Question

Metallic gold is collected from below the anode when a mixture of copper and gold metals is refined by electrolysis. Explain this behavior.

Short answer

In the electrolysis of a copper-gold mixture, metallic gold is collected below the anode due to the differences in redox potentials between the two metals. Copper has a lower redox potential and is more prone to oxidation at the anode and reduction at the cathode. Gold, with a higher redox potential, remains in the electrolyte solution and settles down below the anode due to its higher density.

Step-by-step solution

Understanding electrolysis

Electrolysis is a process that uses an electric current to drive a non-spontaneous chemical reaction. In the case of refining metals, this will involve the dissolution of the metal at the anode (positive electrode) and its deposition at the cathode (negative electrode). This is done in an electrolyte solution which facilitates the flow of ions between the electrodes.

Understanding the behavior of copper and gold in electrolysis

Both copper and gold are metals that have positive redox potentials, which means they are generally more resistant to oxidation (loss of electrons) and more likely to undergo reduction (gain of electrons). In an electrolytic cell, the metal that has the highest redox potential will have the greatest tendency to be reduced at the cathode, while the metal with the lower redox potential will oxidize more readily at the anode.

Comparing redox potentials of copper and gold

Although both copper and gold have positive redox potentials, gold has a higher redox potential than copper. Gold has a reduction potential of +1.50 V while copper has a reduction potential of +0.337 V. This means gold is less prone to oxidation and more prone to reduction than copper.

Explaining the behavior of gold in the electrolysis of copper and gold

Based on the redox potentials, when a mixture of copper and gold is subjected to electrolysis, copper is more likely to be oxidized at the anode compared to gold. This is because copper has a lower redox potential, therefore it will readily lose electrons to form Cu(2+) ions in the electrolyte. At the cathode, gold ions will be more likely to undergo reduction due to their higher redox potential. However, when copper ions are present in the solution, they will preferentially reduce at the cathode, forming copper metal. As a result, gold ions will not efficiently deposit onto the cathode and will instead remain in the electrolyte solution. Since gold has a higher density than the electrolyte solution, the gold ions in the solution will gradually settle down due to gravity, forming a layer of metallic gold below the anode. So in conclusion, metallic gold is collected from below the anode during the electrolysis of a mixture of copper and gold because of the differences in the redox potentials between the two metals. Copper is more likely to be oxidized at the anode and reduced at the cathode, while gold remains in the electrolyte solution and eventually settles down due to its higher density.

Key concepts

Redox potential

Redox potential is a fundamental concept in understanding electrochemical reactions, such as those occurring in electrolysis. It measures the tendency of a chemical species to acquire electrons and be reduced. A higher redox potential indicates a greater likelihood for a substance to gain electrons (undergo reduction), while a lower redox potential suggests a greater tendency for electron loss (oxidation).

In the context of electrolysis, the electrodes and their respective potentials decide which metal will deposit or dissolve. Positive redox potentials mean that metals are more resistant to oxidation. So, between two metals, the one with the higher redox potential will remain unoxidized and more likely to gain electrons at the cathode. In a setup with copper and gold, gold's higher redox potential (\(+1.50 \, \text{V}\)) over copper (\(+0.337 \, \text{V}\)) implies that it is less likely to lose electrons in the presence of copper.

Copper refining

Copper refining through electrolysis involves removing impurities from crude copper to produce high-purity copper. During this process, crude copper acts as the anode, dissolving into the electrolyte under the influence of an electric current. Pure copper then plates onto the cathode.

The crude copper, containing impurities, may also include other metals, like gold. Since copper has a lower redox potential than these impurities, it is easily oxidized to Cu(2+) ions, leaving the more noble metals (such as gold) undisturbed at the anode.

This process effectively purifies the copper, as its ions are selectively deposited on the cathode while the impurities settle differently based on their electrochemical properties or are left in the anode sludge.

Gold refining

Gold refining often involves technologies like electrolysis, ensuring separation from less noble metals. Gold's high redox potential helps it resist oxidation in mixtures with other metals.

In an electrolytic cell containing both copper and gold, gold does not oxidize as readily as copper due to its higher redox potential. Instead, copper oxidizes and moves to the cathode as copper metal.

Since gold remains largely undissolved, it eventually precipitates out of the solution due to its density. Metallic gold collects as a "sludge" or solid deposit at the base of the electrolytic cell, typically below the anode. This makes electrolysis an efficient method for separating gold from its ores or mixed metal alloys, allowing for the extraction of highly pure gold.