Question

Boron can be obtained by various methods but not by (1) Thermal decomposition of \(\mathrm{B}_{2} \mathrm{H}_{6}\). (2) Pyrolysis of \(\mathrm{B} \mathrm{I}_{3}\). (3) Reducing \(\mathrm{BCl}_{3}\) with \(\mathrm{H}_{2}\). (4) Electrolysis of fused \(\mathrm{BCl}_{3}\).

Short answer

The method not used is electrolysis of fused \(\mathrm{BCl}_3\).

Step-by-step solution

Understand the Provided Methods

Review the given methods for obtaining boron: thermal decomposition of \(\mathrm{B}_2 \mathrm{H}_6\), pyrolysis of \(\mathrm{B} \mathrm{I}_3\), reducing \(\mathrm{BCl}_3\) with \(\mathrm{H}_2\), electrolysis of fused \(\mathrm{BCl}_3\).

Analyze Each Method

Evaluate each method for obtaining boron: \(\mathrm{B}_2 \mathrm{H}_6\) can thermally decompose to form boron, \(\mathrm{B} \mathrm{I}_3\) can undergo pyrolysis to yield boron, \(\mathrm{BCl}_3\) can react with \(\mathrm{H}_2\) to produce boron; however, electrolysis of fused \(\mathrm{BCl}_3\) is not a typical method for boron extraction.

Verify the Correct Answer

Note that thermal methods like decomposition and pyrolysis, and chemical reduction with \(\mathrm{H}_2\), are documented methods for producing boron. Electrolysis of fused \(\mathrm{BCl}_3\) is not. Therefore, the method not used for obtaining boron is the electrolysis of fused \(\mathrm{BCl}_3\).

Key concepts

Thermal Decomposition

Thermal decomposition is a process where a compound breaks down into simpler substances when heated. This method can be used to extract boron from boron hydrides like \(\text{B}_2 \text{H}_6\)\r. When heated, \(\text{B}_2 \text{H}_6\)\r decomposes to form elemental boron and hydrogen gas.
\rIt’s an efficient way of obtaining boron because it allows for the controlled release of unwanted elements. Additionally, because the process is purely thermal, no extra reagent is required.
Some important points about thermal decomposition:\r

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• Only requires heat to break down the compound.
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• Does not need additional chemicals.
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• Effective for extracting pure boron.
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\r Understanding this method is crucial, as it highlights how boron can be sourced using just temperature manipulation.

Pyrolysis

Pyrolysis is the chemical decomposition of materials at elevated temperatures in an inert atmosphere. This method is used to decompose boron compounds like \(\text{BI}_3\)\r to extract boron. \(\text{BI}_3\)\r when subjected to high temperatures, breaks down into boron and iodine.
It is crucial to carry out pyrolysis in the absence of oxygen since the presence of oxygen can lead to unwanted reactions.
Key takeaways about pyrolysis:

• Involves high temperature and no oxygen.
• Produces boron and by-product gases.
• Efficient for extracting boron from certain compounds.

Understanding pyrolysis provides insights into another significant method for obtaining boron through thermal processes.

Chemical Reduction

Chemical reduction involves the gaining of electrons by a molecule, atom, or ion. To extract boron, one effective method is reducing \(\text{BCl}_3\)\r with hydrogen. In this process, \(\text{BCl}_3\)\r reacts with \(\text{H}_2\)\r, producing elemental boron and hydrochloric acid.
This method is advantageous because it allows for a direct chemical reaction to extract boron. Significant points about chemical reduction:\r

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• Involves a reducing agent, typically hydrogen.
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• Direct chemical reaction yields boron.
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• Produces useful by-products like HCl.
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\r This method showcases how boron can be obtained efficiently through a straightforward reduction process.

Electrolysis

Electrolysis involves using an electric current to drive a non-spontaneous chemical reaction. While it is a widely used method for extracting metals, it’s not typically used for boron extraction. For instance, electrolysis of fused \(\text{BCl}_3\)\r is not a common method for obtaining boron because the process isn’t efficient or practical for boron.
\rHowever, understanding electrolysis is essential:

• It uses electricity to separate elements.
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• Common for metal extraction but not boron.
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• Requires high-energy input.
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\r Although electrolysis is pivotal for many other elements, it’s not preferred for boron, highlighting the importance of choosing the right extraction method based on the element being targeted.