Problem 70 - A Complete Resource Book in Chemistry for JEE Main

Question

When sodium argentocyanide is treated with zinc dust, silver precipitates because (a) zinc forms a complex readily with cyanide (b) zinc is more electropositive than silver (c) silver is more electropositive than zinc (d) both \(\mathrm{Zn}^{2+}\) and \(\mathrm{Ag}^{+}\)ions have \(\mathrm{d}^{10}\) electronic configuration

Step-by-Step Solution

Answer

(b) zinc is more electropositive than silver.

Step 1: Identify the Key Concept

The key concept here is the displacement reaction which occurs due to the electropositive nature of zinc.

Step 2: Understand Electropositivity

Electropositivity refers to the tendency of an element to lose electrons and form positive ions. Elements that are higher in the electrochemical series are more electropositive.

Step 3: Recall the Electrochemical Series

In the electrochemical series, zinc (Zn) is above silver (Ag), indicating that zinc is more electropositive than silver.

Step 4: Analyze the Reaction Conditions

When sodium argentocyanide is treated with zinc dust, zinc displaces silver due to its higher electropositivity, resulting in the precipitation of silver.

Step 5: Select the Correct Reason

Based on the analysis above, (b) zinc is more electropositive than silver is the correct reason for why silver precipitates.

Key Concepts

Electropositivity

Electropositivity is a fundamental concept in chemistry that refers to the ability of an element to donate electrons and form positive ions, also known as cations. This property is crucial when analyzing reactions, especially displacement reactions.
In such reactions, a more electropositive element displaces a less electropositive one from its compound.

• Highly electropositive elements tend to lose electrons easily and readily form cations.
• They are typically metals located on the left side of the periodic table.
• The higher the electropositivity, the greater the tendency to donate electrons.

Electropositivity plays a key role in determining the behavior of elements during chemical reactions by influencing their reactivity and ability to form various compounds. Understanding this concept helps in predicting the outcomes of reactions, like in the displacement reaction involving zinc and silver.

Electrochemical Series

The electrochemical series is a scientific tool that ranks elements based on their standard electrode potentials. It serves as a visual guide to determine which element can displace another in a chemical reaction.
The series lists elements from most to least electropositive, revealing their tendency to donate electrons.

• Elements higher up in the series can displace elements that are lower down, due to their greater electropositivity.
• Zinc (Zn) is located higher than silver (Ag), demonstrating zinc's ability to displace silver in reactions.
• This hierarchy helps chemists predict the feasibility of reaction events.

Using the electrochemical series, one can easily identify which metal will act as the reducing agent or which metal will undergo oxidation. It's essential for understanding displacement reactions, such as the one between sodium argentocyanide and zinc dust.

Zinc Argentocyanide Reaction

The reaction between sodium argentocyanide and zinc dust is a classic example of a displacement reaction. Here, zinc displaces silver from sodium argentocyanide, causing silver to precipitate.
The mechanism of this reaction highlights important chemical principles:

• As zinc is more electropositive than silver, it can donate electrons more readily.
• Zinc forms a stable complex with cyanide ions, which facilitates the displacement process.
• The displacement leads to the formation of zinc cyanide (\(\text{Zn(CN}_2)\)) and the release of metallic silver (\(\text{Ag} \)) as a precipitate.

This specific reaction not only demonstrates the concept of electropositivity but also showcases the utility of the electrochemical series in predicting the movement of electrons between different elements.