Problem 59 - A Complete Resource Book in Chemistry for JEE Main

Question

When a copper wire is placed in a solution of \(\mathrm{AgNO}_{3}\), the solution acquires blue colour. This is due to the formation of (a) \(\mathrm{Cu}^{+}\)ions (b) \(\mathrm{Cu}^{2+}\) ions (c) soluble complex of copper with \(\mathrm{AgNO}_{3}\) (d) \(\mathrm{Cu}^{-}\)ion by the reduction of \(\mathrm{Cu}\)

Step-by-Step Solution

Answer

(b) \( \text{Cu}^{2+} \) ions form, causing the solution to turn blue.

Step 1: Identify the Reaction

When a copper wire is placed in a solution of silver nitrate, a single displacement reaction takes place between copper and silver nitrate, where copper displaces silver from the solution.

Step 2: Write Down the Chemical Equation

The reaction can be represented as: \[ \text{Cu} (s) + 2\text{AgNO}_3 (aq) \rightarrow \text{Cu(NO}_3)_2 (aq) + 2\text{Ag} (s) \]. Here, copper (Cu) is more reactive than silver (Ag), displacing Ag from the solution.

Step 3: Determine Resulting Ions

The copper in the wire is oxidized to \( \text{Cu}^{2+} \) ions as it forms copper(II) nitrate, which dissolves in the solution, giving it a blue color. The released \( \text{Cu}^{2+} \) ions are responsible for this characteristic color.

Step 4: Analyze the Options

Consider each given option:- (a) \( \text{Cu}^+ \) ions are not formed in this reaction.- (b) \( \text{Cu}^{2+} \) ions match with the reaction steps and result in a blue solution.- (c) No complex with \( \text{AgNO}_3 \) forms; copper forms copper nitrate.- (d) \( \text{Cu}^- \) ion formation is incorrect as copper undergoes oxidation.

Step 5: Choose Correct Answer

Based on the chemical reaction and analysis of options, the correct answer is (b) \( \text{Cu}^{2+} \) ions.

Key Concepts

Copper Reactivity

Copper is an interesting metal due to its moderate reactivity. It's not the most reactive metal, but it can displace some other metals from their compounds in a redox reaction. When copper wire is placed in silver nitrate solution, copper reacts with the silver ions. This is primarily because copper is slightly more reactive than silver. As a result, copper can replace the silver ions in the solution, forming copper nitrate.

This reactivity is based on a concept known as the reactivity series of metals. Metals like potassium and sodium are at the top, being highly reactive, while metals like gold and platinum lie at the bottom, being less so. Copper falls somewhere in the middle, which allows it to take part in reactions where its reactivity is just sufficient to dislodge less reactive metal ions from their solutions. Understanding this aspect of copper's behavior helps us grasp why it can react with silver nitrate, helping students to predict and understand outcomes of particular chemical reactions of metals.

Solution Color Change

The appearance of a blue color when copper wire is placed in silver nitrate solution is a clear indicator of a chemical change. Specifically, the color change is due to the formation of copper(II) ions, denoted as \(\text{Cu}^{2+}\), in solution. As the copper metal reacts and dissolves, these ions are released into the liquid, imparting a characteristic blue hue.

This blue color is a visual cue highlighting the presence of copper ions. It's important because it allows us to visually confirm the occurrence of a reaction without needing specialized equipment. Many copper compounds are known for their striking colors, which makes copper chemistry both fascinating and useful in various applications, such as creating visual evidence of chemical processes or serving as pigments in paints.

Displacement Reaction

The reaction between copper and silver nitrate solution is a classic example of a displacement reaction. Here, a more reactive metal (copper) displaces a less reactive metal (silver) from a compound, \(\text{AgNO}_3\). This results in the formation of silver metal and copper(II) nitrate, as shown in the equation:
\[ \text{Cu} (s) + 2\text{AgNO}_3 (aq) \rightarrow \text{Cu(NO}_3)_2 (aq) + 2\text{Ag} (s) \]

Displacement reactions are central to understanding redox chemistry, as they illustrate the basic principles of oxidation and reduction. In this case, copper undergoes oxidation (loss of electrons), while silver ions are reduced (gain of electrons). Understanding these reactions is crucial as they are used in many industrial processes, such as metal extraction or in galvanization, where one metal is used to protect another from corrosion.