Problem 27 - A Complete Resource Book in Chemistry for JEE Main

Question

A metal chloride solution on mixing with \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) solution gives a yellow precipitate, insoluble in acetic acid. The metal may be (a) zinc (b) lead (c) silver (d) mercury

Step-by-Step Solution

Answer

The metal is lead.

Step 1: Understand the Reaction

When a metal chloride solution reacts with potassium dichromate (\(\mathrm{K}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\)) and forms a yellow precipitate, we need to identify the properties of this precipitation reaction.

Step 2: Recall the Chemistry of the Precipitate

The yellow precipitate formed by the reaction of a metal chloride with \(\mathrm{K}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\) is characteristic of lead chromate (\(\mathrm{PbCrO}_{4}\)). Lead dichromate is insoluble in acetic acid.

Step 3: Connect with the Options

Compare the metal that forms lead chromate with the given options. From our understanding that the yellow precipitate indicates \(\mathrm{PbCrO}_{4}\), match lead (b) from the options given as it forms \(\mathrm{PbCrO}_{4}\), which is insoluble in acetic acid.

Key Concepts

Precipitation Reaction

Precipitation reactions occur when two aqueous solutions combine to form an insoluble solid known as a precipitate. These reactions are common in chemistry and are generally characterized by the following:

• Mixing of aqueous solutions of ionic compounds.
• Formation of a solid product that settles out of the solution.
• The solid formed is insoluble in the solvent present.

When predicting the products of a precipitation reaction, it's important to understand the solubility rules, which help in determining which compounds will precipitate and which will remain dissolved. These rules state which combinations of ions form insoluble products. In the given problem, the yellow precipitate emerges when metal chloride reacts with potassium dichromate, highlighting a characteristic precipitation reaction of the metal involved.

Metal Chlorides

Metal chlorides are compounds that consist of metal cations and chloride anions. These salts are usually soluble in water but can show varied reactions when mixed with other solutions. Here are some general properties:

• The solubility can vary depending on the metal involved.
• They often participate in double displacement reactions.
• React with other compounds leading to the formation of precipitates or gases.

In the problem context, a metal chloride reacts with potassium dichromate to yield a specific colored precipitate. For example, lead(II) chloride reacts with potassium dichromate to form lead dichromate or lead chromate, which is distinctively yellow and insoluble in acetic acid.

Potassium Dichromate Reaction

Potassium dichromate ( K_{2}Cr_{2}O_{7} ) is a common chemical reactant in precipitation reactions due to its ability to oxidize other substances, discolor water, or form distinct precipitates. The reaction of potassium dichromate with metal chlorides, such as in our problem,

• Shows the formation of insoluble chromates when certain metal ions are involved.
• Involves a color change due to the formation of colored precipitates, like that of lead chromate, which is yellow.
• Often provides insights into the properties of the involved metal ions based on the observed precipitate.

This reaction is useful in distinguishing metal chlorides that can form insoluble chromates, allowing identification based on the color and solubility properties of the formed precipitate.