Electroplating, copper purification, environmental links：練習問題

During an electroplating process, the metallic object to be coated is connected to the negative terminal of the D.C. power supply. What is the fundamental chemical process that occurs at the surface of this object?

The electroplating industry generates wastewater containing various metal ions. Chromium(VI) ions (e.g., in \(\text{Cr}_2\text{O}_7^{2-}(aq)\)) are often used in chrome plating baths and are highly toxic. A crucial step in wastewater treatment is reducing \(\text{Cr}_2\text{O}_7^{2-}\) to less toxic \(\text{Cr}^{3+}\) ions, which are then precipitated as chromium(III) hydroxide sludge, \(\text{Cr(OH)}_3\).

Which statement best describes the primary environmental challenge posed by the resulting \(\text{Cr(OH)}_3\) sludge?

An industrial electrolytic cell is used for refining impure copper. The impure copper anode contains significant amounts of zinc, iron, and silver. The electrolyte is an aqueous solution of copper(II) sulfate. Given the standard electrode potentials:

\(E^\circ(Zn^{2+}(aq)/Zn(s)) = -0.76\,V\)
\(E^\circ(Fe^{2+}(aq)/Fe(s)) = -0.44\,V\)
\(E^\circ(Cu^{2+}(aq)/Cu(s)) = +0.34\,V\)
\(E^\circ(Ag^+(aq)/Ag(s)) = +0.80\,V\)

Which of the following statements most accurately describes the electrochemical processes and their associated environmental links during this copper refining?

A metal object is electroplated with copper. During this process, which electrode and reaction type are associated with the surface of the object being coated?

An electrolytic cell is set up to purify a sample of impure copper. The anode consists of copper with small amounts of silver and zinc as impurities. The electrolyte used is an acidified copper(II) sulfate solution. Which of the following statements concerning this electrolysis process are correct?

(1) The concentration of \(Cu^{2+}(aq)\) in the electrolyte decreases during the process.
(2) Silver ions are reduced and deposited on the cathode along with the purified copper.
(3) A sludge containing silver is formed at the bottom of the cell underneath the anode.

In the electrolytic refining of copper, is the impure copper used as the anode or the cathode? State the reason.

Explain why gold and silver are collected as 'anode slime' rather than being oxidised into the electrolyte during the industrial electrolytic purification of copper.

In the industrial purification of copper, the electrolyte contains \(Cu^{2+}(aq)\) and \(H_2SO_4(aq)\). Explain why the concentration of copper(II) ions in the solution decreases slightly over time if the impure copper anode contains reactive impurities such as iron and nickel, and state one environmental consequence of discharging the untreated spent electrolyte.

Steel cans are often electroplated with a thin layer of tin to prevent them from rusting. This process is known as tin-plating.

(a) To plate a steel can with tin, should the can be connected to the positive or negative terminal of the d.c. power supply?

(b) Write the ionic half-equation for the reaction occurring at the anode if a pure tin electrode is used.

(c) State one expected observation at the surface of the steel can during the electroplating process.

(d) Large amounts of acidic wastewater containing dissolved tin(II) ions are produced in this industry. Suggest one method to treat this wastewater before it is discharged into the environment.

The industrial electrolytic refining of impure copper is a critical process for obtaining high-purity copper. The impure copper anode contains various metallic impurities such as zinc, iron, silver, and gold, while a thin strip of pure copper serves as the cathode. The electrolyte is an aqueous solution of copper(II) sulfate.

(a) Explain, with reference to the electrochemical series (or standard electrode potentials), the different fates of zinc, iron, and silver impurities at the impure copper anode during the refining process.

(b) Write the ionic half-equation for the primary reaction occurring at the pure copper cathode.

(c) The anode sludge, a by-product of this process, contains valuable noble metals but also toxic heavy metal compounds. Discuss two distinct environmental challenges posed by the presence of these toxic heavy metals in the sludge if it is not properly managed.

(d) Propose one specific industrial strategy, other than simple recovery of noble metals, that can be employed to treat the heavy metal impurities in the anode sludge to minimize its environmental impact before final disposal.