Chapter 7, Problem 1QC

Summary Introduction

To determine:

Which member of the pair is reduced and which is oxidized, and which pair has more chemical energy and which has less chemical energy in the following pairs:

${\text{NAD}}^{+}/\text{NADH}$; ${\text{FAD}}^{+}/{\text{FADH}}_2$; ${\text{CO}}_2/{\text{C}}_6{\text{H}}_{12}{\text{O}}_6$

Concept introduction:

In oxidation-reduction reactions, the oxidation is loss of electrons and the reduction is the gain of electrons. In couple oxidation-reduction reactions, the loss and gain of electrons always take place together. The transferring of electrons occurs in such a way from one molecule to the other, that one molecule gain electrons; while the other loses them.

Explanation of Solution

A reduced molecule has bonds, which are less electronegative as compared to the oxidized molecule. These molecules are identified by the presence of an increased $\text{C-H}$ bond. It is because of these bonds, the reduced forms releases more chemical energy as compared to their oxidized forms.

In the following pairs:

a) ${\text{NAD}}^{+}/\text{NADH}$, the $\text{NADH}$ molecules are reduced and the ${\text{NAD}}^{+}$ molecules are oxidized, the reduced forms have more chemical energy, so the $\text{NADH}$ molecules have more chemical energy.

b) ${\text{FAD}}^{+}/{\text{FADH}}_2$, the ${\text{FADH}}_2$ molecules are reduced and the ${\text{FAD}}^{+}$ molecules are oxidized., and ${\text{FADH}}_2$ molecules have more chemical energy.

c) ${\text{CO}}_2/{\text{C}}_6{\text{H}}_{12}{\text{O}}_6$, the ${\text{C}}_6{\text{H}}_{12}{\text{O}}_6$ molecules are reduced and the ${\text{CO}}_2$ molecules are oxidized, and the ${\text{C}}_6{\text{H}}_{12}{\text{O}}_6$ molecules have more chemical energy.

Conclusion

In the given pairs, $\text{NADH}$, ${\text{FADH}}_2$, and ${\text{C}}_6{\text{H}}_{12}{\text{O}}_6$ are reduced molecules and ${\text{NAD}}^{+}$, ${\text{FAD}}^{+}$,  and ${\text{CO}}_2$ are the oxidized molecules. The reduced molecules have more chemical energy therefore in the given pairs the reduced molecules will have more chemical energy.