Problem 135 - A Complete Resource Book in Chemistry for JEE Main

Question

The soldiers of Napolean's army, while on Alps during freezing winter, suffered a serious problem as regards to the tin buttons of their uniforms. White metallic tin buttons got converted to grey powder. This transformation is related to (a) an interaction with nitrogen of the air at very low temperatures (b) a change in the crystalline structure of tin (c) an interaction with water vapour contained in the humid air (d) a change in the partial pressure of oxygen in the air.

Step-by-Step Solution

Answer

The transformation is related to (b) a change in the crystalline structure of tin.

Step 1: Understanding the Problem

The soldiers of Napolean's army experienced a conversion of their tin buttons into grey powder. This transformation occurs in a chemical or physical change context, particularly under certain environmental conditions. Our task is to identify the process responsible for this transformation.

Step 2: Analyzing the Options

We have four options: (a) interaction with nitrogen at low temperatures, (b) change in crystalline structure, (c) interaction with water vapor, and (d) change in oxygen's partial pressure. We'll evaluate these to determine which one explains the physical transformation observed.

Step 3: Evaluating Tin's Properties

Tin exists in two allotropes: white (β-tin) and gray (α-tin). At colder temperatures, tin can transform from its metallic form (white) to a non-metallic powdered form (gray), known as tin pest. This change is due to a rearrangement in the crystalline structure, usually occurring at temperatures below 13°C.

Step 4: Connecting Tin Pest with the Problem

The conversion of white tin to gray powder is precisely due to a change in the crystalline structure at low temperatures, similar to what Napolean's soldiers experienced in the Alps. This explains the transformation described in the exercise.

Step 5: Selecting the Correct Option

Given our analysis, the explanation that corresponds to the transformation of tin buttons into gray powder due to low temperatures is option (b) a change in the crystalline structure of tin.

Key Concepts

Tin Allotropes

Tin is fascinating due to its ability to exist in different forms known as allotropes. Allotropes are simply different physical forms of the same element. In the case of tin, it commonly appears as two allotropes:

• White Tin (β-tin): This is the metallic form of tin, which is stable at room temperature and has a shiny surface.
• Gray Tin (α-tin): This is a non-metallic form that appears as a gray powder and becomes stable at low temperatures. The transition from white to gray tin can have significant effects on materials, as seen with Napolean's soldiers.

This change in form is not just a simple cosmetic change but involves a deep structural transformation at the atomic level.

Phase Transformation

The transition from one allotrope to another in tin is known as a phase transformation. This usually involves a change in the crystalline structure of the material.
At colder temperatures, specifically below 13°C, white tin can begin to convert to gray tin.
This transformation is often called 'tin pest'. It is particularly problematic for objects made of tin in cold environments:

• The atomic arrangement in white tin is a tetragonal lattice that transforms into a cubic structure in gray tin.
• Although only a phase shift, this transformation can cause physical deterioration as the material falls apart, turning into powder.

This transformation can be accelerated by impurities or surface defects.

Chemical Properties of Tin

Besides its allotropes and transformations, tin's chemical properties also play a crucial role in its behavior. Tin is generally quite stable and does not react easily with water or oxygen at room temperature, owing to a thin oxide layer forming on its surface. However, under certain conditions, chemical behavior might shift:

• Reaction with Oxygen: In high temperatures, tin can oxidize, changing properties.
• Interaction with Acids and Alkalis: Tin resists corrosion in many substances, but certain strong acids or alkalis can corrode it.
• Brittle Transformation: The phase change to gray tin encompasses a chemical process because it alters tin's structural integrity.

These chemical properties help us understand why tin was historically favored for protective coatings but could still succumb to specific environmental conditions like those in the Alps.