Problem 151 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following are correctly matched? (a) Zone refining - Ultra pure Ge (b) Leaching - Extraction of \(\mathrm{Au}, \mathrm{Ag}\) (c) Cyanide process - Extraction of Au (d) Electrolyte reduction - Extraction of Al

Step-by-Step Solution

Answer

All options (a, b, c, d) are correctly matched.

Step 1: Understanding the Question

We need to determine which of the options are correctly matched. Each option provides a method and a substance related to the extraction or purification process.

Step 2: Evaluating Option (a) - Zone Refining

Zone refining is a technique used to obtain ultra-pure materials, and it is particularly effective for semiconductors like germanium ( sGe ). Thus, option (a) is correctly matched.

Step 3: Evaluating Option (b) - Leaching

Leaching is a method used to extract metals from their ores using a solvent. It is commonly used for extracting gold ( Au ) and silver ( Ag ). Therefore, option (b) is correctly matched.

Step 4: Evaluating Option (c) - Cyanide Process

The cyanide process (also known as cyanidation) is specifically used for extracting gold ( Au ). Therefore, option (c) is also correctly matched.

Step 5: Evaluating Option (d) - Electrolyte Reduction

Electrolytic reduction is used in the extraction of aluminium ( Al ), especially in the Hall-Héroult process. Thus, option (d) is correctly matched.

Key Concepts

Zone Refining

Zone refining is a precise method used to produce ultra-pure materials, especially semiconductors like germanium (Ge) and silicon (Si). The process involves passing a narrow region of molten metal through a long, solid metal bar.

• As the molten zone moves along the bar, impurities concentrate at the liquid-solid boundary, effectively separating them.
• This method enhances the purity by repeatedly sweeping the impurities to one end of the bar.

The zone refining method is critically important in developing high-purity materials needed for electronics and computer technology. With each pass of the molten zone, the concentration of impurities at the solid-liquid interface increases, thereby producing a highly refined product.

Leaching Process

Leaching is an ancient and effective process used to extract metals from their ores using chemical solutions. Often used for metals like gold (Au) and silver (Ag), the ore is first crushed and then dissolved in a solvent.

• The solvent dissolves the metal ions from the ore, leaving behind the waste material, or gangue.
• Further chemical reactions then separate the metal from the solution.

Common solvents used in leaching are acid solutions or cyanides, which are effective at selectively dissolving metals. This method is significant in metal extraction, particularly in gold mining, where it allows for more efficient recovery.

Cyanide Process

The cyanide process, or cyanidation, is a widely used procedure for extracting gold (Au) from its ore. The process involves dissolving gold in a solution of sodium cyanide or potassium cyanide.

• This forms a soluble gold-cyanide complex.
• Zinc metal is often added to the solution to precipitate pure gold out of the complex.

Cyanidation is particularly favored in the gold industry due to its efficiency in recovering gold from low-grade ores. Despite environmental concerns over cyanide use, advancements in recycling and detoxifying the chemical have improved its safety.

Electrolytic Reduction

Electrolytic reduction is a technique predominantly employed in the extraction of metals like aluminium (Al), particularly through the Hall-Héroult process. This method utilizes electric currents to reduce metal ores into pure metal.

• Occurs in electrolysis cells, which consist of a cathode and an anode submerged in a solution or molten state of the metal ore.
• The electric current decomposes the compound into its elemental form at the cathode, effectively extracting the metal.

For aluminium, the process involves the electrolysis of molten aluminium oxide. While energy-intensive, this method is essential for modern aluminium production, accounting for nearly all the world's aluminium output.