Question

Calculate the overall standard electromotive force (ΔE°') and free energy (ΔG°') for the reaction below. Show all work. Pyruvate + NADH + H+ lactate + NAD+

Answer 1

The reaction in question is

pyruvate + NADH + H⁺    lactate + NAD⁺

This redox reaction consists of two half reactions:

pyruvate + 2 H+ + 2 e–    lactate		(reduction half reaction)
NADH + H+    NAD+ + 2H+ + 2 e–		(oxidation half reaction)

The two reduction half reactions are as follows

(1)	pyruvate + 2 H+ + 2 e–    lactate	E°' = –0.190 V
(2)	NAD+ + 2H+ + 2e–    NADH + H+	E°' = –0.320 V

reaction 2 must be written in reverse. When a reduction reaction is reversed to create an oxidation half reaction, the sign of the reduction potential must also be reversed to get an oxidation potential.

NADH + H+    NAD+ + 2H+ + 2e–

E°' = 0.320 V

Now we can calculate the ΔE° of the full reaction

	pyruvate + 2 H+ + 2 e–    lactate	E° = –0.190 V
	NADH + H+    NAD+ + 2H+ + 2e–	E° = 0.320 V
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	pyruvate + NADH + H+    lactate + NAD+	ΔE° = 0.130 V

free energy (ΔG°) for the reaction

is got from the equation

ΔG° = -nFE°

where n = no of electrons exchanged in the reaction, F = Faraday's constant 96458 C/mol

ΔG° = -2 mol e^- x 96458 x 0.130

ΔG° = -25079 J

ΔG° = -25.08 kJ