Problem 156 - A Complete Resource Book in Chemistry for JEE Main

Question

The method of thermal decomposition cannot be used for (a) Au (b) \(\mathrm{Hg}\) (c) \(\mathrm{Mg}\) (d) \(\mathrm{Ag}\)

Step-by-Step Solution

Answer

Thermal decomposition cannot be used for (c) Mg.

Step 1: Understanding Thermal Decomposition

Thermal decomposition is the process in which a compound is broken down into simpler substances when heated. Not all metals or their compounds are suitable for decomposition using heat; typically, only those with relatively low thermal stability and low melting points are amenable to this process.

Step 2: Exploring Thermal Stability

Metals like gold (Au) and silver (Ag) have very high thermal stability, meaning they do not easily break down into other substances upon heating. In contrast, mercury (Hg) compounds can decompose easily due to mercury's lower melting point.

Step 3: Identifying the Exception

Magnesium (Mg) is known for its high thermal stability and high melting point, making it unsuitable for thermal decomposition. When heated, magnesium burns brightly but does not decompose readily.

Step 4: Conclusion

Given these properties, the method of thermal decomposition cannot be effectively used for metals like magnesium (Mg) because it requires higher thermal energy and does not break down into simpler substances through this process.

Key Concepts

Thermal Stability

Thermal stability is a measure of a substance's resistance to decomposition when exposed to heat. High thermal stability implies that a material retains its structure even at elevated temperatures, making it unlikely to decompose into simpler substances easily.
Metals like gold (Au) and silver (Ag) are known for their exceptional thermal stability. They have highly ordered atomic structures that are resistant to heat. This makes them ideal for applications where they need to withstand high temperatures without altering their chemical structure.

• Gold and silver's stability stems from strong metallic bonds that require significant energy to break.
• Metals with high thermal stability generally also have high melting points.

Understanding thermal stability is crucial in predicting whether a compound will decompose thermally, as it directly impacts the level of thermal energy required to facilitate decomposition.

Thermal Energy

Thermal energy is the energy that comes from the temperature of a substance — the higher the temperature, the greater the thermal energy. It's essential for triggering thermal decomposition because it provides the energy needed to break down compounds.
When a substance is heated, its particles gain kinetic energy and start moving faster. This increase in movement can weaken the bonds holding the compound together, potentially leading to decomposition. However, this process depends heavily on the initial thermal stability of the compound or material.

• Thermal energy must be high enough to overcome the energy barriers of the substance’s bonds.
• Substances with low thermal stability require less energy to decompose.

Thus, metals like mercury (Hg), having compounds with lower thermal stability, will decompose with relatively lower thermal energy compared to metals such as magnesium (Mg) or gold (Au).

Metal Properties

The properties of metals, such as melting points and bond strength, play a vital role in their response to thermal decomposition. Metals are known for having unique structural properties that can either facilitate or hinder decomposition.
Melting points are a crucial part of understanding metal properties in thermal decomposition. Metals with low melting points tend to be more amenable to thermal decomposition, as their bonds require less energy to break apart.

• Gold and silver, with high melting points, resist decomposition under normal heating conditions.
• Mercury, with a low melting point, can more readily undergo thermal reactions.

Bonding nature, such as metallic bonds in pure metals, influences how metals handle heating and decomposition, making elements like magnesium resistant, even under high thermal energy conditions.

Decomposition Reaction

A decomposition reaction involves breaking down a compound into simpler substances. In thermal decomposition, heat energy initiates this breakdown, making thermal stability a key factor.
During a decomposition reaction, a single compound transforms into two or more different substances. This can occur through the application of heat, light, or chemical reactions. However, not all compounds are equally susceptible to thermal decomposition.

• Compounds of metals with lower thermal stability are more prone to decompose when heated.
• Decomposition reactions are utilized in various processes, such as in the extraction of metals from ores.

The understanding of decomposition reactions helps in the effective selection of methods for processing or altering materials, particularly in how different metals and compounds behave under heat.