Problem 57 - A Complete Resource Book in Chemistry for JEE Main

Question

One among the following compounds will not give effervescence with sodium carbonate. (a) Phenol (b) \(2,4,6\) - Trinitrophenol (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CO}_{2} \mathrm{H}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{SO}_{3} \mathrm{H}\)

Step-by-Step Solution

Answer

Phenol does not give effervescence with sodium carbonate.

Step 1: Understanding the Reaction

Compounds generally react with sodium carbonate \((\mathrm{Na}_2\mathrm{CO}_3)\) to produce carbon dioxide gas, which causes effervescence. Acids usually give such a reaction.

Step 2: Assess the Nature of Each Compound

Evaluate each option: - (a) Phenol: Though weakly acidic, phenol does not sufficiently react with sodium carbonate to produce CO\(_2\).- (b) 2,4,6-Trinitrophenol: Also known as picric acid, which is a strong acid due to electron-withdrawing nitro groups making it likely to effervesce.- (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CO}_{2} \mathrm{H}\): Benzoic acid, a typical organic acid that will react with \(\mathrm{Na}_2\mathrm{CO}_3\) to release CO\(_2\).- (d) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{SO}_{3} \mathrm{H}\): Sulfonic acids are very strong acids, expected to react with \(\mathrm{Na}_2\mathrm{CO}_3\) and release CO\(_2\).

Step 3: Identify the Non-Effervescing Compound

From the analysis, phenol, a weak acid, does not react with sodium carbonate to give effervescence, unlike the other strong acids listed.

Key Concepts

Acid-Base Reactions

Acid-base reactions are fundamental chemical processes, playing a central role in various scientific fields. These reactions generally involve the transfer of protons from acids to bases. In the case of the sodium carbonate reaction with acids, carbon dioxide gas is often a product. This results in observable bubbling or effervescence.
When an acid mixes with sodium carbonate (\(\text{Na}_2\text{CO}_3\)), the acid donates protons (\(\text{H}^+\)) to the carbonate ion, forming carbonic acid (\(\text{H}_2\text{CO}_3\)). However, carbonic acid is unstable and quickly decomposes into water (\(\text{H}_2\text{O}\)) and carbon dioxide gas (\(\text{CO}_2\)). This reaction is both informative and visually striking, which helps in identifying acidic compounds.

• For example, strong acids will often cause significant effervescence due to their higher ability to donate protons, leading to more \(\text{CO}_2\) production.
• Weak acids, however, might not be able to produce visible effervescence as easily.

Organic Acids

Organic acids are compounds typically containing carbon-oxygen double bonds and often a carboxyl group (\(\text{-COOH}\)). These acids can vary in strength from mildly weak to very strong, all affecting their reactivity with base substances.
Organic acids such as benzoic acid (\(\text{C}_6\text{H}_5\text{CO}_2\text{H}\)) demonstrate their acidity by reacting with bases like sodium carbonate to produce effervescence.

• Benzoic acid is a fine example of a typical organic acid that reacts to form \(\text{CO}_2\), evident through bubbling.
• However, some organic acids like phenol are weaker and don't react with \(\text{Na}_2\text{CO}_3\) to form \(\text{CO}_2\).

The classification of organic acids is crucial in predicting their behavior in reactions. While compounds such as 2,4,6-trinitrophenol (picric acid) are strong due to additional nitro groups withdrawing electrons and enhancing acidity, others like phenol lack sufficient protons to donate.

Chemical Reactivity

Chemical reactivity refers to the ease and speed with which a chemical compound undergoes a reaction to form new products. Understanding reactivity helps anticipate how substances interact under various conditions.
In the exercise related to effervescence with sodium carbonate, the reactivity of organic compounds is under scrutiny:

• Compounds like benzoic acid and sulfonic acids react vigorously with sodium carbonate, as they easily donate protons (\(\text{H}^+\)).
• Strong reactivity leads to the rapid formation of \(\text{CO}_2\) gas, showcasing visible bubbling.
• Conversely, phenol, being less reactive, does not produce effervescence due to its inability to effectively donate protons.

Chemical reactivity in acids is greatly influenced by structural aspects. Strong acids are often potent against bases and are reactive due to the easy availability of protons for reaction. On the other hand, weak acids like phenol show lower reactivity and hence, less or no effervescence.