Problem 38 - Chemistry: Principles and Reactions

Question

Silver is obtained in much the same manner as gold, using \(\mathrm{NaCN}\) solution and \(\mathrm{O}_{2}\), Describe with appropriate equations the extraction of silver from argentite ore, \(A g_{2}\). . (The products are \(\mathrm{SO}_{2}\) and \(\mathrm{Ag}(\mathrm{CN})_{2}^{-}\), which is reduced with zinc.)

Step-by-Step Solution

Answer

Question: Describe the extraction process of silver from its ore argentite using sodium cyanide and oxygen. Answer: The extraction of silver from argentite ore involves the following steps: 1. Formation of Silver Cyanide: Argentite ore reacts with sodium cyanide and oxygen to form silver cyanide and sodium sulfate: \(4Ag_2S + 8NaCN + 2O_2 + H_2O \rightarrow 4Ag(CN)_2^- + 2Na_2SO_4 + 4H^+\) 2. Formation of Sulfur Dioxide: Sodium sulfate reacts with hydrogen ions to form sulfur dioxide: \(2Na_2SO_4 + 4H^+ \rightarrow 2SO_2 + 4Na^{+} + 2H_2O\) 3. Reduction of Silver Cyanide with Zinc: Silver cyanide is reduced using zinc to form solid silver and zinc cyanide: \(2Ag(CN)_2^- + Zn \rightarrow 2Ag + Zn(CN)_4^{2-}\) 4. Silver Recovery: Silver is recovered from the precipitate and can be further purified to yield high-purity silver metal.

Step 1: Formation of Silver Cyanide

The argentite ore, \(Ag_2S\), reacts with the sodium cyanide (\(NaCN\)) and oxygen (\(O_2\)) to form silver cyanide (\(Ag(CN)_2^-\)) and sodium sulfate (\(Na_2SO_4\)). The balanced chemical equation for this reaction is: \(4Ag_2S + 8NaCN + 2O_2 + H_2O \rightarrow 4Ag(CN)_2^- + 2Na_2SO_4 + 4H^+\) Sodium cyanide is a highly toxic compound, so it's crucial to handle it carefully and with proper safety measures.

Step 2: Formation of Sulfur Dioxide

When extracting silver, sulfur dioxide is produced as a byproduct in the reaction between argentite (\(Ag_2S\)) and sodium cyanide (\(NaCN\)), as mentioned above. The balanced chemical equation for the production of sulfur dioxide is: \(4Ag_2S + 8NaCN + 2O_2 + H_2O \rightarrow 4Ag(CN)_2^- + 2Na_2SO_4 + 4H^+\) The \(2Na_2SO_4\) reacts further with \(4H^+\) to form sulfur dioxide (\(SO_2\)): \(2Na_2SO_4 + 4H^+ \rightarrow 2SO_2 + 4Na^{+} + 2H_2O\)

Step 3: Reduction of Silver Cyanide with Zinc

Silver cyanide (\(Ag(CN)_2^-\)) is soluble in water, and thus, it needs to be recovered in the form of a precipitate. For this purpose, zinc is used for the reduction of silver cyanide. The balanced chemical equation for this reaction is: \(2Ag(CN)_2^- + Zn \rightarrow 2Ag + Zn(CN)_4^{2-}\) In this step, silver (\(Ag\)) is recovered as a solid precipitate while the zinc cyanide (\(Zn(CN)_4^{2-}\)) formed is still in solution.

Step 4: Silver Recovery

After the reduction of silver cyanide with zinc, the silver is recovered from the precipitate, which can then be purified further, yielding high-purity silver metal. This marks the end of the extraction process for silver from argentite ore using sodium cyanide and oxygen.

Key Concepts

Chemical Equations

Chemical equations are vital for understanding chemical reactions because they provide a clear and concise representation of the transformations taking place. In the context of extracting silver from argentite, multiple chemical reactions occur in a stepwise manner.

For instance, the first reaction involves argentite ore reacting with sodium cyanide in the presence of oxygen to form silver cyanide, sodium sulfate, hydrogen ions, and water. The balanced chemical equation represents the stoichiometry of reactants and products, ensuring that the law of conservation of mass is upheld. Handling the reactants, especially sodium cyanide, requires adherence to strict safety protocols due to its toxicity.

Silver Cyanide Formation

Silver cyanide formation is a pivotal step in the process of extracting silver from argentite. When argentite ore reacts with sodium cyanide and oxygen, the silver ions in the ore bind with the cyanide ions, forming silver cyanide. Silver cyanide is a dicyanoargentate(\text{I}) complex, denoted by the chemical formula \(Ag(CN)_2^-\).

The formation of this complex is beneficial because silver cyanide is soluble in water, allowing for the separation of silver from other impurities present in the ore. This solubility plays a critical role in the later stages of the process where silver is precipitated from the solution.

Reduction with Zinc

The final stage in the extraction of silver from argentite involves reduction, which is a chemical reaction that results in a decrease in the oxidation state of a substance. In this case, silver cyanide is reduced using zinc metal.

Zinc serves as the reducing agent by giving up electrons to the silver ions in the silver cyanide complex, which precipitates pure silver as a result. The chemical equation for this reaction shows zinc replacing silver in the complex to form zinc cyanide, allowing the solid silver to be separated out. After the reduction, the recovered silver can be purified further to obtain high-quality silver metal. This process demonstrates the practical application of redox chemistry in metallurgy.