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

Question

When \(\mathrm{Cl}_{2}\) is passed through hot conc. \(\mathrm{NaOH}\), Salts \(\mathrm{A}\) and \(\mathrm{B}\) are formed which are separated by fractional crystallization. When 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

\(\mathrm{HClO}_{4}\) (c) is the strongest Bronsted acid formed.

Step 1: Reaction of Cl2 with NaOH

When chlorine, \( \mathrm{Cl}_{2} \), is passed through hot concentrated sodium hydroxide, \( \mathrm{NaOH} \), the reaction produces sodium chloride, \( \mathrm{NaCl} \), and sodium chlorate, \( \mathrm{NaClO}_{3} \). These are the salts \( \mathrm{A} \) and \( \mathrm{B} \) respectively: \[ 3\mathrm{Cl}_{2} + 6\mathrm{NaOH} \rightarrow 5\mathrm{NaCl} + \mathrm{NaClO}_3 + 3\mathrm{H}_{2}\mathrm{O} \]

Step 2: Conversion of NaClO3 to KClO3

When sodium chlorate (\( \mathrm{NaClO}_{3} \), salt \( \mathrm{B} \)) is treated with potassium chloride (\( \mathrm{KCl} \)), potassium chlorate (\( \mathrm{KClO}_{3} \), substance \( \mathrm{C} \)) is produced through a double displacement reaction.

Step 3: Disproportionation of KClO3

Gently heating potassium chlorate (\( \mathrm{KClO}_{3} \), substance \( \mathrm{C} \)) causes it to disproportionate into potassium chloride (\( \mathrm{KCl} \), substance \( \mathrm{D} \)) and potassium perchlorate (\( \mathrm{KClO}_{4} \), substance \( \mathrm{E} \)) according to the reaction: \[ 4\mathrm{KClO}_3 \rightarrow 3\mathrm{KClO}_4 + \mathrm{KCl} \]

Step 4: Reaction of KClO4 with H2SO4

When potassium perchlorate (\( \mathrm{KClO}_{4} \), substance \( \mathrm{E} \)) is heated with concentrated sulfuric acid (\( \mathrm{H}_{2}\mathrm{SO}_{4} \)), perchloric acid (\( \mathrm{HClO}_{4} \), substance \( \mathrm{F} \)), which is the strongest Bronsted acid, is produced. The reaction can be represented as: \[ \mathrm{KClO}_{4} + \mathrm{H}_{2}\mathrm{SO}_{4} \rightarrow \mathrm{HClO}_{4} + \mathrm{KHSO}_{4} \]

Key Concepts

Sodium Chlorate

Sodium chlorate, represented as \( \mathrm{NaClO}_3 \), is an essential chemical salt widely used in various applications. It appears as a white crystalline powder and is primarily known for its role as an herbicide, bleacher, and in the production of chlorine dioxide.
Sodium chlorate is typically produced through the reaction of chlorine gas with hot concentrated sodium hydroxide. This reaction also produces sodium chloride as a byproduct. The reaction can be written as: - \( 3\mathrm{Cl}_2 + 6\mathrm{NaOH} \rightarrow 5\mathrm{NaCl} + \mathrm{NaClO}_3 + 3\mathrm{H}_2\mathrm{O} \)
This transformation involves an oxidation process where chlorine gas is converted into sodium chlorate. In the context of chemical reactions, sodium chlorate frequently participates in redox or disproportionation reactions, where it might transform into substances with different oxidation states under certain conditions. This characteristic is crucial in industrial processes where sodium chlorate is utilized to extract metals from ores or in paper industry packaging.

Potassium Chlorate

Potassium chlorate, \( \mathrm{KClO}_3 \), is a chemical compound composed of potassium, chlorine, and oxygen. It’s known for its oxidizing properties and is often utilized in matches, fireworks, and explosives due to its ability to decompose and release oxygen under heat.
Potassium chlorate is formed through a double displacement reaction between sodium chlorate and potassium chloride: - \( \mathrm{NaClO}_3 + \mathrm{KCl} \rightarrow \mathrm{KClO}_3 + \mathrm{NaCl} \)
The resulting potassium chlorate can undergo disproportionation when gently heated. This process breaks down potassium chlorate into potassium perchlorate and potassium chloride, each with a different chlorine oxidation state. This feature makes it a versatile compound in both laboratory and industrial applications. Potassium chlorate’s role in reactions as a source of oxygen is invaluable, especially where rapid combustion or oxidation is essential.

Potassium Perchlorate

Potassium perchlorate, \( \mathrm{KClO}_4 \), is a compound often employed in pyrotechnics and as a propellant in rocketry. This salt is formed when potassium chlorate undergoes a disproportionation reaction, resulting in both potassium perchlorate and potassium chloride. The equation for this process is: - \( 4\mathrm{KClO}_3 \rightarrow 3\mathrm{KClO}_4 + \mathrm{KCl} \)
As an oxidizing agent, potassium perchlorate provides oxygen to support the combustion of fuels, making it vital in many pyrotechnic formulations. It is preferred over other chlorates in applications where stability is paramount since it is less sensitive to friction and impact.
In addition, potassium perchlorate is useful in chemical reactions where high energy output or vigorous reactions are desirable, particularly under controlled laboratory conditions. Its relatively lower solubility in water also distinguishes it, effectively managing its release in specific applications.

Perchloric Acid

Perchloric acid, \( \mathrm{HClO}_4 \), is a powerful Bronsted acid recognized as one of the strongest acids in existence. It is generated when potassium perchlorate is heated with concentrated sulfuric acid: - \( \mathrm{KClO}_4 + \mathrm{H}_2\mathrm{SO}_4 \rightarrow \mathrm{HClO}_4 + \mathrm{KHSO}_4 \)
Due to its intense acidity, perchloric acid is used in analytical chemistry for preparing compounds with high purity levels and is a crucial component in research laboratories.
Its ability to donate a proton easily makes it particularly notable among acids. However, its strong oxidizing property means it should be handled with caution, as it can react explosively with organic materials.
Additionally, perchloric acid's utility also extends to the etching of metals in electronics manufacturing and catalyzing sophisticated organic synthesis reactions. This versatility highlights its significance across various chemistry and industrial applications.