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

Question

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

Step-by-Step Solution

Answer

The method of thermal decomposition cannot be used for Magnesium (\(\mathrm{Mg}\)).

Step 1: Understand Thermal Decomposition

Thermal decomposition involves breaking down a compound into simpler substances when it is heated. Metals that decompose on heating are generally those that form less stable oxides. For this exercise, consider which metals form stable oxides that will not easily decompose.

Step 2: Metal Reactivity and Stability

Evaluate the stability of the oxides of the given metals. Gold ((Au)) and Mercury ((Hg)) form less stable oxides that can be decomposed by heat. However, Magnesium ((Mg)) forms a highly stable oxide ((MgO)) which generally does not decompose on heating.

Step 3: Conclusion

Based on the stability of the metal oxides, the method of thermal decomposition cannot be effectively used for Magnesium ((Mg)) as it forms a stable oxide that does not easily decompose.

Key Concepts

Metal oxides stability

Metal oxides vary in terms of stability, which significantly affects their behavior during thermal decomposition. When a metal oxide is stable, it remains in its compound form even when subjected to heat.
This is because stable oxides have strong bonds between the metal and oxygen atoms, making them resistant to decomposition.
For example, magnesium oxide (MgO) is a highly stable oxide. The bonds in MgO are so robust that they do not break easily with heat, meaning magnesium oxide does not decompose into magnesium and oxygen under typical heating conditions.

• Stable oxides resist heat-induced decomposition.
• The strength of metal-oxygen bonds plays a key role in stability.
• Magnesium oxide (MgO) is an example of a very stable metal oxide.

Metal reactivity

Metal reactivity refers to how quickly or violently a metal reacts with other substances, including how their oxides behave when heated.
Generally, reactive metals like alkali metals form less stable oxides, which can decompose more readily under heat. However, this is not a universal rule as some less reactive metals can also form unstable oxides.
Gold (Au) and mercury (Hg) are classic examples of metals with low reactivity but form oxides that decompose upon heating. Their oxides are less stable because the metal-oxygen bonds are weaker, making decomposition more feasible when exposed to thermal energy.

• Reactivity often aligns with unstable oxide formation.
• Stable oxides are not always formed by low-reactivity metals.
• Gold and mercury oxides are easily decomposed by heat.

Chemical decomposition

Chemical decomposition is the process where a single compound breaks down into simpler components. Thermal decomposition is a specific type where heat induces this process.
Not all metal oxides undergo this change when heated. The likelihood of decomposition is determined by the stability of the compound.
With stable oxides like MgO, heat fails to break down the structure. However, less stable oxides such as those of gold and mercury will disintegrate, reverting to their elements. Understanding this aspect is crucial in chemistry, as it guides the selection of metals for various processes based on their decomposition tendencies.

• Thermal decomposition needs heat to break down compounds.
• Compound stability dictates decomposition potential.
• Understanding decomposition guides industrial applications.