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

(c) a change in the crystalline structure of tin.

Step 1: Understanding the Problem

You need to determine the reason why the tin buttons transformed from white metallic tin to grey powder during freezing winter. Each option explores a different chemical or physical condition affecting tin.

Step 2: Evaluating Option (a)

Option (a) suggests that the transformation is due to a change in the partial pressure of oxygen in the air. Since oxygen changes are more related to oxidation, it is unlikely to cause tin to change crystalline form under these conditions.

Step 3: Evaluating Option (b)

Option (b) proposes that the interaction with water vapour in humid air led to the transformation. Interaction with water might lead to corrosion, but not necessarily a change in crystalline structure.

Step 4: Evaluating Option (c)

Option (c) discusses a change in the crystalline structure of tin. Tin can transform from a metallic form (white beta-tin) to a non-metallic form (grey alpha-tin) at low temperatures, which is known as 'tin pest'.

Step 5: Evaluating Option (d)

Option (d) considers interaction with nitrogen at low temperatures. This is less likely because nitrogen is generally inert and transformations involving crystalline structure would not usually involve nitrogen.

Step 6: Making a Decision

After evaluating all options, option (c) is the best choice. The transformation of tin buttons during cold temperatures fits well with the known phenomenon of the tin pest, which is a change in the crystalline structure of tin.

Key Concepts

Crystalline Structure

The crystalline structure of a material is the specific arrangement of atoms that repeat periodically in three dimensions, forming the structure of a crystal. Tin, a common metallic element, showcases interesting behavior in its crystalline form. At normal temperatures, tin exists in the white, metallic structure known as beta-tin. This structure gives tin its familiar metallic properties, like luster and conductivity.

However, at lower temperatures, particularly under freezing conditions, tin can undergo a fundamental transformation in its crystalline structure. This transformation is from beta-tin to alpha-tin, where the orderly arrangement of atoms shifts, significantly affecting tin’s properties.

• Beta-tin is metallic and conductive.
• Alpha-tin, also known as grey tin, is a brittle and non-metallic.

This shift in structure, from a well-ordered metallic arrangement to a powdery form, is critical in understanding materials like tin that are sensitive to temperature variations. In Napoleon's army, such transformation at cold temperatures led to the decay of tin buttons, a problem now famously known as tin pest.

Phase Transformation

Phase transformation is the process of changing one form of a substance into another, where each form is called a phase. For many materials, including tin, these transformations can be driven by changes in temperature. The unique aspect of tin is how it transforms so visibly and dramatically at low temperatures.

This type of transformation is critical in metallurgy, the study and technology of metals. In tin's case, when cooled below 13.2°C, beta-tin (white tin) begins to convert into alpha-tin (grey tin), altering its phase.

• The beta-tin phase is stable at room temperature and above.
• Alpha-tin becomes stable at very low temperatures, like those in the cold Alps.

The phase transformation is not immediate and can be slow, depending on environmental factors such as temperature and humidity. Understanding phase transformations enables us to predict and manage materials’ behavior in varying conditions, crucial in applications like engineering and manufacturing.

Chemical Reactions at Low Temperature

Chemical reactions can behave differently under varying temperatures, which can influence how materials transform. In the case of tin buttons exposed to the severe cold of the Alps, the transformation was not driven by a typical chemical reaction like oxidation, which involves oxygen, but by a physical change due to low temperatures.

At temperatures below 13.2°C, tin chemically changes its state due to a shift in its atomic arrangement, leading to the transformation from beta-tin to alpha-tin. Here, temperature acts as the key catalyst rather than the traditional reactions seen in other scenarios, such as oxidation involving oxygen or rust involving iron.

• Low temperatures can often slow down chemical reactions, but in tin, they trigger a shift.
• This shift does not involve external chemical agents like oxygen, nitrogen, or water vapor. It is a notable exception in material science because the transformation is internal.

The phenomenon signifies the differences in chemical reactivity depending on environmental temperature conditions. This showcases the importance of considering environmental factors when evaluating materials for specific conditions, like severe cold or heat, to ensure stability and functionality.