Problem 96 - Chemistry: The Central Science in SI Units, Global Edition

Question

A mixture of copper and gold metals 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. 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 electrorefining, tellurium impurities will likely go into solution as \(\mathrm{Te}^{4+}\) ions, given its standard reduction potential of 0.57 V is higher than that of copper but lower than gold. The tellurium impurities will then plate out on the cathode along with gold but before copper.

Step 1: 1. Familiarize with Electrorefining Process

Electrorefining is a process in which an impure metal is purified through the use of an electrolytic cell. In this case, a copper-gold mixture with tellurium impurities is being refined. When a voltage is applied, the impure metal reacts at the anode, and the pure metal deposits at the cathode. Impurities can either settle to the bottom as slag or enter the solution, depending on their properties.

Step 2: 2. List provided information

We know the following information from the problem statement: - The standard reduction potential for tellurium, \(E_{\mathrm{red}}^\circ(\mathrm{Te}^{4+}/\mathrm{Te})\), is 0.57 V. - We are trying to determine the fate of tellurium impurities during electrorefining.

Step 3: 3. Compare standard reduction potentials

To determine the probable fate of tellurium impurities, we need to compare its standard reduction potential with that of copper. The standard reduction potentials for copper and gold are as follows: - Copper: \(E_{\mathrm{red}}^\circ(\mathrm{Cu}^{2+}/\mathrm{Cu}) = +0.337 \mathrm{~V}\) - Gold: \(E_{\mathrm{red}}^\circ(\mathrm{Au}^{3+}/\mathrm{Au}) = +1.498 \mathrm{~V}\) Since the standard reduction potential of tellurium is higher than that of copper but lower than gold, tellurium is more likely to be reduced than copper, but not as likely as gold.

Step 4: 4. Determine the fate of tellurium impurities

We now consider the three possible scenarios: 1. Impurities fall to the bottom, unchanged: This situation would occur if tellurium neither oxidizes nor reduces during the electrorefining process, which is unlikely given the provided reduction potential. 2. Impurities go into solution as ions: This happens if tellurium oxidizes more readily than copper. Comparing reduction potentials, we find that tellurium's potential is higher than copper's. Therefore, tellurium is more likely to be reduced than copper, implying that tellurium ions would enter the solution and not turn to slag. 3. Impurities plate out on the cathode: This situation can occur when a metal with a higher reduction potential than copper plates out on the cathode during the process. The reduction potential of tellurium is higher than copper but lower than gold, which means tellurium will have a tendency to plate out on the cathode along with gold but before copper. Based on the discussion above, the probable fate of tellurium impurities during electrorefining is to go into solution as \(\mathrm{Te}^{4+}\) ions and then plate out on the cathode.

Key Concepts

Standard Reduction Potential

The standard reduction potential is a measure of the tendency of a chemical species to gain electrons and thereby be reduced. It is represented as the electrode potential, under standard conditions of 1M concentration, 1 atmosphere pressure, and at a temperature of 25°C. The standard reduction potential is essential in predicting the direction of electron flow in electrochemical cells. It informs us about which species can oxidize others and which can be reduced by others.

In the context of electrorefining, such as in the exercise where we need to refine copper and gold from impurities, the standard reduction potentials of different metals and impurities play a crucial role. By comparing their values, we can determine which substances will be reduced or oxidized during the process.

• If a metal's standard reduction potential is higher, it is more ‘eager’ to gain electrons compared to a metal with a lower standard reduction potential.
• For example, tellurium has a standard reduction potential of 0.57 V, which is higher than copper's 0.337 V. This implies tellurium ions are more likely to gain electrons and get reduced back to solid form.
• In contrast, gold, with its significant potential of 1.498 V, will be preferentially reduced before both copper and tellurium.

Understanding these values helps determine whether certain impurities will precipitate, remain in solution, or deposit on the cathode during electrorefining.

Copper Refining

Copper refining through electrorefining is an effective process to obtain pure copper from its ore or mixtures containing impurities like gold. The primary mechanism involves using an electrolytic cell where impure copper acts as the anode and pure copper at the cathode.

Here's how the process unfolds:

• When the voltage is applied, copper at the anode oxidizes to form copper ions ( ext{Cu}^{2+}), which go into the solution.
• Meanwhile, at the cathode, copper ions gain electrons and deposit back as pure copper metal.

This continuous cycling leads to the accumulation of pure copper, while impurities either settle as sludge at the bottom of the cell or remain in solution based on their reduction potentials. In our specific scenario, the standard reduction potential of tellurium indicates that during electrorefining, tellurium is more likely to join gold ions in plating out at the cathode due to its intermediate reduction potential value between that of copper and gold.

This precise understanding allows the selective removal and deposition of metals, efficiently refining copper while dealing with its associated impurities.

Impurities in Metallurgy

Impurities in metallurgy refer to unwanted elements or compounds present within metals and alloys, which can affect their quality, performance, and refinement. The challenge in metallurgy is to remove or manage these impurities to achieve desired purity levels for industrial applications.

Different impurities behave differently during metallurgical processes, like electrorefining, based on their chemical properties.

• Some impurities, like gold, have very high standard reduction potentials, meaning they preferentially form stable elemental metals and deposit at the cathode.
• Others, such as copper and tellurium, require careful management as they both have potential to be reduced but to different extents depending on their standard reduction potentials.

In electrorefining, impurities present in the anode material can either oxidize and remain in the solution or reduce and deposit on the cathode. Taking tellurium as an example, its reduction potential implies it can be effectively separated by allowing it to plate out along with gold. This management of impurities ensures the purity of the refined metal, in this case, copper, enhancing its quality for subsequent industrial applications.