Thursday, March 27, 2014

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. ...

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!

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