Problem 126 - A Complete Resource Book in Chemistry for JEE Main

Question

The species present in solution when \(\mathrm{CO}_{2}\) is dissolved in water are (a) \(\mathrm{CO}_{2}, \mathrm{H}_{2} \mathrm{CO}_{3}, \mathrm{HCO}_{3}=\mathrm{CO}_{3}^{2=}\) (b) \(\mathrm{H}_{2} \mathrm{CO}_{3}, \mathrm{CO}_{3}^{2-}\) (c) \(\mathrm{CO}_{3}^{2-}, \mathrm{HCO}_{3}^{-}\) (d) \(\mathrm{CO}_{2}, \mathrm{H}_{2} \mathrm{CO}_{3}\)

Step-by-Step Solution

Answer

The species present are \(\mathrm{CO}_2\), \(\mathrm{H}_2\mathrm{CO}_3\), \(\mathrm{HCO}_3^-\), and \(\mathrm{CO}_3^{2-}\); option (a) is correct.

Step 1: Understand the context

When carbon dioxide (\(\mathrm{CO}_2\)) is dissolved in water, it undergoes a series of equilibria and forms different chemical species. These species include dissolved \(\mathrm{CO}_2\), carbonic acid (\(\mathrm{H}_2\mathrm{CO}_3\)), bicarbonate ion (\(\mathrm{HCO}_3^-\)), and carbonate ion (\(\mathrm{CO}_3^{2-}\)).

Step 2: Analyze the equilibrium reactions

The dissolution of \(\mathrm{CO}_2\) in water is represented by these reactions: \[ \mathrm{CO}_2 + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{H}_2\mathrm{CO}_3 \] Then, \(\mathrm{H}_2\mathrm{CO}_3\) can dissociate into \(\mathrm{HCO}_3^-\) and \(\mathrm{H}^+\): \[ \mathrm{H}_2\mathrm{CO}_3 \rightleftharpoons \mathrm{HCO}_3^- + \mathrm{H}^+ \] Additional dissociation can occur from \(\mathrm{HCO}_3^-\) to \(\mathrm{CO}_3^{2-}\): \[ \mathrm{HCO}_3^- \rightleftharpoons \mathrm{CO}_3^{2-} + \mathrm{H}^+ \]

Step 3: Identify species present in solution

From the reactions provided, the species present are \(\mathrm{CO}_2\), \(\mathrm{H}_2\mathrm{CO}_3\), \(\mathrm{HCO}_3^-\), and \(\mathrm{CO}_3^{2-}\). This matches option (a), which includes all these species.

Step 4: Conclusion of the analysis

Since the equilibria involve each of these species, the correct answer is option (a).

Key Concepts

Carbon Dioxide Dissolution

When carbon dioxide (\(\mathrm{CO}_2\)) is dissolved in water, a fascinating series of reactions takes place. It is not just about carbon dioxide mixing with water but more about how it interacts chemically. Initially, carbon dioxide reacts with water forming carbonic acid. Here is what happens in the first step:

• The chemical reaction is: (\(\mathrm{CO}_2 + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{H}_2\mathrm{CO}_3\)).

It's important to note that carbonic acid itself is quite unstable in its pure form. It shows up more as a point of transition between the starting materials, carbon dioxide and water, and other products like bicarbonate and carbonate ions.
This dynamic between dissolving (\(\mathrm{CO}_2\)) and forming (\(\mathrm{H}_2\mathrm{CO}_3\)) is a key part of carbon dioxide dissolution chemistry.

Acid-Base Equilibria

Once carbonic acid (\(\mathrm{H}_2\mathrm{CO}_3\)) forms, it can engage in further reactions, generating various ions through a process known as acid-base equilibria. Acid-base equilibria refer to the balance among acidic and basic species. Here’s how carbonic acid contributes:

• It first dissociates into a hydrogen ion and a bicarbonate ion:(\(\mathrm{H}_2\mathrm{CO}_3 \rightleftharpoons \mathrm{HCO}_3^- + \mathrm{H}^+\)).


• Next, the bicarbonate ion can further dissociate to form a carbonate ion and another hydrogen ion:(\(\mathrm{HCO}_3^- \rightleftharpoons \mathrm{CO}_3^{2-} + \mathrm{H}^+\)).

Through these reactions, we see the transformation from carbonic acid to bicarbonate, and eventually, carbonate.
This balance of acids and bases in solution defines the mixture of species that exist when (\(\mathrm{CO}_2\)) is dissolved in water.

Carbonate Chemistry

Carbonate chemistry is integral to understanding carbon dioxide's behavior in aqueous environments, especially due to the presence of carbonate ions (\(\mathrm{CO}_3^{2-}\)). Carbonate chemistry encompasses several equilibria that describe a system involving carbon dioxide, carbonic acid, bicarbonates, and carbonates. Each species plays a crucial role in neutralizing acids and bases:

• The carbonate ion is a two-time negatively charged ion that results from the dissociation reactions of carbon dioxide in water.
• It acts as a base, capable of interacting with hydrogen ions to reform bicarbonates when necessary, thus maintaining balance in the solution.

Moreover, carbonate chemistry is vital for buffering systems, where it helps regulate pH levels in natural waters.
This complex interplay of equilibria and ionic species underscores why carbonate chemistry is essential for ecosystems, including oceans and freshwater systems.
Understand these concepts aids in appreciating how environmental changes might disrupt these delicate balances.