An important redox couple in living systems based on nicotinamide adenine dinucl

Question

An important redox couple in living systems based on nicotinamide adenine dinucleotide, or NAD for short. The half reaction may be written as follows: NAD^+ + H^+ + 2e^- NADH E = -0.320 at pH 7.00 (the biochemical standard state) Nitrogen fixation is the reduction of dinitrogen in air to ammonia: N2(g) + 8H^+ +6e^- = 2NH4^+ E = +0.274 V (a) Combine the two half reactions in this question and the preceding one to produce a balanced redox reaction in which NADH reduces to N2 to ammonia. Also, write the cell notation for the redox reaction. (b) Calculate the cell potential at pH 7.00. (c) Is the reaction spontaneous at pH 7.00.

Explanation / Answer

Reaction 1 can be written as

NADH---> NAD+ H++2e- Eo= 0.320 (2)

multiplying equation (2) by 3

3NADH----> 3NAD++ 3H+ + 6e- (3) E0= 0.320 V

N2+ 8H++ 6e- ----> 2NH4+ (4) E0= 0.274 V

combing the equations (3) and equation (4)

3NADH+N2+5H+ ---> 3NAD+ + 2NH4+ E0= 0.320+ 0.274 = 0.594

DelG is indication of spontanity of the reaction

DelG=-nFE sicne E is +ve

DelG is always negative indicating the spontatnity of the reactiuon.

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