Problem 30 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

The following reactions represent the reduction of \(\mathrm{IO}_{3}^{-}\) ion into \(\mathrm{I}^{-}\) ion in acidic and basic medium. Predict in which medium \(\mathrm{IO}_{3}^{-}\) ion will act as a better oxidizing agent? \(\mathrm{IO}_{3}^{-}+6 \mathrm{H}^{+}+6 \mathrm{e}^{-} \rightarrow \mathrm{I}^{-}+3 \mathrm{H}_{2} \mathrm{O}\) \(E^{\circ}=+0.907 \mathrm{~V}\) \(\mathrm{IO}_{3}^{-}+3 \mathrm{H}_{2} \mathrm{O}+6 \mathrm{e}^{-} \rightarrow \mathrm{I}^{-}+6 \mathrm{OH}^{-}\) \(E^{\circ}=+0.260 \mathrm{~V}\) (a) Acid medium (b) Basic medium (c) Equally in both (d) Not predictable

Step-by-Step Solution

Answer

The \text{IO}_3^{-} ion will act as a better oxidizing agent in an acidic medium (a).

Step 1: Compare Standard Reduction Potentials

The effectiveness of an oxidizing agent is determined by its standard reduction potential (\(E^{\text{o}}\text{ value}\)). A higher reduction potential indicates a better oxidizing agent because it is more likely to gain electrons and be reduced. In the given exercise, we compare the reduction potentials for the reduction of \text{IO}_3^- ion in acidic medium (\(E^{\text{o}} = +0.907 \text{ V}\)) and basic medium (\(E^{\text{o}} = +0.260 \text{ V}\)).

Step 2: Predict the Better Oxidizing Agent

Since the standard reduction potential of the \text{IO}_3^{-} ion in an acidic medium (\(E^{\text{o}} = +0.907 \text{ V}\)) is higher than in a basic medium (\(E^{\text{o}} = +0.260 \text{ V}\)), \text{IO}_3^{-} ion will act as a better oxidizing agent in acidic medium.

Key Concepts

Standard Reduction Potential

Understanding the standard reduction potential is key to predicting the behavior of oxidizing agents. The standard reduction potential, represented as \(E^\circ\), measures a substance's tendency to gain electrons and thus be reduced during a redox reaction.

Consider it as a numeric value that you can compare across different chemical species; a higher \(E^\circ\) values signifies a greater likelihood of the substance acting as an oxidizing agent. When comparing the \(E^\circ\) values of the \(\text{IO}_3^-\) ion in acidic (\(E^^\circ = +0.907 \text{V}\)) versus basic (\(E^\circ = +0.260 \text{V}\)) mediums, the acidic medium's higher value indicates a stronger oxidizing ability. This is because in the context of the \(E^\circ\) value, the \(IO_3^-\) ion is more inclined to accept electrons (get reduced) in the presence of acid, implying its better performance as an oxidizing agent in that environment.

Redox Reactions

Redox reactions are chemical processes in which the oxidation state of atoms is changed due to the transfer of electrons between them. Within this framework, oxidation refers to the loss of electrons, while reduction is the gain of electrons. These reactions are essential in understanding how oxidizing agents work.

In the context of our exercise, the \(\text{IO}_3^-\) ion transforms into the \(\text{I}^-\) ion either by gaining electrons in an acidic or basic medium. This electron gain, or reduction, is facilitated by another species losing electrons (oxidation)—this is where the oxidizing agent comes into play. An effective oxidizing agent will readily attract electrons from another substance, thereby 'oxidizing' it. The standard reduction potential helps us measure just how good an oxidizing agent it is.

Chemical Equilibrium

In chemical reactions, the concept of equilibrium refers to the state where the rates of the forward and reverse reactions are equal, resulting in no net change in the concentration of the reactants and products over time. It's important when considering redox reactions within a battery or electrochemical cell, where the equilibrium can determine voltage outputs.

In the analysis of our exercise, equilibrium helps us understand that although the \(IO_3^-\) ion can act as an oxidizing agent in both mediums, it does so more effectively in acid. This is because, at equilibrium, the higher standard reduction potential in an acidic medium reflects a greater propensity for the forward reaction to occur, demonstrating that the \(IO_3^-\) ion is a stronger oxidizing agent under those conditions.