas using the HSC data base value of 136.5, the dG is 24 kJ/mol (again, what you plotted for HSC). Which (if either) value is correct, I don't know, but I suspect this is the source of the discrepancy.

[endamcglynn]

Dear Eltee,

thanks for your help and indeed you are absolutely correct, the dHf [ZnO(g)] value from IVTANTHERMO is 206.655 kJ/mol, close to the 209.6 kJ/mol value you calculated. The D_0 value for ZnO(g) comes out as 242.9kJ/mol (2.52 eV) using your calculation in reverse with the HSC dHf [ZnO(g)] = 136.5 kJ/mol. I don't recognise the 2.52 eV D_0 value from any literature sources. Perhaps it was indeed an "agreed" or averaged value from the older IVTANTHERMO database. I'll see if I can dig this information out myself.

Many thanks again,

Enda

[LesG]

See Clemmer, D. E.; Dalleska, N. F.; Armentrout, P. B. J. Chem. Phys. 1991, 95, 7263 for ZnO(g) thermochemistry.

Do(OK) = 155 kJ/mol, (298K) 158 +/- 4 kJ/mol.

LesG