09-214 Physical Chemistry Name______KEY____________________

Quiz 5 (15 pts) March 2, 2001

R = 8.314 J/K mol

F = 96500 coulombs

1 joule = 1 volt coulomb

Some biochemists' standard reduction electrode potentials at 25° C are given below.

 

Oxidant/reductant

Electrode reaction

Eo' (V)

Horseradish peroxidase

HRP[Fe(III)] + e- ® HRP[Fe(II)]

-0.271

NAD+/NADH

NAD+ + H+ + 2e- ® NADH

-0.320

 

For the hypothetical oxidation of nicotinamide adenine dinucleotide ,NADH, by horseradish peroxidase, HRP[Fe(III)], , calculate D G° ' and the equilibrium constant at 25° C and pH 7.

Oxidation step: NADH ® NAD+ + H+ + 2e- Eo' = +0.320 V (2 pts)

Reduction: HRP[Fe(III)] + e- ® HRP[Fe(II)] - Eo' = -0.271 V

We need two e: 2 HRP[Fe(III)] + 2e- ® 2 HRP[Fe(II)] - Eo' = -0.271 V (2pts)

Adding the 1st and 3rd equations gives

NADH + 2 HRP[Fe(III)] ® NAD+ + H+ + 2 HRP[Fe(II)] Eo' = +0.049 V (2 pts)

D G° ' = -nFEo' = -2(96500 C)(0.049 V) = -9.46 X 103 C V = -9.46 X 103 J (3 pts)

K'eq = e-D G° '/RT = e-(-9460)/(8.314)(298) = e+3.82 = 4.55 X 101 (2 pts)

What is the balanced equation for the overall reaction?

 

 

NADH + 2 HRP[Fe(III)] ® NAD+ + H+ + 2 HRP[Fe(II)] (2 pts)

 

 

If the reaction were actually carried out in an electrolytic cell, which electrode reaction would correspond to that occurring at the anode?

Oxidation occurs at the anode: NADH ® NAD+ + H+ + 2e- (2pts)