Problem 14 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

A solution of a metal ion when treated with KI gives a red precipitate which dissolves in excess KI to give a colourless solution. Moreover, the solution of metal ion on treatment with a solution of cobalt (II) thiocyanate gives rise to a deep blue crystalline precipitate. The metal ion is (a) \(\mathrm{Pb}^{2+}\) (b) \(\mathrm{Hg}^{2+}\) (c) \(\mathrm{Cu}^{2+}\) (d) \(\mathrm{Co}^{2+}\)

Step-by-Step Solution

Answer

The metal ion is \(\text{Hg}^{2+}\).

Step 1: Identify Red Precipitate

The problem states that a red precipitate forms when the metal ion solution reacts with KI. This is characteristic of a reaction producing mercury iodide, \(\text{HgI}_2\), which is red and known to follow this behavior.

Step 2: Dissolves in Excess KI

Mercury iodide \(\text{HgI}_2\) is known to dissolve in excess KI because it forms a complex, \[\text{HgI}_4^{2-}\text{(aq)},\text{ which is colorless.}\] This matches the description given in the problem.

Step 3: Reaction with Cobalt (II) Thiocyanate

The solution when treated with cobalt (II) thiocyanate gives a deep blue precipitate. This behavior is consistent with the formation of mercury(II) thiocyanate complexes which are typically associated with such coloration changes.

Step 4: Identification of Metal Ion

Only \(\text{Hg}^{2+}\) ion, from the options given, fits both the red precipitate formation with KI and the deep blue precipitate with cobalt (II) thiocyanate. Therefore, the correct answer is \(\text{Hg}^{2+}\).

Key Concepts

Metal Ion Identification

In inorganic chemistry, identifying metal ions in solution is a fundamental task, accomplished through various reactions yielding distinct changes in color or precipitate formation. Each metal ion possesses unique properties leading to specific results when interacting with particular reagents.
For this problem, the identification process begins by analyzing the type of reaction and the resulting product when the metal ion is treated with a reagent. Clues from these observations lead to deducing the correct identity of the metal ion through a systematic elimination of options based on observed characteristics. In our case, the dissolution behavior of the precipitate and further reactions with other compounds help narrow down the possibilities, finally indicating the metal ion as mercury (II) or \( \text{Hg}^{2+} \).

Precipitate Formation

Precipitate formation is a common occurrence in inorganic chemistry when two solutions are mixed, resulting in the formation of an insoluble solid. In this scenario, when the metal ion reacts with potassium iodide (KI), a red precipitate of mercury iodide (\( \text{HgI}_2 \)) forms.
This red precipitate is a key indicator in identifying the metal ion since different metal ions form unique precipitates with characteristic colors. The red hue of mercury iodide is one such distinct marker, making it easier to differentiate from other potential products.

• Color: Reddish hue, which is specific to mercury iodide.
• Reactions: Involves mixing a salt solution, here KI, with the metal ion solution, leading to formation of a solid precipitate.

Complex Formation with KI

When the red precipitate of mercury iodide is treated with an excess of KI, it dissolves to form a complex ion, \( \text{HgI}_4^{2-} \). This is a noteworthy transformation, where the previously insoluble precipitate now forms a soluble complex.
The chemistry here involves a coordination reaction where additional iodide ions coordinate with the inner mercury ion. The resulting complex ion is colorless, contrasting with the initial red precipitate, making it identifiable in lab settings.

• Dissolution: Requires excess KI to dissolve the red precipitate.
• Complex: A colorless aqueous solution evidence of complex formation.

Reaction with Cobalt (II) Thiocyanate

A fascinating aspect of this exercise is the metal ion's reaction with cobalt (II) thiocyanate, culminating in a deep blue crystalline precipitate. This specific reaction is characteristic of mercury (II) forming a complex with thiocyanate ions, which alters its visible spectrum resulting in a deep blue color.
The change from a colorless solution to a vivid blue precipitate is significant, assisting in differentiation between potential metal ions.
In conclusion, the correct metal ion among given options, forming distinctly colored precipitates with both KI and cobalt (II) thiocyanate, is \( \text{Hg}^{2+} \).