Nitrogen free energy change

A precise treatment of the free energy change of protonation, expressed in terms of the partition functions and the various energy components, can be found in a discussion of the basic strengths of nitrogen heterocycles by Chalvet, O., Daudel, R., and Peradejordi, F., J. Chim. Phys. 59, 709 (1962). In their treatment, as in the present case, possible effects due to relative differences in molecular partition functions are neglected. 

N2(g) + i02(g) -> NO(g), AG° = AG , NO = 87 kj/mol By definition, AGf for a compound equals the free energy change that would accompany the formation of 1 mol of that compound from its elements in their standard states. NO (and some other oxides of nitrogen) have weaker bonds as compared to the triple bond of N2 and the double bond of 02. Because of this, NO (and some other oxides of nitrogen) have higher (positive) free energies of formation as compared to the relatively stable N2 and 02 molecules. 

In environmental engineering, it is customary to call the substance oxidized as the electron donor and the substance reduced as the electron acceptor. The electron donor is normally considered as food. In the context of nitrogen removal, the foods are the nitrites, nitrates, and ammonia. Equation (15.10) is an example of an electron donor reaction. Zn is the donor of the electrons portrayed on the right-hand side of the half-cell reaction. On the other hand, the reverse of the equation is an example of an electron acceptor reaction. Zn would be the electron acceptor. McCarty (1975) derived values for free energy changes of half-reactions for various electron donors and acceptors utilized in a bacterial systems. The ones specific for the nitrogen species removal are shown in Table 15.2. 

In order to calculate the free-energy changes following the PMF description of the proton transfer, we used the QM/MM-MFEP method [195]. The reaction path was divided into six steps and the reaction coordinate along this path was chosen to be the distance between the acidic proton Ha to the amine nitrogen N. Step 0 was the neutral and step 5 the zwitterionic glycine. The Ha- -N distances of steps 1-4 were taken from snapshots of BOMD-MD dynamics. 

Ammonia can react with oxygen gas to form nitrogen dioxide and water, (a) Write a balanced chemical equation for this reaction, (b) Use tabulated data to determine the free energy change for the reaction and comment on its spontaneity, (c) Use tabulated data to calculate the enthalpy change of the reaction. 

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