Problem 95 - Chemistry: The Central Science

Question

Metallic gold is collected from below the anode when crude copper metal is refined by electrolysis. Explain this behavior.

Step-by-Step Solution

Answer

During the electrolysis of crude copper, metallic gold is collected below the anode because it has a lower reactivity and oxidation potential than copper. Gold does not participate in the oxidation or reduction reactions during electrolysis and remains unoxidized in the sludge that forms below the anode. As the process continues, concentrations of gold and other less reactive metals increase and eventually precipitate out for collection.

Step 1: Understanding Electrolysis

Electrolysis is a process where electrical energy is used to drive a non-spontaneous redox reaction. In this process, an electric current is passed through an electrolyte, causing the oxidation and reduction to occur at the anode and cathode, respectively. The anode is the positive electrode where oxidation occurs, while the cathode is the negative electrode where reduction occurs.

Step 2: Electrolysis of Copper

During the electrolysis of crude copper, the copper is oxidized at the anode and then reduced at the cathode to form pure copper. During this process, impurities in the crude copper anode dissolve into the electrolytic solution and accumulate in the form of sludge. Gold, being one of the impurities in the crude copper, also dissolves during the electrolysis process.

Step 3: Gold Accumulation below the Anode

Gold has a lower reactivity than copper and therefore, it does not preferentially oxidize in the electrolytic cell. So, it remains unoxidized in the sludge that forms below the anode during electrolysis. As the process continues, the concentrations of gold and other less reactive metals increase, and eventually, they will precipitate out and get collected at the bottom of the anode. In other words, gold does not participate in the oxidation or reduction reactions during electrolysis and gathers below the anode because it has a lower oxidation potential than copper. This behavior of gold allows for its extraction and purification from the crude copper through the electrolysis process.