Problem 19 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The species present in solution when \(\mathrm{CO}_{2}\) is dissolved in water are \([\mathbf{2 0 0 6}-\mathbf{5 M},-1]\) (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

Option (a) is correct: \(\mathrm{CO}_{2}, \mathrm{H}_{2}\mathrm{CO}_{3}, \mathrm{HCO}_{3}^{-}, \mathrm{CO}_{3}^{2-}\).

Step 1: Understanding CO2 Dissolution

When carbon dioxide (\(\mathrm{CO}_{2}\)) is dissolved in water, it undergoes a series of reactions. The initial dissolution leads to the formation of carbonic acid (\(\mathrm{H}_{2}\mathrm{CO}_{3}\)) by the reaction: \(\mathrm{CO}_{2} + \mathrm{H}_{2}\mathrm{O} \rightleftharpoons \mathrm{H}_{2}\mathrm{CO}_{3}\).

Step 2: Dissociation of Carbonic Acid

Carbonic acid (\(\mathrm{H}_{2}\mathrm{CO}_{3}\)) further dissociates into bicarbonate ions (\(\mathrm{HCO}_{3}^{-}\)) and hydrogen ions (\(\mathrm{H}^{+}\)): \(\mathrm{H}_{2}\mathrm{CO}_{3} \rightleftharpoons \mathrm{HCO}_{3}^{-} + \mathrm{H}^{+}\).

Step 3: Further Ionization into Carbonate

The bicarbonate ion (\(\mathrm{HCO}_{3}^{-}\)) can further ionize to form carbonate ions (\(\mathrm{CO}_{3}^{2-}\)) and additional hydrogen ions: \(\mathrm{HCO}_{3}^{-} \rightleftharpoons \mathrm{CO}_{3}^{2-} + \mathrm{H}^{+}\).

Step 4: Listing All Present Species

From the above steps, the species present in the solution when \(\mathrm{CO}_{2}\) is dissolved in water are \(\mathrm{CO}_{2}\), \(\mathrm{H}_{2}\mathrm{CO}_{3}\), \(\mathrm{HCO}_{3}^{-}\), and \(\mathrm{CO}_{3}^{2-}\).

Step 5: Choosing the Correct Option

Based on all the dissociations and ionizations described, the correct set of species is option (a): \(\mathrm{CO}_{2}\), \(\mathrm{H}_{2}\mathrm{CO}_{3}\), \(\mathrm{HCO}_{3}^{-}\), and \(\mathrm{CO}_{3}^{2-}\).

Key Concepts

Carbonic Acid Formation

When carbon dioxide (\(\mathrm{CO}_{2}\)) dissolves in water, an important chemical reaction occurs involving the formation of carbonic acid (\(\mathrm{H}_{2}\mathrm{CO}_{3}\)). This process is fascinating and begins when the dissolved \(\mathrm{CO}_{2}\) reacts with water molecules. The reaction can be represented by the equation:
\[\mathrm{CO}_{2} + \mathrm{H}_{2}\mathrm{O} \rightleftharpoons \mathrm{H}_{2}\mathrm{CO}_{3}\]
This reaction is reversible, which means it can go in both directions. The carbonic acid formed is relatively weak and unstable. Thus, it doesn't stay long in its complete form. Instead, it gets ready to break down into other ionic species, starting a chain of reactions that affect the chemistry of the solution significantly. Understanding this initial step is crucial, as it sets the stage for the following reactions involving bicarbonate and carbonate ions.

Bicarbonate Ionization

Once carbonic acid (\(\mathrm{H}_{2}\mathrm{CO}_{3}\)) is formed, it undergoes ionization. This means it breaks apart to form bicarbonate ions (\(\mathrm{HCO}_{3}^{-}\)) and hydrogen ions (\(\mathrm{H}^{+}\)):
\[\mathrm{H}_{2}\mathrm{CO}_{3} \rightleftharpoons \mathrm{HCO}_{3}^{-} + \mathrm{H}^{+}\]
This is the first ionization step, where one molecule of carbonic acid releases a hydrogen ion. The bicarbonate ion is more stable compared to carbonic acid, making this step favorable in solutions. Details are important here because the amount of \(\mathrm{H}^{+}\) ions affects the acidity of the solution, which is key in many biological and chemical processes. In the bicarbonate system, this process acts as a buffer, helping maintain stable pH levels in natural systems like blood and seawater.

Carbonate Ion

The bicarbonate ions (\(\mathrm{HCO}_{3}^{-}\)) can further undergo ionization. This second ionization leads to the formation of carbonate ions (\(\mathrm{CO}_{3}^{2-}\)) and yet another hydrogen ion:
\[\mathrm{HCO}_{3}^{-} \rightleftharpoons \mathrm{CO}_{3}^{2-} + \mathrm{H}^{+}\]
This step is essential in the carbon cycle and influences the hardness of water. The carbonate ion is typically found in solid forms like limestone and is crucial for marine organisms that build shells and skeletons. As with the previous steps, these reactions have a direct impact on the pH level of the solution. Understanding this final step completes the picture of how carbon dioxide in water can affect both the chemistry of the water itself and various natural processes, highlighting the interconnectedness of chemical reactions and environmental stability.