Problem 52 - 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

The correct answer is (c): Gold forms complexes which are soluble.

Step 1: Understand the Process

The hydrometallurgical process for extracting gold involves dissolving the solid mineral in a suitable extractant. This process is highly reliant on the chemical properties of gold that allow it to form certain compounds or complexes.

Step 2: Identify Gold's Key Property

Gold's key property in the hydrometallurgical extraction process is its ability to form complexes, particularly with cyanide ions. These complexes are characterized by their solubility in water, a critical factor for the extraction process.

Step 3: Eliminate Non-Relevant Options

Options (a) and (b) are not directly related to the hydrometallurgical process: (a) 'being electropositive' is more relevant to electrochemical processes, and (b) 'being less reactive' is a general property but not directly involved in forming soluble complexes. Similarly, (d) forming salts that are water soluble is not specific to the hydrometallurgical process used for gold.

Step 4: Choose the Correct Answer

The correct option is (c), which states that gold forms complexes that are soluble. This characteristic is crucial for its recovery through the hydrometallurgical process.

Key Concepts

Gold Extraction

Gold extraction is typically done by a method known as hydrometallurgy. In this process, gold is extracted from its ore using an aqueous chemistry approach. This involves the use of a liquid solvent to dissolve a solid substance to recover valuable metals. The process is especially effective for extracting metals like gold because it can selectively target and dissolve metals while leaving the rest of the rock behind.
The main advantage of this method is its ability to handle low-grade ores that are not economically viable using traditional methods. Additionally, a hydrometallurgical approach requires less energy than pyrometallurgical processes, where metals are extracted by heat. This makes it an environmentally friendlier option compared to alternatives.

Complex Formation

Complex formation is a fundamental concept in the extraction of gold using a hydrometallurgical process. Here, the gold ion forms a complex ion with other molecules or ions, which changes its physical and chemical properties. A prominent example involves cyanide, which forms a highly stable complex with gold:

• The cyanide ion ( ext{CN}^-) binds with gold ( ext{Au}), creating a gold-cyanide complex, ext{[Au(CN)_2]^-}.
• This complex is highly soluble in water, a key requirement for dissolving gold from its ore in the solution.
• The stability of the gold-cyanide complex is essential because it simplifies the separation of gold from the ore, enhancing extraction efficiency.

The formation of such complexes can significantly increase the efficiency of metal extraction by making it easier to transport and recover metals from a solution.

Solubility Concepts

Solubility plays a critical role in the hydrometallurgical extraction of gold. It refers to the ability of a substance to dissolve in a solvent and is crucial for the efficient extraction and recovery of metals. Here’s how solubility fits into the process:

• When gold forms complexes such as the gold-cyanide complex, this significantly enhances the solubility of gold in water.
• The solubility of these complexes ensures that as much gold as possible can be dissolved from the ore, maximizing the yield.
• Once dissolved, further chemical reactions can be initiated to recover the gold from the solution, such as through the precipitation process.

Understanding solubility concepts ensures we can optimize the conditions under which gold is extracted, making the process both more economical and sustainable.