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: Understand the Substances Involved

Sodium argentocyanide is a complex compound containing silver, and zinc dust is an active metal. We need to consider the chemical reaction between them, which causes silver to precipitate.

Step 2: Recall Basic Principles of Electropositivity

Electropositivity refers to the ability of an atom to donate electrons during a chemical reaction. In this context, we need to compare the electropositivity of zinc and silver.

Step 3: Identify Relative Electropositivity

Zinc is more electropositive than silver. This means zinc can donate electrons more readily, allowing it to replace silver in the complex, thereby precipitating silver during the reaction.

Step 4: Conclusion from Principles

Given that zinc is more electropositive, this leads to a displacement reaction. Zinc displaces silver from sodium argentocyanide, allowing silver to form a solid precipitate.

Key Concepts

Electropositivity

Electropositivity is a fundamental concept in chemistry that refers to the tendency of an element to donate electrons during a chemical reaction. In a displacement reaction, the element that is more electropositive is typically the one that will drive the reaction by replacing another element.
For example, zinc is more electropositive than silver. This means zinc has a greater tendency to lose electrons compared to silver.

• This property allows zinc to participate actively in reactions by becoming positively charged as it loses electrons.
• As a result, in a reaction involving sodium argentocyanide and zinc dust, zinc can effectively donate its electrons more readily to displace silver, leading to the precipitation of silver.

Understanding electropositivity helps predict reactions and outcomes based on an element's position in the reactivity series.

Zinc Reactions

Zinc is known for its reactive nature, especially when it comes into contact with certain compounds. In the context of the sodium argentocyanide complex, zinc's ability to replace other metals is instrumental.
When zinc dust is added to this complex, a displacement reaction occurs.

• Zinc finds it easier to react due to its higher electropositivity.
• This reaction replaces the silver present in the complex with zinc.
• The replacement happens because zinc atoms donate electrons to the complex, thereby ejecting silver ions out of the compound.

This type of reaction is characterized by zinc's readiness to undergo oxidation, highlighting its chemical reactivity and the principles of electropositivity driving the process.

Precipitation in Complexes

Precipitation reactions are a key part of chemistry that help in isolating elements or compounds from a solution. When zinc reacts with sodium argentocyanide, the outcome is the formation of solid silver.
In these scenarios, several principles of chemistry come into play:

• The replacement of silver by zinc, due to its higher electropositivity, triggers the formation of a solid silver precipitate.
• Since zinc forms stronger bonds within the cyanide complex, the silver ions are left without a binding partner and thus fall out of the solution as a solid.
• Precipitation happens when the ionic product exceeds the solubility product, leading to the solid formation.

Understanding such precipitation reactions can assist in anticipating the outcomes when reacting different metals and complexes.