Question

The correct option(s) to distinguish nitrate salts of \(\mathrm{Mn}^{2+}\) and \(\mathrm{Cu}^{2+}\) taken separately is (are) (A) \(\mathrm{Mn}^{2+}\) shows the characteristic green colour in the flame test (B) Only \(\mathrm{Cu}^{2+}\) shows the formation of precipitate by passing \(\mathrm{H}_{2} \mathrm{~S}\) in acidic medium (C) Only \(\mathrm{Mn}^{2+}\) shows the formation of precipitate by passing \(\mathrm{H}_{2} \mathrm{~S}\) in faintly basic medium (D) \(\mathrm{Cu}^{2+} / \mathrm{Cu}\) has higher reduction potential than \(\mathrm{Mn}^{2+} / \mathrm{Mn}\) (measured under similar conditions)

Short answer

Option B (Only \text{\

Step-by-step solution

Understanding the options

First, understand what each option is implying about the properties of manganese(II) and copper(II) salts. These properties are related to color identification and reaction with hydrogen sulfide in different pH environments.

Option A Analysis

Check the validity of the statement that manganese(II) ions display a characteristic green color in the flame test. Research or check a reliable source to confirm that manganese doesn't exhibit a green flame and it's known that manganese does not give a distinctive flame color.

Option B Analysis

Research or use a chemical reference to confirm whether copper(II) ions form a precipitate with hydrogen sulfide in an acidic medium. The formation of a black precipitate of copper sulfide confirms that this option is a correct way to distinguish between manganese(II) and copper(II).

Option C Analysis

Research shows that manganese(II) ions react with hydrogen sulfide in a basic medium to form pinkish-brown manganese(III) sulfide. This reaction does not occur in the faintly basic medium, so this option is incorrect.

Option D Analysis

Consider the standard reduction potentials of copper and manganese. Copper indeed has a higher reduction potential than manganese which makes it easier to reduce, and thus can be distinguished based on this property.

Key concepts

Flame Test

The flame test is a qualitative analytical procedure used to detect the presence of certain metal ions in compounds based on the color of the flame produced when the substance is heated. Each metal ion can produce a distinctive color when vaporized in flame due to the excitement of electrons.

For instance, copper ions typically produce a blue-green flame, while sodium gives off a bright yellow. The test is simple and can be done using just a few drops of a solution placed on a clean wire loop, which is then held in a Bunsen burner flame. In the context of the original exercise, manganese ions do not show a characteristic green color, as mentioned erroneously in option (A), which makes this option incorrect for distinguishing between manganese(II) and copper(II) nitrate salts.

The flame test is an important tool in analytical chemistry but has limitations due to its qualitative nature and interference by other ions. Therefore, it's often used in conjunction with other tests for a more accurate analysis.

Precipitation Reaction

A precipitation reaction occurs when two soluble substances react together in solution to form an insoluble solid called a precipitate. Precipitates are often formed when two ionic solutions are mixed and an insoluble compound is produced, which sinks out of the solution.

In analytical chemistry, precipitation reactions are useful for separating and identifying ions. For example, when discussing nitrate salts of manganese(II) and copper(II), passing hydrogen sulfide gas (H2S) through their solutions can lead to different outcomes. If a black precipitate forms, as it does with copper(II) in an acidic medium, we confirm the presence of copper(II) ions, which makes option (B) a correct method for differentiation.

Impact of pH on Precipitation

Manganese(II), in contrast, does not form a precipitate when exposed to hydrogen sulfide in a faintly basic medium, disproving option (C) as a valid test. The pH level of the solution can greatly influence the solubility of the products and thus the outcome of precipitation reactions.

Standard Reduction Potential

The standard reduction potential (SRP) is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. It is measured under standard conditions, which include a temperature of 298 K, a 1 M concentration, and a pressure of 1 atm for gases.

The SRP values are usually given with respect to the standard hydrogen electrode (SHE), which has a potential of 0 volts. A higher SRP indicates a greater likelihood that the species will gain electrons and be reduced.

Comparative SRP for Metal Ions

In the context of distinguishing between manganese(II) and copper(II), option (D) suggests using their SRP values. Copper(II) ions have a higher reduction potential compared to manganese(II) ions indicating copper(II) is more readily reduced to copper metal. This difference in SRP provides a valuable insight for chemists, allowing them to predict the feasibility of redox reactions and form a basis for distinguishing between different metal ions more scientifically.