Problem 13 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Sodium thiosulphate is prepared by [1996-1 Mark] (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) neutralising \(\mathrm{H}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}\) solution with \(\mathrm{NaOH}\) (d) boiling \(\mathrm{Na}_{2} \mathrm{SO}_{3}\) solution with \(\mathrm{S}\) in acidic medium

Step-by-Step Solution

Answer

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

Step 1: Identify Key Information

First, identify that the question is asking how to prepare sodium thiosulphate. Sodium thiosulphate has the chemical formula \( \text{Na}_2 \text{S}_2 \text{O}_3 \). It's known for being a compound formed under specific conditions when \( \text{Na}_2 \text{SO}_3 \) (sodium sulfite) reacts with sulfur, often in an alkaline solution.

Step 2: Analyze Option B

The option 'boiling \( \text{Na}_2 \text{SO}_3 \) solution with \( \text{S} \) in alkaline medium' aligns with the typical industrial preparation of sodium thiosulphate. In this process, sodium sulfite reacts with sulfur in the presence of an alkaline medium to form sodium thiosulphate:\[ \text{Na}_2 \text{SO}_3(aq) + \text{S}(s) \rightarrow \text{Na}_2 \text{S}_2 \text{O}_3(aq) \]

Step 3: Evaluate Other Options

Consider other options to ensure completeness:- Reducing \( \text{Na}_2 \text{SO}_4 \) with \( \text{H}_2 \text{S} \) (option A) does not typically result in sodium thiosulphate.- Neutralizing \( \text{H}_2\text{S}_2\text{O}_3 \) with \( \text{NaOH} \) (option C) is unlikely to directly form sodium thiosulphate.- Boiling \( \text{Na}_2 \text{SO}_3 \) with \( \text{S} \) in an acidic medium (option D) is not the standard condition for thiosulphate preparation.

Step 4: Select the Correct Answer

Based on typical chemical reactions and industry practices, the correct option for preparing sodium thiosulphate is:**(b) boiling \( \text{Na}_2 \text{SO}_3 \) solution with \( \text{S} \) in alkaline medium**.

Key Concepts

Sodium Sulfite Reaction

Sodium sulfite, or \( \text{Na}_2 \text{SO}_3 \), is a chemical compound that plays a crucial role in the preparation of sodium thiosulphate. In this reaction, sodium sulfite is combined with elemental sulfur. This is not a simple mixing; a chemical reaction takes place to form a new compound. This reaction is important industrially because it helps in forming sodium thiosulphate, which has many uses, such as in photography, medicine, and even as a dechlorination agent in water treatment. Here are some steps to help understand the reaction:

• Sodium sulfite is typically in an aqueous solution form. This means the substance is dissolved in water.
• Sulfur is added to this aqueous solution of sodium sulfite.
• The reaction between these substances requires specific conditions to proceed optimally.

The reaction itself is represented by the equation:\[ \text{Na}_2 \text{SO}_3(aq) + \text{S}(s) \rightarrow \text{Na}_2 \text{S}_2 \text{O}_3(aq) \] This equation describes how sodium sulfite reacts with solid sulfur to form sodium thiosulphate. Keeping track of the states of matter - aqueous (\(aq\)) and solid (\(s\)) - is crucial for understanding this reaction.

Alkaline Medium

An alkaline medium is an essential condition for the formation of sodium thiosulphate in this context. "Alkaline" implies that the environment has a higher pH, meaning it is basic rather than acidic. This is usually achieved by adding bases such as sodium hydroxide \(\text{(NaOH)}\) in the solution.The presence of an alkaline medium favors certain reactions over others because it influences the ionic state of the reacting components:

• Ensures the stability of sodium sulfite against unexpected reactions that might occur in acidic conditions.
• Allows better interaction between sodium sulfite and sulfur, facilitating the conversion to sodium thiosulphate.
• An alkaline environment helps new compounds maintain their structure during and after synthesis.

Therefore, managing the pH and providing a stable alkaline environment is crucial for the effective preparation of sodium thiosulphate. This is often done industrially by controlling the additives and maintaining a consistent pH.

Chemical Reactions

Chemical reactions involve the transformation of substances, and they are key processes in chemistry. For the preparation of sodium thiosulphate, a specific chemical reaction takes place between sodium sulfite and sulfur. Chemical reactions like this involve breaking and forming bonds to create new compounds. It's important to recognize the indicators of a chemical reaction:

• Formation of a new substance: Here, sodium thiosulphate is the new compound formed.
• Change in energy, such as heat and light, indicating a reaction's occurrence.
• Specific conditions must be met, such as temperature or pH, as seen with the alkaline medium.

Chemical reactions are the basis of countless processes in both nature and industrial applications. Understanding them includes recognizing the reactants and products and knowing why certain conditions are necessary for the reaction to happen. When comprehended fully, these reactions lay the foundation for synthesizing various useful compounds, including sodium thiosulphate.