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 $$\mathrm{Te}^{4+}(a q)+4 \mathrm{e}^{-} \longrightarrow \mathrm{Te}(s) \quad E_{\mathrm{red}}^{\circ}=0.57 \mathrm{~V}$$ Given this information, describe the probable fate of tellurium impurities during electrorefining.

Short answer

The tellurium impurities are more likely to be reduced and deposited onto the cathode before copper ions during the electrorefining process, as tellurium has a higher standard reduction potential (\(0.57\) V) compared to copper (\(0.34\) V). The presence of tellurium impurities may negatively affect the effectiveness of the copper purification in the electrorefining process. However, it is important to consider that actual behavior of the elements in the electrorefining solution may differ depending on factors like temperature, concentration, and pH.

Step-by-step solution

Understanding electrorefining

Electrorefining is an electrochemical process used to purify metals, such as copper, by removing impurities from crude copper. The process involves placing the impure copper as the anode and a pure copper as the cathode in an electrolytic cell. When a voltage is applied, the copper ions in the solution are reduced and deposited onto the cathode, while impurities in the anode either remain as sludge or go into the solution, depending on their standard reduction potential.

Analyzing standard reduction potential

Given the standard reduction potential of tellurium is \(E_{\mathrm{red}}^{0}=0.57 \mathrm{~V}\), we notice it has a positive value. This means that tellurium is more likely to be reduced (gain electrons) when compared to hydrogen (\(E_{\mathrm{red}}^{0}=0\)).

Comparing tellurium with copper

The most important factor to consider in electrorefining is the reduction potential of the metal being refined. In this case, that metal is copper. The standard reduction potential for copper ions, \(\mathrm{Cu}^{2+}\), to be reduced to solid copper, \(\mathrm{Cu}(s)\), is \(E_{\mathrm{red}}^{0}=0.34 \mathrm{~V}\) (given in a standard electrode potential table).

Determine the fate of tellurium impurities

Comparing the standard reduction potentials of tellurium (\(0.57\) V) and copper (\(0.34\) V), we can see that tellurium has a higher reduction potential. This means that tellurium is more likely to be reduced (gain electrons) than copper ions in the electrorefining process. However, it is important to note that these reduction potentials are for standard conditions (1 M concentration of ions). The actual behavior of the elements in the electrorefining solution may differ depending on factors like temperature, concentration, and pH. Nevertheless, the basis of our analysis indicates that the tellurium impurities will likely be reduced and deposited onto the cathode before copper ions during the electrorefining process. This means that the electrorefining process may not purify the copper as effectively due to the presence of tellurium impurities.

Key concepts

Tellurium Impurities

In electrorefining, the presence of impurities plays a significant role in the purification process. Tellurium is one such impurity found in crude copper, which presents a challenge during the purification process due to its chemical properties.
Tellurium is not like typical impurities that easily separate from copper during electrorefining. Instead, due to its high standard reduction potential, tellurium can actually interfere with the copper purification process.

• Its presence in the anode may lead it to undergo reduction and potential deposition onto the cathode, where purified copper is being deposited.
• This results in contamination of what should be pure copper layers, potentially decreasing the efficiency of the refining process.

Understanding the behavior of tellurium impurities helps in planning further refinement steps to achieve the desired level of purity in the copper product.

Standard Reduction Potential

Standard reduction potential (SRP) is a key concept in electrochemistry, indicating the tendency of a chemical species to gain electrons, thus being reduced. It serves as an important indicator when predicting reactions that will occur in an electrochemical cell.
The standard reduction potential for tellurium, given as 0.57 V, helps us understand its behavior versus other elements like copper, which has a standard reduction potential of 0.34 V.

• A higher reduction potential means an element is more likely to gain electrons and undergo reduction first.
• In electrorefining, elements with higher SRP than the noble metal being refined can deposit earlier at the cathode, potentially compromising the purity of the refined metal.

By comparing the SRP of copper and tellurium, it's evident why tellurium might deposit accidentally during the copper electrorefining process.

Copper Purification

Copper electrorefining is a meticulous process aimed at obtaining high-purity copper from crude sources. The process uses an electrolytic cell setup where impure copper is the anode, and pure copper is the cathode.
During electrorefining:

• When a voltage is applied, copper ions from the anode dissolve into the solution and then reduce and deposit onto the cathode as pure copper.
• Ideally, impurities with lower reduction potentials remain in the solution or form a sludge at the bottom of the cell.

However, impurities like tellurium, with high reduction potentials, can "outcompete" copper ions and also deposit onto the cathode.
The tactics to counter this include precise control of electrolytic conditions, ensuring that tellurium concentration remains low, and secondary refining procedures if needed.
Ultimately, understanding and controlling the electrochemical behaviors at play is crucial in achieving the desired copper purity levels.