Question

The following half reactions play important roles in metabolism. \\[ \begin{array}{c} ^{1} /_{2} \mathrm{O}_{2}+2 \mathrm{H}^{+}+2 e^{-} \rightarrow \mathrm{H}_{2} \mathrm{O} \\ \mathrm{NADH}+\mathrm{H}^{+} \rightarrow \mathrm{NAD}^{+}+2 \mathrm{H}^{+}+2 e^{-} \end{array} \\] Which of these two is a half reaction of oxidation? Which one is a half reaction of reduction? Write the equation for the overall reaction. Which reagent is the oxidizing agent (electron acceptor)? Which reagent is the reducing agent (electron donor)?

Short answer

The oxidation half-reaction is \(\text{NADH} + \text{H}^+ \rightarrow \text{NAD}^+ + 2 \text{H}^+ + 2 e^-\). The reduction half-reaction is \(\frac{1}{2} \text{O}_2 + 2 \text{H}^+ + 2 e^- \rightarrow \text{H}_2 \text{O}\). The overall reaction is \(\text{NADH} + \frac{1}{2} \text{O}_2 + \text{H}^+ \rightarrow \text{NAD}^+ + \text{H}_2 \text{O}\). The oxidizing agent is \( \frac{1}{2} \text{O}_2 \) and the reducing agent is \( \text{NADH} \).

Step-by-step solution

Identify the Half-Reaction of Oxidation

Oxidation is the loss of electrons. In the reaction \(\text{NADH} + \text{H}^+ \rightarrow \text{NAD}^+ + 2 \text{H}^+ + 2 e^-\), \(\text{NADH}\) loses electrons, so this is the oxidation half-reaction.

Identify the Half-Reaction of Reduction

Reduction is the gain of electrons. In the reaction \(\frac{1}{2} \text{O}_2 + 2 \text{H}^+ + 2 e^- \rightarrow \text{H}_2 \text{O}\), \(\frac{1}{2} \text{O}_2\) gains electrons to form \(\text{H}_2 \text{O}\). Therefore, this is the reduction half-reaction.

Write the Equation for the Overall Reaction

Combine the oxidation half-reaction and the reduction half-reaction: \[ \text{NADH} + \text{H}^+ \rightarrow \text{NAD}^+ + 2 \text{H}^+ + 2 e^- \] \[ \frac{1}{2} \text{O}_2 + 2 \text{H}^+ + 2 e^- \rightarrow \text{H}_2 \text{O} \] To get the overall reaction, cancel the 2 electrons on both sides: \[ \text{NADH} + \text{H}^+ + \frac{1}{2} \text{O}_2 + 2 \text{H}^+ \rightarrow \text{NAD}^+ + 2 \text{H}^+ + \text{H}_2 \text{O} \] Combine like terms: \[ \text{NADH} + \frac{1}{2} \text{O}_2 + \text{H}^+ \rightarrow \text{NAD}^+ + \text{H}_2 \text{O} \]

Identify the Oxidizing Agent

The oxidizing agent is the substance that accepts electrons. In this case, \(\frac{1}{2} \text{O}_2 + 2 \text{H}^+\) accepts electrons to become \(\text{H}_2 \text{O}\), so \(\frac{1}{2} \text{O}_2\) is the oxidizing agent.

Identify the Reducing Agent

The reducing agent is the substance that donates electrons. Here, \(\text{NADH}\) donates electrons, so \(\text{NADH}\) is the reducing agent.

Key concepts

Half-Reactions

In chemistry, particularly in redox reactions, a half-reaction shows either the oxidation or the reduction process separately. Together, they make up the overall redox reaction. A half-reaction for oxidation involves the loss of electrons. For example, in the reaction: \[ \text{NADH} + \text{H}^+ \rightarrow \text{NAD}^+ + 2 \text{H}^+ + 2 e^- \], NADH is being oxidized as it loses electrons.
A half-reaction for reduction involves the gain of electrons. For instance, in the reaction: \[ \frac{1}{2} \text{O}_2 + 2 \text{H}^+ + 2 e^- \rightarrow \text{H}_2 \text{O} \], oxygen gains electrons to become water.
By writing equations for these half-processes, one can easily visualize which molecules are gaining or losing electrons.

Oxidizing Agent

Oxidizers, or oxidizing agents, play a crucial role in redox reactions. They are the substances that accept electrons during these processes. An oxidizing agent undergoes reduction by gaining electrons from another substance. For instance, in the reaction \[ \frac{1}{2} \text{O}_2 + 2 \text{H}^+ + 2 e^- \rightarrow \text{H}_2 \text{O} \], the oxygen molecule, \(\frac{1}{2} \text{O}_2 \), acts as the oxidizing agent because it accepts electrons and gets reduced to water.
This acceptance of electrons helps drive the redox reaction forward and is essential in many metabolic processes.

Reducing Agent

Reducing agents are substances that donate electrons in redox reactions. By losing electrons, reducing agents undergo oxidation. In the reaction: \[ \text{NADH} + \text{H}^+ \rightarrow \text{NAD}^+ + 2 \text{H}^+ + 2 e^- \], NADH acts as the reducing agent because it donates electrons and becomes oxidized to NAD+.
Reducing agents provide the necessary electrons needed to reduce other substances, making them crucial for maintaining balance in chemical reactions, especially in metabolic pathways where energy is transferred and transformed.

Electron Transfer

Electron transfer is the movement of electrons from one molecule or atom to another in a redox reaction. This process is fundamental to metabolic reactions, as it allows the transfer of energy and the transformation of substances.
In the overall redox reaction, the electrons from NADH are transferred to oxygen, facilitating the formation of water: \[ \text{NADH} + \text{H}^+ + \frac{1}{2} \text{O}_2 \rightarrow \text{NAD}^+ + \text{H}_2 \text{O} \].
This transfer not only releases energy but also converts NADH to NAD+ and oxygen to water.
Such electron transfers are key to processes like cellular respiration, which generate the energy required for various cellular functions.