Problem 161 - A Complete Resource Book in Chemistry for JEE Main

Question

Amount of oxalic acid present in a solution can be determined by its titration with \(\mathrm{KMnO}_{4}\) solution in the presence of \(\mathrm{H}_{2} \mathrm{SO}_{4} .\) The titration gives unsatisfactory result when carried out in the presence of \(\mathrm{HCl}\), because of (a) Gets oxidized by oxalic acid to chlorine (b) Furnishes \(\mathrm{H}^{+}\)ions in addition to those from oxalic acid (c) Reduces permanganate to \(\mathrm{Mn}^{2+}\) (d) Oxidises oxalic acid to carbon dioxide and water

Step-by-Step Solution

Answer

The correct option is (c): \(\mathrm{HCl}\) reduces permanganate to \(\mathrm{Mn}^{2+}\).

Step 1: Understanding the Problem

We need to understand why using \(\mathrm{HCl}\) in the titration with oxalic acid and \(\mathrm{KMnO}_4\) gives unsatisfactory results. We have to choose from the given options (a through d) what happens when \(\mathrm{HCl}\) is present.

Step 2: Analyzing the Chemical Reaction

In this titration, oxalic acid \((\mathrm{H}_2\mathrm{C}_2\mathrm{O}_4)\) is oxidized by \(\mathrm{KMnO}_4\) in an acidic medium. Usually, sulfuric acid \((\mathrm{H}_2\mathrm{SO}_4)\) is used to provide an acidic environment, but the role of hydrochloric acid \((\mathrm{HCl})\) needs to be evaluated.

Step 3: Evaluating Options with \(\mathrm{HCl}\)

We consider each option: \(\mathrm{HCl}\) could get oxidized to chlorine gas, which is option (a). Option (b) suggests \(\mathrm{HCl}\) provides additional \(\mathrm{H}^+\) ions, which is acceptable for reaction completion. Option (c) says \(\mathrm{HCl}\) reduces permanganate ion, leading to undesired products, making this a key unsatisfactory result.

Step 4: Identifying the Correct Option

Options (a) and (c) are the most concerning because they involve side reactions with \(\mathrm{HCl}\). Option (c), reducing \(\mathrm{KMnO}_4\) to \(\mathrm{Mn}^{2+}\), is undesirable because it interferes with the intended reaction path, making the titration results unsatisfactory.

Step 5: Conclusion

The presence of \(\mathrm{HCl}\) leads to its reduction of \(\mathrm{KMnO}_4\) to \(\mathrm{Mn}^{2+}\), which directly interferes with the titration of oxalic acid.

Key Concepts

Oxidation-Reduction Reactions

Oxidation-reduction reactions, often referred to as redox reactions, are fundamental to understanding many chemical processes. In a redox reaction, one substance is oxidized (loses electrons), and another is reduced (gains electrons). These reactions are crucial in titrations, such as when determining the concentration of oxalic acid using potassium permanganate (\( \mathrm{KMnO}_4 \)).In the specific titration of oxalic acid (\( \mathrm{H}_2\mathrm{C}_2\mathrm{O}_4 \)), the oxalic acid is the reducing agent and is oxidized to carbon dioxide (\( \mathrm{CO}_2 \)), whereas the permanganate ion (\( \mathrm{MnO}_4^- \)) is reduced to manganese ion (\( \mathrm{Mn}^{2+} \)). This type of reaction requires a correct acidic medium to facilitate the transfer of electrons between the reactants, ensuring that the target reactions proceed as planned.

Chemical Reaction Analysis

Analyzing chemical reactions involves studying the reactants, products, and the conditions required for a reaction to proceed effectively. For the titration of oxalic acid with potassium permanganate, it's essential to understand that reactions occur optimally in an acidic environment.The presence of (\( \mathrm{H}_2\mathrm{SO}_4 \)) (sulfuric acid) is preferred as it provides the necessary (\( \mathrm{H}^+ \)) ions without participating in side reactions that might alter the reaction pathway. However, when (\( \mathrm{HCl} \)) (hydrochloric acid) is used instead, it creates additional side reactions. One major side reaction is the reduction of permanganate ions to manganese ions, which does not involve oxalic acid and alters the balance of the intended titration, leading to inaccurate results.

Role of HCl in Titration

The role of hydrochloric acid (\( \mathrm{HCl} \)) in the titration process of oxalic acid must be clearly understood to avoid errors. Although (\( \mathrm{HCl} \)) can furnish extra (\( \mathrm{H}^+ \)) ions, its tendency to engage in undesirable side reactions makes it less ideal for this titration.One key issue is that (\( \mathrm{HCl} \)) can be oxidized to chlorine gas, which disrupts the redox balance needed for accurate titrations. More critically, (\( \mathrm{HCl} \)) reduces (\( \mathrm{KMnO}_4 \)) to (\( \mathrm{Mn}^{2+} \)) without involving oxalic acid. Due to this, it's advised to utilize acids like (\( \mathrm{H}_2\mathrm{SO}_4 \)) that maintain the integrity of the principal reaction without promoting side reactions.