Problem 154 - A Complete Resource Book in Chemistry for JEE Main

Question

When \(\mathrm{Cl}_{2}\) is passed through hot conc. \(\mathrm{NaOH}\), Salts A and \(\mathrm{B}\) are formed which are separated by fractional crystallization. When \(\mathrm{B}\) is treated with a calculated amount of \(\mathrm{KCl}\), a substance \(\mathrm{C}\) is produced. When \(\mathrm{C}\) is gently heated, it disproportionates to give \(\mathrm{D}\) and \(\mathrm{E}\). When \(\mathrm{E}\) is heated with conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\), the strongest bronsted acid \(\mathrm{F}\) is formed which is isoelectronic with \(\mathrm{H}_{2} \mathrm{SO}_{4}\). Identify \(\mathrm{F}\). (a) \(\mathrm{HClO}_{2}\) (b) \(\mathrm{HClO}_{3}\) (c) \(\mathrm{HClO}_{4}\) (d) \(\mathrm{HCl}\)

Step-by-Step Solution

Answer

The strongest Brønsted acid F is \(\mathrm{HClO}_4\) (c).

Step 1: Reaction of Cl2 with hot con. NaOH

When chlorine (\(\text{Cl}_2\)) reacts with hot concentrated sodium hydroxide (\(\text{NaOH}\)), it forms two salts: sodium chloride (\(\text{NaCl}\)) and sodium chlorate (\(\text{NaClO}_3\)). The reaction is as follows:\[3\text{Cl}_2 + 6\text{NaOH} \rightarrow 5\text{NaCl} + \text{NaClO}_3 + 3\text{H}_2\text{O}\]

Step 2: Understanding Salt B Action with KCl

Salt \(\text{B}\) is sodium chlorate (\(\text{NaClO}_3\)). When \(\text{NaClO}_3\) is treated with potassium chloride (\(\text{KCl}\)), potassium chlorate (\(\text{KClO}_3\)) is formed by a simple ion exchange:\[\text{NaClO}_3 + \text{KCl} \rightarrow \text{KClO}_3 + \text{NaCl}\]

Step 3: Disproportionation of C

Substance \(\text{C}\) is potassium chlorate (\(\text{KClO}_3\)). When heated gently, it disproportionates to give potassium chloride (\(\text{KCl}\)) and potassium perchlorate (\(\text{KClO}_4\)):\[4\text{KClO}_3 \rightarrow 3\text{KClO}_4 + \text{KCl} + 3\text{O}_2\]

Step 4: Heating E with conc. H2SO4

Here, \(\text{E}\) is potassium perchlorate (\(\text{KClO}_4\)). When heated with concentrated sulfuric acid (\(\text{H}_2\text{SO}_4\)), it forms perchloric acid (\(\text{HClO}_4\)):\[\text{KClO}_4 + \text{H}_2\text{SO}_4 \rightarrow \text{HClO}_4 + \text{KHSO}_4\]

Step 5: Identify the strongest Bronsted acid F

The strongest Brønsted acid \(\text{F}\) formed is perchloric acid (\(\text{HClO}_4\)). Perchloric acid is indeed isoelectronic with sulfuric acid (\(\text{H}_2\text{SO}_4\)). Both acids contain a central atom bonded to four oxygen atoms.

Key Concepts

Chemical Reactions

In the fascinating world of chemistry, chemical reactions are transformations where reactants turn into products. These reactions involve rearrangements of atoms to form new substances. Such interactions are foundational, enabling everything from basic life processes to complex industrial manufacturing. A simple example is the reaction of chlorine gas (\(\text{Cl}_2\)) with hot concentrated sodium hydroxide (\(\text{NaOH}\)). This produces sodium chloride and sodium chlorate:

• Sodium chloride (\(\text{NaCl}\)) is a common salt.
• Sodium chlorate (\(\text{NaClO}_3\)) is useful in various chemical applications.

Keeping in mind that temperature and concentration can significantly influence the outcome of chemical reactions, it's clear that these conditions helped create two different products from the reaction above. Each reaction follows a specific pathway or mechanism, often dictated by specific reaction conditions.

Fractional Crystallization

Fractional crystallization is a process used to separate mixtures based on differences in solubility. When salts like sodium chloride (\(\text{NaCl}\)) and sodium chlorate (\(\text{NaClO}_3\)) are formed, separating them involves a clever technique known as fractional crystallization. This process exploits the fact that different substances crystallize at different temperatures and concentrations.
During crystallization, as the solution cools, compounds with lower solubility begin to form solid crystals first. By carefully controlling temperature, concentration, and sometimes using solvents, we can separate substances based on when they crystallize out of solution. This method is particularly useful in purifying substances and is a cornerstone technique in both research and industrial settings.

Brønsted Acids

Brønsted acids are substances that can donate a proton (\(\text{H}^+\)). This concept is central to understanding acid-base chemistry. In the reaction series from sodium chlorate transformation, perchloric acid (\(\text{HClO}_4\)) emerges as the strongest Brønsted acid.

• Perchloric acid is known for its strength and is highly reactive.
• It is isoelectronic with sulfuric acid (\(\text{H}_2\text{SO}_4\)), meaning both share the same electron configuration structure d they contain a central atom bonded to four oxygen atoms.
• The strong proton donation ability of \(\text{HClO}_4\) characterizes it as a Brønsted acid, highlighting its reactivity and applications in various chemical processes.

The reactivity and behavior of Brønsted acids like perchloric and sulfuric acids illustrate fundamental acid-base chemistry concepts that are vital for students to grasp.