Problem 96 - Chemistry: The Central Science

Question

The crude copper that is subjected to electrorefining contains tellurium as an impurity. The standard reduction potential between tellurium and its lowest common oxidation state, \(\mathrm{Te}^{4+}\), is $$ \mathrm{Te}^{4+}(a q)+4 e^{-} \longrightarrow \mathrm{Te}(s) \quad E_{\text {iod }}^{s}=0.57 \mathrm{~V} $$ Given this information, describe the probable fate of tellurium impurities during electrorefining. Do the impurities fall to the bottom of the refining bath, unchanged, as copper is oxidized, or do they go into solution as ions? If they go into solution, do they plate out on the cathode?

Step-by-Step Solution

Answer

During the electrorefining process, tellurium impurities will go into solution as \(\mathrm{Te}^{4+}\) ions and plate out on the cathode due to their higher reduction potential (\(E_\text{iod}^{s}=0.57V\)) compared to copper (\(E^{0}=0.34V\)). The impurities will not fall to the bottom of the refining bath or remain unchanged.

Step 1: Write down the reaction for copper electrorefining

The overall reaction of copper electrorefining is a redox process, involving the oxidation of copper in an impure anode and reduction in a pure cathode: Anode: \(\mathrm{Cu}(s) \longrightarrow \mathrm{Cu}^{2+}(aq) + 2 e^-\) Cathode: \(\mathrm{Cu}^{2+}(aq) + 2 e^- \longrightarrow \mathrm{Cu}(s)\)

Step 2: Determine the standard reduction potential of copper

The standard reduction potential for the half-reaction at the cathode, \(\mathrm{Cu}^{2+}(aq) + 2 e^- \longrightarrow \mathrm{Cu}(s)\), is \(E^{0}=+0.34V\).

Step 3: Compare the reduction potentials

We now compare the reduction potential of tellurium, \(E_\text{iod}^{s}=0.57V\), to the reduction potential of copper, \(E^{0}=0.34V\). Since the standard reduction potential of tellurium is higher than that for copper, it is more thermodynamically favorable for tellurium to be reduced.

Step 4: Determine the fate of tellurium impurities during electrorefining

During electrorefining, the tellurium impurities would undergo reduction more easily compared to copper due to their higher reduction potential. This means that tellurium impurities would go into solution as \(\mathrm{Te}^{4+}\) ions and then plate out on the cathode. The impurities would not fall to the bottom of the refining bath or remain unchanged. To summarize, during the electrorefining process, tellurium impurities will go into solution as ions and plate out on the cathode due to their higher reduction potential compared to copper.

Key Concepts

Standard Reduction Potential

Understanding the standard reduction potential is key to predicting how different elements behave in electrochemical reactions. This value is a measure of the tendency of a chemical species to acquire electrons and be reduced. In the context of electrorefining, elements with higher standard reduction potentials are more likely to gain electrons and deposit on the cathode.

For example, tellurium has a standard reduction potential of +0.57 V, which means it has a greater likelihood of being reduced than copper, which has a standard reduction potential of +0.34 V. Therefore, when both are present in the same electrochemical cell, tellurium will be reduced and plated on the cathode first. This hierarchical plating is crucial in purifying metals, as it ensures that only the desired metal is deposited while impurities are left in the solution or as residue.

Redox Process

A redox process, shorthand for reduction-oxidation reaction, involves the transfer of electrons between two substances. It is a fundamental concept in chemistry, particularly in the field of electrochemistry. Every redox reaction consists of two half-reactions: oxidation, where a substance loses electrons, and reduction, where another substance gains those electrons.

In copper electrorefining, the redox process entails the oxidation of copper at the anode to form copper ions, and the reduction of these ions at the cathode to form pure copper metal. The movement of electrons through the external circuit and ions through the solution completes the electrical circuit, allowing the current to flow and facilitating the electrorefining process.

Copper Electrorefining

Copper electrorefining is a technique for purifying metals, especially copper, in which electric current is used to drive the oxidation of an impure metal anode and reduction at a separate pure metal cathode. During this process, copper is plated out onto the cathode, resulting in a higher purity copper than was originally at the anode.

Impurities that might be present in the crude copper, such as tellurium, are usually either less noble, which causes them to remain in solution, or they have higher standard reduction potentials, causing them to plate out onto the cathode before copper does. In the case of tellurium, because its standard reduction potential is higher than copper's, it will plate out on the cathode, which must then be treated to remove the tellurium deposits.