Problem 98 - Chemistry Matter and Change

Question

Will NAD + oxidize \(\mathrm{Fe}^{2+}\) to \(\mathrm{Fe}^{3+}\) ? Explain your answer.

Step-by-Step Solution

Answer

NAD+ will not oxidize Fe²⁺ to Fe³⁺. This is because the redox potential of Fe³⁺/Fe²⁺ (+0.77 V) is more positive than that of NAD+/NADH (-0.32 V). In fact, the reaction will proceed in the opposite direction, with NADH acting as a reducing agent to reduce Fe³⁺ to Fe²⁺.

Step 1: Identify Redox Potentials

First, let's identify the redox potentials of NAD+/NADH and Fe²⁺/Fe³⁺. These values can usually be found in a standard reduction potentials table. Take note of the redox potential values for the following half-reactions: 1. NAD+ + 2e⁻ + H⁺ → NADH 2. Fe³⁺ + e⁻ → Fe²⁺

Step 2: Compare Redox Potentials

Now that we have the redox potentials of the species involved, we need to compare them. Determine which half-reaction has a more positive redox potential. Remember that the half-reaction with the more positive redox potential will act as an oxidizing agent and proceed in the direction as written in the table (reduction). Redox potential of NAD+/NADH: -0.32 V Redox potential of Fe³⁺/Fe²⁺: +0.77 V Since the redox potential of Fe³⁺/Fe²⁺ (+0.77 V) is more positive than that of NAD+/NADH (-0.32 V), Fe³⁺ will act as an oxidizing agent, while NADH will act as a reducing agent.

Step 3: Determine the Resulting Reaction

As Fe³⁺ acts as an oxidizing agent and NADH as a reducing agent, the resulting reaction will be: Fe³⁺ + e⁻ → Fe²⁺ NADH → NAD+ + 2e⁻ + H⁺ Combining both half-reactions, we get the overall reaction: Fe³⁺ + NADH + H⁺ → Fe²⁺ + NAD+

Step 4: Conclusion

Since the redox potential of Fe³⁺/Fe²⁺ is more positive than that of NAD+/NADH, it will act as an oxidizing agent and NADH as a reducing agent. Therefore, NAD+ will not oxidize Fe²⁺ to Fe³⁺. Instead, NADH will reduce Fe³⁺ to Fe²⁺ in the overall reaction.

Key Concepts

Reduction Potential

Reduction potential is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. It is often measured in volts (V) and can be found in tables of standard electrode potentials.

In the context of the exercise, we look at the reduction potentials of NAD⁺/NADH and Fe³⁺/Fe²⁺. The reduction potential for the Fe³⁺/Fe²⁺ couple is +0.77 V, while that for NAD⁺/NADH is -0.32 V.

This difference is crucial because the half-reaction with a higher, more positive reduction potential will proceed as a reduction in a redox process. The more positive reduction potential indicates a greater tendency to gain electrons.

Oxidizing Agent

An oxidizing agent, or oxidant, is a substance that has the ability to oxidize other substances, meaning it gains electrons from those other substances during a chemical reaction.

In redox chemistry, an oxidizing agent receives electrons and, in the process, induces oxidation of another species. It is effectively reduced itself.

• Fe³⁺ in the Fe³⁺/Fe²⁺ couple acts as an oxidizing agent in this scenario because it will accept electrons from NADH.
• This acceptance of electrons turns Fe³⁺ into Fe²⁺, meanwhile, NADH is oxidized to NAD⁺.

Thus, in this exercise, Fe³⁺ acts as an excellent oxidizing agent due to its higher reduction potential.

Half-Reactions

Half-reactions are the two parts of a redox reaction, which show the oxidation or reduction processes separately. Each half-reaction involves electrons, showing either their gain or loss.

In our example, the half-reactions are:

• Fe³⁺ + e⁻ → Fe²⁺, which represents the reduction process.
• NADH → NAD⁺ + 2e⁻ + H⁺, representing oxidation.

These half-reactions detail the specific electron transfer in a redox process, making it clearer which species are being oxidized and reduced.

By balancing these half-reactions, we can determine the overall redox reaction.

Electrode Potentials

Electrode potentials refer to the potential difference measured at an electrode used in redox reactions. Electrode potentials help in determining which way the electron flow will occur in a cell.

For these redox reactions:

• The Fe³⁺/Fe²⁺ has an electrode potential of +0.77 V, indicating its strong tendency to gain electrons and undergo reduction.
• On the other hand, the NAD⁺/NADH couple has an electrode potential of -0.32 V, showing that it has a weaker tendency to gain electrons compared to Fe³⁺.

These values help in predicting the direction of a chemical reaction and whether a reaction is spontaneous. Electrode potentials provide straightforward comparisons that are essential for determining the feasibility of reactions such as in our exercise.