Problem 44 - General Chemistry: Principles and Modern Applications

Question

In the metallurgical extraction of silver and gold, an alloy of the two metals is often obtained. The alloy can be separated into Ag and Au either with concentrated \(\mathrm{HNO}_{3}\) or boiling concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4},\) in a process called parting. Write chemical equations to show how these separations work.

Step-by-Step Solution

Answer

1. Reaction of Ag and Au with Nitric Acid: \(3Ag + 4HNO3 -> 3AgNO3 + NO + 2H2O\) and \(Au + HNO3 -> Au + HNO3\). 2. Reaction of AgNO3 with Water: \(3AgNO3+ 3H2O -> 3Ag + 4NO + 6H2O\). 3. Reaction of Ag and Au with Sulphuric Acid: \(2Ag + H2SO4 -> Ag2SO4 +SO2 +2H2O\) and \(Au + H2SO4 -> Au + H2SO4\).

Step 1: Reaction of Ag and Au with Nitric Acid

When Ag and Au react with nitric acid, the nitric acid oxidizes Ag to Ag+ and Au remains unaffected due to its low reactivity. The reaction can be written as follows: \(3Ag + 4HNO_3 -> 3AgNO_3 + NO + 2H_2O\) and \(Au + HNO_3 -> Au + HNO_3\) (No reaction takes place)

Step 2: Reaction of AgNO3 with Water

The silver nitrate formed in the first step will further react with water to form silver, nitric oxide and water. The reaction can be written as follows: \(3AgNO3+ 3H2O -> 3Ag + 4NO + 6H2O\)

Step 3: Reaction of Ag and Au with Sulphuric Acid

When the alloy is reacted with hot concentrated sulphuric acid, the silver from the alloy reacts with the sulphuric acid to produce silver sulfate and SO2, while gold again remains unaffected. The reaction can be written as follows: \(2Ag + H2SO4 -> Ag2SO4 +SO2 +2H2O\) and \(Au + H2SO4 -> Au + H2SO4\) (No reaction takes place)

Key Concepts

Silver Separation

Silver separation is a fascinating process in the metallurgical extraction of precious metals. When silver is mixed with gold, creating an alloy, we often need to separate the two for various applications. This can be achieved through a chemical process called parting. In this process, a suitable reagent like \(HNO_3\) is used to oxidize silver, turning it into silver ions (Ag^+). This change doesn't affect gold since it doesn't react with nitric acid. Hence, nitric acid selectively separates silver from the mixture by converting it into silver nitrate (AgNO_3).
The chemical equation for this reaction is:
\[3Ag + 4HNO_3 \rightarrow 3AgNO_3 + NO + 2H_2O\]
This technique is essential for the refining of metals to get pure silver from the alloy.
It's interesting to note that not only does this reaction make the silver soluble, it also leaves gold in its solid form, ready for further processing.

Gold Separation

Gold separation in metallurgical processes entails obtaining pure gold from an alloy, such as one composed of both gold and silver. Despite the complexity of co-existing metals, reagents like concentrated \(HNO_3\) or \(H_2SO_4\) are effective.
During the separation process, gold's outstanding resistance to these acids means it remains unreacted. This unique property allows gold to remain solid while silver converts into soluble compounds. As such, gold does not participate in the chemical reactions with either nitric acid or sulphuric acid as silver does. This characteristic aids in parting, where gold remains unchanged, and can be removed from the solution after silver is separated.
Utilizing these specific reactions, metallurgists successfully achieve gold separation efficiently, respecting the precious metal's unique inertness to these strong acids.

Nitric Acid Reaction

The reaction of silver with nitric acid is a pivotal step in metallurgy, particularly in the separation of silver from gold. Concentrated nitric acid (HNO_3) acts as a powerful oxidizing agent. When silver and gold alloy is exposed to HNO_3, only silver oxidizes, forming silver nitrate (AgNO_3) and releasing gases like nitric oxide (NO).
Here's the chemical reaction:
\[3Ag + 4HNO_3 \rightarrow 3AgNO_3 + NO + 2H_2O\]
As this occurs, gold remains unreacted due to its lower reactivity, thereby facilitating the separation process. It's a clear demonstration of the selective nature of chemical reactions based on metal reactivity.
This technique is both efficient and precise and is an example of how controlled chemical reactions can lead to successful metal recovery and purification. Understanding these reactions deeply is crucial for those studying or working in metallurgical sciences.

Sulphuric Acid Reaction

In metallurgical processes, using concentrated sulphuric acid (\(H_2SO_4\)) is another viable method for separating silver from gold. When the silver-gold alloy is subjected to hot \(H_2SO_4\), a critical reaction takes place. Silver reacts with this acid to form silver sulfate (Ag_2SO_4), while sulfur dioxide (SO_2) and water are produced as by-products. All this time, gold remains unresponsive to \(H_2SO_4\), maintaining its pure form.
The reaction is as follows:
\[2Ag + H_2SO_4 \rightarrow Ag_2SO_4 + SO_2 + 2H_2O\]
This process is particularly advantageous in scenarios where recovering silver in its sulfate form is necessary. The non-reactivity of gold simplifies recovery, as it remains unchanged and easily extractable after this reaction completes.
Understanding the role of acids in separating metals highlights the intricate balance of chemical properties utilized in industrial applications, showcasing the ingenuity in metallurgical techniques.