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: Understanding the Problem

We need to determine why silver precipitates when sodium argentocyanide is treated with zinc dust. The options give us reasons related to the behavior of zinc and silver in this chemical reaction.

Step 2: Comparing Electropositivity

Electropositivity refers to an element's tendency to lose electrons and form positive ions. In this context, the more electropositive metal will tend to displace the less electropositive metal from a solution. Zinc is a more electropositive metal than silver.

Step 3: Result of Electropositive Comparison

Since zinc (Zn) is more electropositive than silver (Ag), it can displace silver from the sodium argentocyanide complex, leading to silver precipitating out of the solution. This is due to the greater tendency of zinc to form cations compared to silver.

Key Concepts

Electropositivity

In the world of electrochemistry, electropositivity is a key term that describes how easily an element can lose electrons to form positive ions. Metals like zinc and silver have unique positions in the electrochemical series based on their electropositivity.

• An element with higher electropositivity tends to lose electrons more readily.
• When placed in a competitive environment, a more electropositive metal will have the ability to displace a less electropositive metal from its compound.

In our case, zinc is more electropositive than silver. This means zinc has a higher tendency to lose electrons and form cations than silver does. Thus, when both zinc and silver are in a solution, zinc is more likely to "kick out" silver from the complex. This ability to replace silver in sodium argentocyanide highlights the basic concept of electropositivity where zinc's tendency to form ions is stronger than that of silver.

Displacement reaction

A displacement reaction, particularly in the context of metals, is a type of reaction where a more reactive metal displaces a less reactive metal from its compound. This is exactly what happens when zinc dust is added to sodium argentocyanide.

• Here, zinc, being more electropositive, replaces silver in the argentocyanide complex.
• The metal with a higher tendency to form positive ions (zinc) takes the place of the metal with a lesser tendency (silver).

The end result of such a reaction is often a visible change; in this case, the formation of metallic silver precipitating out of the solution. This displacement is a classic example of a single replacement reaction that you can observe physically, where zinc's reactivity leads to silver being set free from its complex state.

Complex formation

Complex formation in chemistry refers to the creation of a structure composed of several components. In the context of sodium argentocyanide, we are dealing with a complex ion containing silver and cyanide ions.

• Complex ions are typically formed when a central metal ion binds with one or more molecules or ions—called ligands—via coordinate covalent bonds.
• Sodium argentocyanide is an example where the silver ion (\(\text{Ag}^+\)) is part of a larger entity formed with cyanide ions.

When zinc dust is added to the solution, the electropositivity difference results in zinc forming its complex, freeing silver ions. The essential understanding of complex formation is crucial to predicting and explaining reactions where elements are liberated or precipitated.