Problem 58 - A Complete Resource Book in Chemistry for JEE Main

Question

Sodium thiosulphate is prepared by (a) reducing \(\mathrm{Na}_{2} \mathrm{SO}_{4}\) solution with \(\mathrm{H}_{2} \mathrm{~S}\) (b) boiling \(\mathrm{Na}_{2} \mathrm{SO}_{3}\) solution with \(\mathrm{S}\) in alkaline medium (c) neutralizing \(\mathrm{H}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}\) solution with \(\mathrm{NaOH}\) (d) boiling \(\mathrm{Na}_{2} \mathrm{SO}_{3}\) solution with \(\mathrm{S}\) in acidic medium

Step-by-Step Solution

Answer

(b) Boiling \( \text{Na}_2\text{SO}_3 \) solution with \( \text{S} \) in alkaline medium.

Step 1: Understanding Sodium Thiosulphate

Sodium thiosulphate is a chemical compound with the formula \( \text{Na}_2\text{S}_2\text{O}_3 \). It is often produced from sodium sulfate \( \text{Na}_2\text{SO}_4 \), sodium sulfite \( \text{Na}_2\text{SO}_3 \), or sodium bisulfite, in the presence of sulfur, commonly in an alkaline medium.

Step 2: Evaluating the Options

Let's evaluate each given preparation method:- (a) Reducing \( \text{Na}_2\text{SO}_4 \) solution with \( \text{H}_2\text{S} \): This involves a different reduction process and typically does not produce thiosulphate.- (b) Boiling \( \text{Na}_2\text{SO}_3 \) solution with \( \text{S} \) in alkaline medium: This reaction is a standard method for producing sodium thiosulphate, as it combines sodium sulfite with sulfur under alkaline conditions to form \( \text{Na}_2\text{S}_2\text{O}_3 \).- (c) Neutralizing \( \text{H}_2\text{S}_2\text{O}_3 \) solution with \( \text{NaOH} \): This reaction would neutralize the thiosulfuric acid to produce sodium thiosulphate, but it's not a typical method used due to the instability of \( \text{H}_2\text{S}_2\text{O}_3 \).- (d) Boiling \( \text{Na}_2\text{SO}_3 \) solution with \( \text{S} \) in acidic medium: The acidic environment would interfere with the formation of \( \text{Na}_2\text{S}_2\text{O}_3 \), favoring other side reactions.

Step 3: Selecting the Correct Method

Based on the analysis, option (b) is the most accurate method for preparing sodium thiosulphate. By boiling sodium sulfite with sulfur in an alkaline medium, you effectively produce \( \text{Na}_2\text{S}_2\text{O}_3 \). The alkaline conditions help to stabilize the thiosulphate ions formed during the reaction.

Key Concepts

Sodium Sulfite

Sodium sulfite, chemically known as \( \text{Na}_2\text{SO}_3 \), plays a crucial role in producing sodium thiosulphate. It serves as a base reactant in the preparation process. In sodium thiosulphate production, sodium sulfite is combined with elemental sulfur. This combination typically occurs in an alkaline medium, which supports efficient reaction and stabilization of the product.
Sodium sulfite itself is a common chemical used in many applications, such as in food preservation as an antioxidant. In chemistry, it acts as a reducing agent, which means it can donate electrons to other substances. This property makes it handy in various reactions, including the formation of sodium thiosulphate. Its presence in the reaction ensures the formation of the desired compound efficiently in an industrial setting.

Alkaline Medium

An alkaline medium refers to a basic environment, usually contributed by a base like sodium hydroxide \( \text{NaOH} \). This medium is essential in producing sodium thiosulphate because it stabilizes the reactive thiosulfate ions. An alkaline environment is indicated by a pH greater than 7. Such conditions are necessary to facilitate the reaction between sodium sulfite and sulfur, enabling the formation of sodium thiosulphate.
In the specific process of generating sodium thiosulphate, boiling a solution containing sodium sulfite and sulfur in an alkaline medium ensures that the reaction proceeds correctly. The alkaline conditions prevent unwanted side reactions and allow for the effective mixing and reaction of the components. This controlled setting ensures that the desired product, sodium thiosulphate, is formed efficiently and safely.

Chemical Reactions

Chemical reactions are processes where one set of chemical substances is transformed into another. In the context of sodium thiosulphate preparation, the key chemical reaction involves sodium sulfite \((\text{Na}_2\text{SO}_3)\) reacting with elemental sulfur \((S)\) in an alkaline medium. The balanced chemical equation for this reaction is:
\[ \text{Na}_2\text{SO}_3 + S \rightarrow \text{Na}_2\text{S}_2\text{O}_3 \]
This reaction shows the transformation of reactants into a new compound, sodium thiosulphate \((\text{Na}_2\text{S}_2\text{O}_3)\). The process highlights the conversion of sulfite and sulfur into thiosulphate ions under specific conditions.
The efficiency of the reaction often depends on various factors such as temperature, concentration, and the presence of an alkaline medium. These conditions ensure that the products are formed without much interference from competing reactions. Understanding these conditions is crucial in industrial settings to optimize the production of sodium thiosulphate.

Sulfur

Sulfur is a key element in the production of sodium thiosulphate. It is a yellow, non-metallic chemical element and is represented by the symbol \( S \). In the preparation of sodium thiosulphate, sulfur reacts with sodium sulfite \( \text{Na}_2\text{SO}_3 \) under alkaline conditions.
In this reaction, sulfur serves as a reactant that combines with sodium sulfite to form sodium thiosulphate \( \text{Na}_2\text{S}_2\text{O}_3 \). The ability of sulfur to form multiple bonds with oxygen and other atoms makes it an essential component in many industrial and chemical reactions.
Beyond its role in producing sodium thiosulphate, sulfur is widely used in various applications, including the manufacture of sulfuric acid, vulcanization of rubber, and even as a fungicide. Its versatility and reactivity make it valuable in both industrial and laboratory settings.