Problem 90 - A Complete Resource Book in Chemistry for JEE Main

Question

A metal nitrate on reaction with KI gives black precipitate and with excess of KI gives orange solution. The metal is (a) \(\mathrm{Hg}^{2+}\) (b) \(\mathrm{Pb}^{2+}\) (c) \(\mathrm{Cu}^{2+}\) (d) \(\mathrm{Bi}^{3+}\)

Step-by-Step Solution

Answer

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

Step 1: Identify the Reaction

When a metal nitrate reacts with KI, it can undergo a double displacement reaction to form a precipitate. For this problem, the metal ion that gives a black precipitate of metal iodide when reacting with KI is critical.

Step 2: Recognize Metal Iodide Properties

Understand which metals form a black precipitate with KI. The black precipitate generally indicates the formation of mercuric iodide (HgI2). Initially, \( \text{Hg}I_2 \) forms a black precipitate with KI. Upon adding excess KI, \( \text{Hg}I_2 \) dissolves to form \[ \text{K}_2[\text{HgI}_4] \], which is orange.

Step 3: Analyze Options Against Observations

Evaluate which metal from the list can form a black precipitate with KI and an orange solution with excess KI. From the listed options, \( \text{Hg}^{2+} \) forms a black precipitate of \( \text{HgI}_2 \) that turns orange with excess KI.

Step 4: Select the Correct Metal

Based on the properties and reactions discussed, \( \text{Hg}^{2+} \) satisfies both conditions: forming a black precipitate and an orange solution with excess KI. Other options do not undergo these exact reactions.

Key Concepts

Double Displacement Reactions

A double displacement reaction, often called a metathesis reaction, occurs when two compounds exchange ions in an aqueous solution. This results in the formation of two new compounds. These reactions can be represented by the general equation: \[ AB + CD \rightarrow AD + CB \]In the context of our exercise, when a metal nitrate reacts with potassium iodide (KI), the metal cation (for example, \(\mathrm{Hg}^{2+}\)) pairs with the iodide anion. This ion exchange can lead to the formation of new substances, like a precipitate, indicating the occurrence of a double displacement reaction. Such reactions are pivotal in qualitative inorganic analysis because they can help to identify unknown substances by observing the products formed from the reaction.

• These reactions are typically driven by one or more of the products being a solid precipitate.
• A gas or a non-ionizable compound like water can also form, driving the reaction forward.

Recognizing these reactions helps in predicting the outcomes of mixing different chemical solutions.

Precipitate Formation

The formation of a precipitate is a common result of double displacement reactions and is key in identifying chemical substances. When two aqueous solutions are mixed, if one of the products is insoluble in water, it forms a solid known as a precipitate. This solid is often an indicator of a specific reaction taking place.
In our problem, when potassium iodide (KI) reacts with a metal nitrate, it results in the formation of a black precipitate. Such black precipitates are often associated with certain metal iodides. For instance, mercuric iodide (\(\text{HgI}_2\)) often forms a striking black precipitate initially, which is a distinct identifier of its presence.

• The solubility product constant (\(K_{sp}\)) of the resulting compound influences whether a precipitate forms.
• A low \(K_{sp}\) indicates low solubility, hence more likelihood of precipitate formation.

Understanding precipitate formation is crucial in analyzing and identifying chemical reactions in qualitative analysis.

Metal Nitrates

Metal nitrates are chemical compounds consisting of metal cations combined with the nitrate anion (\(\text{NO}_3^-\)). These compounds are typically highly soluble in water, making them useful in reactions that require the formation of ionic solutions.
In the problem presented, the metal nitrate initially reacts with potassium iodide. The metal in these nitrates can vary, but common metal nitrates include those of lead, mercury, and copper. Their solubility makes them versatile reagents in various chemical reactions, particularly in displacement reactions where they provide the metal ions.

• The solubility and reactivity of metal nitrates make them pivotal in predicting the outcome of reactions.
• Understanding the properties of these nitrates can help us predict the precipitates and colors we might observe.

The ability to rapidly dissolve and react is what makes metal nitrates an integral part of qualitative inorganic analysis.

Iodide Reactions

Iodides, particularly potassium iodide (KI), are notable for their role in qualitative inorganic analysis, often serving as reactants that help identify other ions through colorimetric and precipitate-specific reactions. The distinctive color changes and precipitate formations that occur with iodide reactions make them a staple in chemical testing.In the given exercise, adding KI to a metal nitrate results in an initial black precipitate, which later turns orange upon adding excess KI due to the formation of \(\text{K}_2[\text{HgI}_4]\). This color change is a critical observation in identifying the specific metal involved, as it points to the presence of \(\text{Hg}^{2+}\) ions forming complex iodides.

• Iodine's high reactivity can lead to vivid color changes in solutions.
• The formation of complex ions, like \(\text{K}_2[\text{HgI}_4]\), can dramatically alter the color of solutions.

Thus, understanding iodide reactions aids significantly in the identification and confirmation of metallic ions in a solution.