Problem 17 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

In the leaching method, bauxite ore is digested with a concentrated solution of \(\mathrm{NaOH}\) that produces ' \(\mathrm{X}^{\prime}\). When \(\mathrm{CO}_{2}\) gas is passed through the aqueous solution of ' \(X\) ', a hydrated compound 'Y' is precipitated. ' \(X\) ' and ' \(Y\) ' respectively are. [Main Online April 15, 2018 (II)] (a) \(\mathrm{Na}\left[\mathrm{Al}(\mathrm{OH})_{4}\right]\) and \(\mathrm{Al}_{2}\left(\mathrm{CO}_{3}\right)_{3} \cdot \mathrm{xH}_{2} \mathrm{O}\) (b) \(\mathrm{Al}(\mathrm{OH})_{3}\) and \(\mathrm{Al}_{2} \mathrm{O}_{3} \cdot \mathrm{xH}_{2} \mathrm{O}\) (c) \(\mathrm{NaAlO}_{2}\) and \(\mathrm{Al}_{2}\left(\mathrm{CO}_{3}\right)_{3} \times \mathrm{xH}_{2} \mathrm{O}\) (d) \(\mathrm{Na}\left[\mathrm{Al}(\mathrm{OH})_{4}\right]\) and \(\mathrm{Al}_{2} \mathrm{O}_{3} \cdot \mathrm{xH}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

(d) \(\mathrm{Na[Al(OH)_{4}]\) and \(\mathrm{Al_{2}O_{3} \cdot \mathrm{xH_{2}O]}\)

Step 1: Understanding the leaching process of bauxite

Leaching is a process used to extract aluminum from bauxite ore. In this process, bauxite ore (composed mainly of Al2O3) is treated with a concentrated solution of NaOH.

Step 2: Reaction of bauxite with NaOH

When bauxite is digested with a concentrated solution of NaOH, sodium aluminate ( \( \mathrm{Na[Al(OH)_{4}] \) ) is produced. This is the compound 'X' in the problem.

Step 3: Reaction of 'X' with CO2

The sodium aluminate solution reacts with CO2 gas when it is passed through, which results in the precipitation of aluminum hydroxide, \( \mathrm{Al(OH)_{3} \). Once precipitated, it can then hydrate to form different hydrated forms of aluminum oxide.

Step 4: Identifying the correct compounds 'X' and 'Y'

From the reactions:- 'X' is sodium aluminate: \( \mathrm{Na[Al(OH)_{4}]\)- 'Y' is the precipitated compound, which is hydrated aluminum oxide ( \( \mathrm{Al_{2}O_{3} \cdot \mathrm{xH_{2}O} \) ).Therefore, the correct choice for 'X' and 'Y' is option D.

Key Concepts

Leaching Process of Bauxite Regarding Sodium Aluminate

The initial step in extracting aluminum from bauxite ore involves a process called leaching. In this process, bauxite, which is primarily composed of aluminum oxide (Al\(_2\)O\(_3\)), is treated with a strong solution of sodium hydroxide (NaOH). When bauxite is exposed to the concentrated NaOH solution, it results in the formation of sodium aluminate, chemically represented as Na[Al(OH)\(_4\)]. Sodium aluminate becomes the product of interest in this reaction, often referred to as compound 'X' in various exercises.

This transformation is crucial as it forms the basis from which industries can further extract pure aluminum. The advantage of forming sodium aluminate is that it's soluble in the alkaline solution, which distinguishes it from other impurities found in bauxite ore that remain insoluble. This step effectively separates the aluminum content from other elements, simplifying the extraction process later on.

Role of Aluminum Hydroxide in the Process

After the creation of sodium aluminate in the leaching process of bauxite, the next important step involves a reaction with carbon dioxide (CO\(_2\)). When CO\(_2\) gas is bubbled through the sodium aluminate solution, aluminum hydroxide (Al(OH)\(_3\)) begins to precipitate out as a solid. This reaction marks the formation of 'Y', the next subject of interest.

The aluminum hydroxide precipitate is key in the refining process, as it is an intermediate that eventually leads to the production of pure aluminum. This stage is crucial because it converts soluble sodium aluminate back into a solid form, which can then be collected and further processed. Once aluminum hydroxide is dried and subjected to high temperatures, it dehydrates to form aluminum oxide (Al\(_2\)O\(_3\)), a useful precursor in aluminum metal manufacturing.

Bauxite Ore Extraction: The Path from Ore to Metal

Bauxite ore extraction is a significant industrial process that involves converting the raw ore into usable aluminum. It begins with mining bauxite, the primary source of aluminum, which contains a high percentage of aluminum oxide. The process of extraction involves several stages to ensure the efficient recovery of aluminum metal from bauxite.

One major step is leaching, where bauxite is converted into a soluble form using sodium hydroxide, leading to the creation of sodium aluminate. This is followed by precipitating aluminum hydroxide, which can then go through additional processing. Post precipitation, aluminum hydroxide can be calcined, meaning it is heated at high temperatures to remove water molecules. This results in the formation of alumina, which is later subjected to electrolysis to produce pure aluminum. - Bauxite ore is rich in aluminum oxide - Leaching converts it into sodium aluminate - Further processing produces aluminum hydroxide - Final steps lead to aluminum metal production

This entire journey from bauxite to aluminum metal involves a series of chemical reactions and physical processes, showcasing the complex but fascinating transformation of a natural resource into a highly useful material.