Question

1.Energetics of respirations

A. Compute the free energy that is released upon transfer of a pair of electrons between the following donor/acceptor pairs. Unless specified otherwise, assume standard reaction conditions; standard reduction potentials are listed below. Provide answers in kJ/mol.

i. NO3-/NO2- to 1⁄2 O2/H2O
ii. SO42-/HSO3- to NO3-/NO2-
iii. 2H+/H2 to NO3-/NO2-
iv. 2H+/H2 to N2O/N2
v. So/HS- to 1⁄2 O2/H2O
vi. Like iii, except with H2 present at only .01 atm

Standard reduction potentials (from Appendix 1)

2H+/H2 NO3-/NO2- So/HS-
1⁄2 O2/H2O SO42-/HSO3- N2O/N2

-0.41 +0.43 -0.27 +0.82 -0.52 +1.36

Answer 1

I will do calculations for the first one to give you guidelines to do it. Try to do the rest by yourself following the same procedure:

i. NO₃^- / NO₂^- to 1/2 O₂ / H₂O

E° = +0.43 V E° = +0.82 V

NO₃^- + H₂O + 2e^- → NO₂^- + 2OH^-

1/2O₂ + 2H⁺ + 2e^- → H₂O

E° = Ered - EOxi

As E° from NO3 is lower this means that this will experience the oxidation, (lower reduction potential) so:

E° = 0.82 - 0.43 = 0.39 V

Now, use the following expression:

= -nFE where e is the number of electrons transfered and F the faraday constant (96500) so:

= -2 * 96500 * 0.39

= -75270 J/mol or simplu -75.27 kJ/mol

Try to do the rest yourself, Hope this helps you and if you get stuck tell me and I'll help you out.