Problem 25 - Chemistry in 30 days

Question

Which of the following metal carbonates is decomposed on heating? (a) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (b) \(\mathrm{MgCO}_{3}\) (c) \(\mathrm{K}_{2} \mathrm{CO}_{3}\) (d) \(\mathrm{Rb}_{2} \mathrm{CO}_{3}\)

Step-by-Step Solution

Answer

\( \mathrm{MgCO}_{3} \) decomposes on heating.

Step 1: Understanding Thermal Decomposition

Thermal decomposition is a chemical reaction where a compound breaks down into simpler substances when heated. Metal carbonates decompose into metal oxides and carbon dioxide gas. For example, \( \mathrm{CaCO}_{3} \) decomposes into \( \mathrm{CaO} \) and \( \mathrm{CO}_{2} \). However, not all metal carbonates decompose easily. The ease of decomposition depends on the metal involved.

Step 2: Identify Metals with Stable Carbonates

Alkali metal carbonates (such as \( \mathrm{Na}_{2} \mathrm{CO}_{3} \), \( \mathrm{K}_{2} \mathrm{CO}_{3} \), and \( \mathrm{Rb}_{2} \mathrm{CO}_{3} \)) are generally stable and do not decompose easily upon heating. This is due to the low strength of the lattice energy and high thermal stability of these carbonates.

Step 3: Determine Less Stable Metal Carbonates

Alkaline earth metal carbonates, such as \( \mathrm{MgCO}_{3} \), decompose more readily upon heating. These metals have higher lattice energies compared to alkali metals, making their carbonates less stable when heated.

Step 4: Analyzing Given Options

From the options given: \( \mathrm{Na}_{2} \mathrm{CO}_{3} \), \( \mathrm{MgCO}_{3} \), \( \mathrm{K}_{2} \mathrm{CO}_{3} \), and \( \mathrm{Rb}_{2} \mathrm{CO}_{3} \), note that \( \mathrm{MgCO}_{3} \) is the only alkaline earth metal carbonate, while the others are alkali metal carbonates.

Step 5: Conclusion

Given our understanding, \( \mathrm{MgCO}_{3} \) is the compound that decomposes upon heating. It decomposes into \( \mathrm{MgO} \) and \( \mathrm{CO}_{2} \). Thus, option (b) is the correct answer.

Key Concepts

Metal Carbonates

Metal carbonates are a type of chemical compound that consist of a metal ion combined with the carbonate ion \((\text{CO}_3^{2-})\). They are often found in various materials like rocks and minerals.
When heated, many metal carbonates undergo a reaction known as thermal decomposition.
During this process, metal carbonates break down into metal oxides and carbon dioxide gas is released. For instance, the decomposition of calcium carbonate (\(\text{CaCO}_3\)) results in calcium oxide (\(\text{CaO}\)) and carbon dioxide (\(\text{CO}_2\)).

• Metal carbonates are used in many industrial applications due to their ability to decompose, producing either oxides useful in ceramics or carbon dioxide used in various chemical processes.
• This breakdown occurs because the heat provides the energy required to weaken and break the bonds within the carbonate compound.
• Different metal carbonates decompose at different temperatures, primarily based on the stability of the metal-carbonate combination.

Understanding which metal carbonates decompose easily is key to using them in processes that require specific reactions, such as the production of lime from limestone.

Alkali Metal Carbonates

Alkali metal carbonates are the compounds formed by combining alkali metals from Group 1 of the periodic table with the carbonate ion \((\text{CO}_3^{2-})\).
Common examples include sodium carbonate (\(\text{Na}_2\text{CO}_3\)), potassium carbonate (\(\text{K}_2\text{CO}_3\)), and rubidium carbonate (\(\text{Rb}_2\text{CO}_3\)).

• Unlike some other metal carbonates, alkali metal carbonates are generally more stable and do not decompose as easily upon heating.
• This stability is due to their low lattice energy, which means that the ions are content in their crystal structure and resist breaking apart with heat.
• The thermal stability of these compounds makes them useful in applications such as glass production and as common laboratory reagents.

Because of their stability, these carbonates require high temperatures to decompose. They typically stay intact under normal heating conditions, distinguishing them from their less stable counterparts, such as the alkaline earth metal carbonates.

Alkaline Earth Metal Carbonates

Different from alkali metals, alkaline earth metals (found in Group 2 of the periodic table) form carbonates that are more prone to decomposition upon heating.
Examples include magnesium carbonate (\(\text{MgCO}_3\)) and calcium carbonate (\(\text{CaCO}_3\)).

• These compounds have higher lattice energies than alkali metal carbonates, making them less stable when exposed to heat.
• Upon heating, alkaline earth metal carbonates readily decompose into their corresponding metal oxides and release carbon dioxide.
• For instance, magnesium carbonate decomposes to form magnesium oxide (\(\text{MgO}\)) and carbon dioxide (\(\text{CO}_2\)).

This propensity to decompose makes alkaline earth metal carbonates a valuable resource for producing metal oxides, which are essential components in various industrial processes, such as manufacturing cement and removing impurities in metal ores.