Problem 51 - A Complete Resource Book in Chemistry for JEE Main

Question

Gold is extracted by hydrometallurgical process, based on its property (a) of being electropositive (b) of being less reactive (c) to form complexes which are soluble (d) to form salts which are water soluble

Step-by-Step Solution

Answer

(c) to form complexes which are soluble

Step 1: Examine chemical properties of gold

Gold is a noble metal, meaning it is resistant to corrosion and oxidation. It tends not to react easily with other substances.

Step 2: Consider gold's ability to form complexes

In hydrometallurgical processes, gold is known for forming stable complexes, particularly with cyanide ions. This is the key to its extraction as these complexes are soluble, allowing for separation from the ore.

Step 3: Eliminate unrelated properties

Properties like being electropositive or forming salts, which are not directly related to the solubility seen in its complex formation, are less relevant to gold extraction in this process.

Step 4: Select the most fitting property

Given that hydrometallurgical processes rely on the dissolution and separation of metals based on solubility, the ability of gold to form complexes that are soluble is the crucial attribute for its extraction.

Key Concepts

Hydrometallurgy

Hydrometallurgy is a fascinating method used in the extraction of metals, including gold. It leverages aqueous chemistry to extract metals from their ores, using water-based solutions. The process focuses on the propensity of metals to dissolve in specific solvents.
Key Steps in Hydrometallurgy for Gold Extraction:

• Leaching: This is the initial phase where the metal is dissolved in a solvent. For gold, cyanide solution is commonly used, as gold forms a soluble complex with cyanide.
• Concentration and Purification: The dissolved metal is concentrated and purified from impurities.
• Recovery: Finally, the metal is recovered from the solution, often through precipitation or electrolysis.

Hydrometallurgy is preferred for gold extraction due to its efficiency in processing low-grade ores and its relatively minimal environmental impact compared to traditional methods like smelting.

Complex Formation

Gold is well-known for forming complexes, particularly with ligands such as cyanide ions. In chemistry, a complex is a compound composed of a central atom or ion bonded to surrounding molecules or ions, called ligands.
The Importance of Complex Formation in Gold Extraction:

• The ability of gold to form complexes significantly enhances its solubility in certain solvents, facilitating easier separation from other materials.
• Cyanide ions rapidly form a stable and soluble complex with gold, known as the ext{[Au(CN)$_ ext{2}$]$^ ext{-}$} ion, crucial in the extraction process.
• This formation is a primary step in extracting gold using hydrometallurgical methods, as it converts gold into a state that dissolves in the solution.

The formation of these soluble complexes is vital because it allows for selective separation and extraction of gold from its ore in an efficient manner.

Solubility of Gold Complexes

The solubility of gold complexes in aqueous solutions is a cornerstone of its hydrometallurgical extraction process. Gold's tendency to form soluble complexes is what makes its extraction feasible and effective.
Understanding Solubility of Gold Complexes:

• Gold complexes are often formed with cyanide due to gold's ability to stabilize in a dissolved state within a liquid solution.
• The complex formed, ext{[Au(CN)$_ ext{2}$]$^ ext{-}$}, exhibits significant solubility, allowing gold to separate from the original ore material.
• This property is crucial, as it means gold can be extracted and recovered efficiently from even low-grade ores using aqueous solutions.
• Recovery processes exploit this solubility to precipitate gold selectively from the solution.

The ability of gold to dissolve as a complex ion in aqueous mediums underpins the success of the hydrometallurgical extraction method, facilitating a higher yield from ore materials.