2. Many biological reactions are electron transfer reactions and their spontanei

Question

2. Many biological reactions are electron transfer reactions and their spontaneity can be determined by calculating the potential, E degree?, and triangle G degree? (the prime (') indicates biochemical conditions of pH = 7) of the reaction. For the following half reactions, write the balanced equation and determine a) the free energy at 25 degree C b) which direction the reaction occurs spontaneously, and c) for the spontaneous reaction identify the oxidizing agent and reducing agent. (6 points) NAD^+ H^+ +2 e^- right arrow NADH E degree' = -0.315 V Pyruvate^- +2H^+ + 2 e right arrow lactate^- E degree? = -0.185 V

Explanation / Answer

The balanced equation will be,

2NADH + 2pyruvate- + 2H+ -----------> 2lactate- + 2NAD+ + 2H+

a) Free energy calculation

Ecell = -0.185 -(-0.315) = 0.13 V

delta G = -nFE

n = moles of e- from balanced redox reaction which is 2 here

F = Faraday's constant = 96,485 C/mol

Feed all the values, delta G = -(2 x 96485 x 0.13) = -25086 kJ

b) Therefore, since Ecell is positive and ?G is negative, this reaction is spontaneous. The direction of the reaction is forward.

c) pyruvate is the oxidizing agent and NADH is the reducing agent.

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