Problem 196 - A Complete Resource Book in Chemistry for JEE Main

Question

The soldiers of Napolean army, while at 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 power. This transformation is related to (a) a change in the partial pressure of oxygen in the air (b) an interaction with water vapour contained in the humid air (c) a change in the crystalline structure of tin. (d) an interaction with nitrogen of the air at very low temperatures

Step-by-Step Solution

Answer

The transformation is due to a change in the crystalline structure of tin (option c).

Step 1: Understanding the Problem

Napolean's army encountered an issue with their tin buttons transforming from white metallic tin to grey powder during the freezing winter. We need to determine why this transformation occurred from the given options.

Step 2: Evaluating the Options - Change in Partial Pressure of Oxygen

Option (a) suggests that a change in the partial pressure of oxygen caused the transformation. Tin, however, is not known for reacting significantly with oxygen to cause such a transformation, so this option is unlikely.

Step 3: Evaluating the Options - Interaction with Water Vapour

Option (b) implies that the transformation was due to an interaction with water vapour in humid air. While tin can react with water, this would often result in corrosion rather than a complete transformation into powder.

Step 4: Evaluating the Options - Change in Crystalline Structure

Option (c) suggests a change in the crystalline structure of tin caused the transformation. At low temperatures, tin undergoes a phase change known as "tin pest" where white tin (β-tin) changes to grey tin (α-tin), resulting in the powdery form.

Step 5: Evaluating the Options - Interaction with Nitrogen

Option (d) attributes the transformation to an interaction with nitrogen at low temperatures. Tin is relatively non-reactive with nitrogen, making this option improbable.

Step 6: Conclusion

After evaluating all the options, the most logical explanation is a change in the crystalline structure of tin, which matches the phenomenon of tin pest.

Key Concepts

Crystalline Structure Change

When Napolean's army faced issues with the transformation of their tin buttons, it was largely due to a change in the crystalline structure of tin. This transformation is known as the "tin pest." Tin, which is initially in the form of white metallic tin (β-tin), can change into a different form called grey tin (α-tin). This structural shift happens at low temperatures, causing the tin to become brittle and powdery.

This structural change occurs because each form of tin has a different crystalline lattice arrangement:

• β-tin is stable at temperatures above 13.2°C and has a tetragonal crystalline structure.
• α-tin becomes stable below 13.2°C and has a cubic structure, which is less dense and weaker.

The lower density and altered structure of α-tin produce the powdery consistency observed in the buttons.

Phase Transformation

Phase transformation is a critical concept in understanding materials like tin that possess multiple structural phases. In the case of tin pest, we observe a phase transformation from β-tin to α-tin, driven by temperature changes.

Phase transformations involve:

• A shift from one crystalline or molecular phase to another.
• Changes in properties such as density, hardness, and volume.
• Specific conditions like temperature or pressure influencing the transformation.

For tin, the phase transformation leads to significant physical changes. The once solid and metallic β-tin transforms to the more brittle and crumbly α-tin at lower temperatures.

Low Temperature Effect

The primary trigger for the transformation of tin in Napolean's army buttons was the cold environment in the Alps. Low temperatures can have pronounced effects on materials, particularly metals.

As temperatures drop:

• Materials may undergo phase transformations, like tin turning from β to α phase.
• The reduced kinetic energy in atoms lowers the threshold for structural changes.
• Thermal expansion is minimized, influencing material properties.

In the case of tin, the cold temperatures initiated and accelerated the transformation process resulting in the notorious tin pest.

Napolean's Army Buttons

Napolean's army had a notable historical problem due to the phenomenon known as tin pest. It is suspected that one of the reasons for the army's difficulties during their campaign in Russia was the deterioration of their tin buttons.

The journey involved traversing extremely cold regions:

• The tin buttons, designed for durability, weakened and crumbled in the cold.
• Key components of uniforms failed, contributing to logistical and morale issues.
• Surviving accounts highlight this failure as a significant hardship endured by the soldiers.

Understanding this event in history underscores the importance of material science and the impacts of environmental conditions on military equipment.