Compute the overall standard potential (in V) for transfer of electrons from the following pairs of donors and acceptors. Rank them from most...

All you have to do is find a reference table for the known standard potentials of some substances. When you do that, you just add the reduction and oxidation potentials of the pair (if a substance is going through oxidation, remember to reverse its sign, as the tables are made with the reduction potential).

I did my search and below is all the calculations done for each pair.

1. H2/Fe3+ = -0.41 + 0.77 = 0.36


2. H2S/O2 = -0.22 + 0.82 = 0.60


3. CH4/NO3- = -0.24 + 0.75 = 0.51


4. H2/O2 = -0.41 + 0.82 = 0.41


5. Fe2+/O2 = 0.77 + 0.82 = 1.59


6. H2S/NO3 = -0.22 + 0.75 = 0.53

With all the potentials known for each pair, you just have to order them. Thus, we get:

Fe2+/O2 > H2S/O2 > H2S/NO3 > CH4/NO3- > H2/O2 > H2/Fe3+

Take a moment to see that the ones paired with O2 are the most energetic. This is due to oxygen being a highly oxidizing agent (the reason why rust is such a big problem).

Also, note that these calculations may vary, as the standard potential depends on the temperature/external conditions, so some tables may present you with different values for the standard potential of a substance!