Problem 141 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following carbonates cannot evolve \(\mathrm{CO}_{2}\) on heating? (a) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (b) \(\mathrm{Rb}_{2} \mathrm{CO}_{3}\) (c) \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) (d) \(\mathrm{MgCO}_{3}\)

Step-by-Step Solution

Answer

\( \mathrm{Na}_{2} \mathrm{CO}_{3} \) cannot evolve \( \mathrm{CO}_{2} \) on heating.

Step 1: Understand the Question

We need to determine which of the listed carbonates will not decompose to release carbon dioxide \( \mathrm{CO}_{2} \) upon heating. Some carbonates are stable and do not decompose easily.

Step 2: Review Carbonate Stability

Carbonates of alkali metals (like \( \mathrm{Na} \) and \( \mathrm{Rb} \)) generally do not decompose to release \( \mathrm{CO}_{2} \) easily because they are stable. However, exceptions exist for \( \mathrm{Li}_2\mathrm{CO}_3 \), which decomposes due to lithium's similarity with magnesium.

Step 3: Decomposition Reaction

For the given carbonates:- \( \mathrm{Na}_2 \mathrm{CO}_3 \) does not decompose to release \( \mathrm{CO}_2 \).- \( \mathrm{Rb}_2 \mathrm{CO}_3 \) is also stable and does not release \( \mathrm{CO}_2 \).- \( \mathrm{Li}_2 \mathrm{CO}_3 \) decomposes, releasing \( \mathrm{CO}_2 \).- \( \mathrm{MgCO}_3 \) decomposes to release \( \mathrm{CO}_2 \).

Step 4: Find the Correct Answer

Based on carbonate stability and whether they decompose to release \( \mathrm{CO}_2 \), the correct answer is the carbonate which does not decompose, which is \( \mathrm{Na}_2 \mathrm{CO}_3 \).

Key Concepts

Alkali Metal Carbonates

Alkali metal carbonates are compounds composed of an alkali metal and the carbonate ion (\(\text{CO}_3^{2-}\)). These metals include lithium (\(\text{Li}\)), sodium (\(\text{Na}\)), potassium (\(\text{K}\)), rubidium (\(\text{Rb}\)), and cesium (\(\text{Cs}\)).

These carbonates are generally stable at high temperatures compared to other metal carbonates. This is because alkali metals are less reactive with oxygen, which keeps the carbonate held together more tightly. Here are a few important points to remember about them:

• They typically do not break down easily when heated.
• An exception is \(\text{Li}_2\text{CO}_3\), due to lithium’s unique chemical behavior, which resembles that of magnesium.

The resemblance of lithium to magnesium, known as the "diagonal relationship," makes its carbonate less stable, causing \(\text{Li}_2\text{CO}_3\) to decompose at relatively lower temperatures compared to other alkali metal carbonates.

Decomposition Reactions

Decomposition reactions are chemical processes where a compound breaks down into simpler substances. For carbonates, this usually involves the breakdown of the compound into a metal oxide and carbon dioxide (\(\text{CO}_2\)).

Let's break down how this occurs with **carbonate decomposition**:

• If a carbonate decomposes, it must form a gas (\(\text{CO}_2\)) and a solid metal oxide.
• This reaction generally requires heat input to overcome the carbonate's stability.

For example, if you heat \(\text{MgCO}_3\), it decomposes based on the equation:
\[ \text{MgCO}_3 \rightarrow \text{MgO} + \text{CO}_2 \]
Diamond formation patterns and their chemistries can vary across different compounds, altering heat decomposition thresholds.

Thermal Stability of Carbonates

The thermal stability of carbonates refers to their resistance to decomposing when heated. In general, the stability of metal carbonates depends on the strength of the bond between the metal ion and the carbonate ion (\(\text{CO}_3^{2-}\)).

**Alkali Metals:**

• Alkali metal carbonates are usually highly stable due to strong electron affinity of \(\text{CO}_3^{2-}\).
• \(\text{Na}_2\text{CO}_3\) and \(\text{Rb}_2\text{CO}_3\) are excellent examples of such remarkable thermal stability.
• These do not release \(\text{CO}_2\) even at high temperatures, illustrating strong structural integrity.

**Exceptions to Stability:**

• \(\text{Li}_2\text{CO}_3\), unlike its alkali cousins, will decompose to \(\text{Li}_2\text{O}\) and \(\text{CO}_2\).
• This happens because the bond between lithium and the carbonate ion is weaker than the others.