Question

Write plausible equations for the following reactions occurring in the hydrometallurgy of the coinage metals. (a) Copper is precipitated from a solution of copper(II) sulfate by treatment with \(\mathrm{H}_{2}(\mathrm{g})\) (b) Gold is precipitated from a solution of \(\mathrm{Au}^{+}\) by adding iron(II) sulfate. (c) Copper(II) chloride solution is reduced to copper(I) chloride when treated with \(\mathrm{SO}_{2}(\mathrm{g})\) in acidic solution.

Short answer

The balanced chemical reactions are: \n (a) \(\mathrm{CuSO}_{4}(\mathrm{a.q.}) + \mathrm{H}_{2}(\mathrm{g}) \rightarrow \mathrm{Cu(s)} + \mathrm{H}_{2}\mathrm{SO}_{4}(\mathrm{a.q.})\) \n (b) \(2 \, \mathrm{Au}^{+}(\mathrm{a.q.}) + 3 \, \mathrm{FeSO}_{4}(\mathrm{a.q.}) \rightarrow 2 \, \mathrm{Au(s)} + 3 \, \mathrm{Fe}^{2+}(\mathrm{a.q.}) + 3 \, \mathrm{SO}_{4}^{2-}(\mathrm{a.q.})\) \n (c) \(2 \, \mathrm{CuCl}_{2}(\mathrm{a.q.}) + \mathrm{SO}_{2}(\mathrm{g}) + 2 \, \mathrm{H}^{+}(\mathrm{a.q.}) \rightarrow 2 \, \mathrm{CuCl}(\mathrm{s}) + \mathrm{H}_{2}\mathrm{SO}_{4}(\mathrm{a.q.})\)

Step-by-step solution

Write Equation for Precipitation of Copper

Copper(II) sulfate reacts with hydrogen gas to precipitate copper. The reaction can be written as: \[ \mathrm{CuSO}_{4}(\mathrm{a.q.}) + \mathrm{H}_{2}(\mathrm{g}) \rightarrow \mathrm{Cu(s)} + \mathrm{H}_{2}\mathrm{SO}_{4}(\mathrm{a.q.}) \] This equation is already balanced.

Write Equation for Precipitation of Gold

Gold ions react with iron(II) sulfate to precipitate gold. The reaction can be written as: \[ 2 \, \mathrm{Au}^{+}(\mathrm{a.q.}) + 3 \, \mathrm{FeSO}_{4}(\mathrm{a.q.}) \rightarrow 2 \, \mathrm{Au(s)} + 3 \, \mathrm{Fe}^{2+}(\mathrm{a.q.}) + 3 \, \mathrm{SO}_{4}^{2-}(\mathrm{a.q.}) \] Here, we needed to balance the equation by multiplying the gold ions and gold solids by 2, and iron(II) sulfate, iron ions and sulfate ions by 3.

Write Equation for Reduction of Copper(II) Chloride

Copper(II) chloride is reduced to copper(I) chloride when treated with sulfur dioxide gas in acidic solution. The reaction can be written as follows: \[ 2 \, \mathrm{CuCl}_{2}(\mathrm{a.q.}) + \mathrm{SO}_{2}(\mathrm{g}) + 2 \, \mathrm{H}^{+}(\mathrm{a.q.}) \rightarrow 2 \, \mathrm{CuCl}(\mathrm{s}) + \mathrm{H}_{2}\mathrm{SO}_{4}(\mathrm{a.q.}) \] This equation was balanced by multiplying copper(II) chloride and copper(I) chloride by 2.

Key concepts

Precipitation Reactions

Precipitation reactions are fundamental chemical reactions where a solid, known as the precipitate, forms from a solution. This process occurs when two solutions, each containing different ions, are mixed and an insoluble compound is produced. The insoluble compound falls out of the solution as a fine solid or precipitate. This happens because the product of the concentrations of the two ions exceeds the solubility product, leading to the crystallization of the solid.

In hydrometallurgy, precipitation reactions are used to recover metals from solutions. For example, in one of the given reactions, copper(II) sulfate reacts with hydrogen gas to form copper metal and sulfuric acid. The chemical equation is:

• \( \mathrm{CuSO}_{4}(\mathrm{aq}) + \mathrm{H}_{2}(\mathrm{g}) \rightarrow \mathrm{Cu(s)} + \mathrm{H}_{2}\mathrm{SO}_{4}(\mathrm{aq}) \)
  

These types of reactions are not only crucial in metallurgical processes but are also frequently observed in laboratory experiments and industrial applications.

Redox Reactions

Redox reactions, short for reduction-oxidation reactions, involve the transfer of electrons between two substances. This simultaneous process includes one substance undergoing oxidation (loss of electrons) and another undergoing reduction (gain of electrons). Redox reactions are essential for understanding various chemical processes, including corrosion, combustion, and many biological processes.

In the hydrometallurgy of coinage metals, redox reactions help in extracting valuable metals from their ores. For example, when gold is precipitated from a solution of \(\mathrm{Au}^{+}\) by adding iron(II) sulfate, a redox reaction occurs where gold ions are reduced to solid gold, and iron(II) is oxidized:

• \( 2 \, \mathrm{Au}^{+}(\mathrm{aq}) + 3 \, \mathrm{FeSO}_{4}(\mathrm{aq}) \rightarrow 2 \, \mathrm{Au(s)} + 3 \, \mathrm{Fe}^{2+}(\mathrm{aq}) + 3 \, \mathrm{SO}_{4}^{2-}(\mathrm{aq}) \)
  

This equation shows how valuable metals can be recovered through redox chemistry, utilizing their ability to switch oxidation states. Understanding these reactions allows chemists to develop efficient methods to extract and purify metals.

Coinage Metals

Coinage metals refer to a group of metals that have been historically used for minting coins. These include copper, silver, and gold, which are valued for their durability, resistance to corrosion, and aesthetic appeal. These metals are located in the same group on the periodic table and share similar physical and chemical characteristics.

The significance of coinage metals in hydrometallurgy lies in their ability to undergo various chemical reactions, such as precipitation and redox reactions, which are used to refine and recover these metals from their ores. For example, in the reduction of copper(II) chloride to copper(I) chloride using sulfur dioxide in acidic conditions, copper undergoes a change in its oxidation state, highlighting its chemical versatility:

• \( 2 \, \mathrm{CuCl}_{2}(\mathrm{aq}) + \mathrm{SO}_{2}(\mathrm{g}) + 2 \, \mathrm{H}^{+}(\mathrm{aq}) \rightarrow 2 \, \mathrm{CuCl}(\mathrm{s}) + \mathrm{H}_{2}\mathrm{SO}_{4}(\mathrm{aq}) \)
  

Such reactions are crucial for the purification and processing of coinage metals, ensuring their availability and quality for industrial and ornamental uses.