Problem 40 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

When propionic acid is treated with aqueous sodium bicarbonate, \(\mathrm{CO}_{2}\) is liberated. The ' \(\mathrm{C}\) ' of \(\mathrm{CO}_{2}\) comes from (a) methyl group (b) carboxylic acid group (c) methylene group (d) bicarbonate

Step-by-Step Solution

Answer

The 'C' in CO2 comes from the bicarbonate (d).

Step 1: Identify Reactants and Products

The reaction involves propionic acid ( 2 H 5 COOH) and sodium bicarbonate ( 3 HCO ) as reactants. One of the products is carbon dioxide ( 2 ). 2

Step 2: Understand the Reaction Process

Propionic acid ( C oH COOH) reacts with sodium bicarbonate ( NaHCO 3) in an acid-base reaction to form carbon dioxide ( CO 2), water ( O), and sodium propionate ( NaC H COO ).

Step 3: Determine Carbon Source in CO2

The carbon in the carbon dioxide ( CO ) comes from the sodium bicarbonate ( NaHCO ), specifically from its bicarbonate ion ( HCO 3 ). The reaction is: C H COOH + NaHCO → C H COONa + H + CO .

Step 4: Answer the Question

Based on the reaction analysis, the carbon atom in the 2 product originates from the bicarbonate ( HCO 3 ) ion.

Key Concepts

Acid-base reactions

Acid-base reactions are fundamental processes in chemistry where an acid and a base interact to produce new substances. The essence of these reactions lies in the transfer of protons (H extsuperscript{+} ions).
For instance, in the reaction discussed, propionic acid (a carboxylic acid) serves as the acid, while sodium bicarbonate acts as the base.

• When an acid like propionic acid gives up a proton, it forms its conjugate base, in this case, sodium propionate.
• The bicarbonate ion in sodium bicarbonate accepts this proton, thereby forming carbonic acid, which spontaneously decomposes to give carbon dioxide and water.
• This results in the liberation of carbon dioxide gas (\(CO_2\)), illustrating a classic gas-evolving acid-base reaction.

These reactions are not only important in laboratory settings but also play a key role in biological systems and industrial applications.

Carboxylic acids

Carboxylic acids are a group of organic compounds characterized by the presence of a carboxyl group (-COOH). This functional group is highly versatile and reactive, making carboxylic acids significant in organic chemistry.

• The carboxyl group consists of a carbonyl (C=O) and a hydroxyl (O-H) group connected to the same carbon atom.
• The acidic nature arises from the ability of the carboxyl group to donate a proton (H\(^+\)), making these compounds ideal reactants in many types of chemical reactions.
• In our example, propionic acid (CH\(_3\)CH\(_2\)COOH) reacts with sodium bicarbonate, highlighting the functional utility of the carboxyl group in acid-base reactions.

Carboxylic acids are found in nature and are useful in the manufacture of polymers, pharmaceuticals, and numerous other chemicals. Understanding their behavior in reactions is crucial for a deeper understanding of organic chemistry.

Carbon dioxide formation

Carbon dioxide formation is a common occurrence in many chemical reactions, especially those involving carbonates and bicarbonates reacting with acids.

• When bicarbonate ions (\(HCO_3^-\)) combine with protons (\(H^+\)) from an acid, carbonic acid is transiently formed.
• Carbonic acid (H\(_2\)CO\(_3\)) is unstable and rapidly decomposes into water (H\(_2\)O) and carbon dioxide (\(CO_2\)), which is released as a gas.
• This process is vital, not just in the laboratory but also in natural processes such as respiration and the carbon cycle.

In the exercise, the liberation of \(CO_2\) serves as a pivotal observation, indicating the source of the carbon within the bicarbonate ion. This simple reaction mechanism is an excellent illustration of how equilibrium in chemical reactions can result in gas formation.