Predicting

Performing and recording

Analyzing and interpreting

Communicating results

S K I L L F O C U S

542 MHR • Unit 5 Electrochemistry

Electroplating

You have learned that electroplating is a process in which a metal is deposited, or plated,onto the cathode of an electrolytic cell. In thisinvestigation, you will build an electrolytic celland electrolyze a copper(II) sulfate solution toplate copper onto the cathode. You will useFaraday’s law to relate the mass of metal deposited to the quantity of electricity used.

QuestionDoes the measured mass of copper plated ontothe cathode of an electrolytic cell agree with themass calculated from Faraday’s law?

PredictionPredict whether the measured mass of copperplated onto the cathode of an electrolytic cellwill be greater than, equal to, or less than themass calculated using Faraday’s law.

Materials150 mL 1.0 mol/L HNO3 in a 250 mL beaker120 mL acidified 0.50 mol/L CuSO4 solution

(with 5 mL of 6 mol/L H2SO4 and 3 mL of 0.1 mol/L HCl added)

drying oven, or acetone in a wash bottle3 cm × 12 cm × 1 mm Cu strip50 cm 16-gauge bare solid copper wire250 mL beakeradjustable D.C. power supply with ammeterdeionized water in a wash bottlefine sandpaper2 electrical leads with alligator clipselectronic balance

Safety Precautions

• Nitric acid is corrosive. Also, note that theCuSO4 solution contains sulfuric acid andhydrochloric acid. Wash any spills on yourskin with plenty of cold water. Inform yourteacher immediately.

• Avoid touching the parts of the electrodes that have been washed with nitric acid.

• Acetone is flammable. Use acetone in the fume hood.

• Make sure your hands and your lab bench aredry before handling any electrical equipment.

Procedure1. Clean off any tarnish on the copper strip

by sanding it gently. Dip the bottom of the copper strip in the nitric acid for a fewseconds, and then rinse off the strip carefullywith deionized water. Avoid touching the section that has been cleaned by the acid.

2. Place the copper strip in the beaker, with the clean part of the strip at the bottom. Bendthe top of the strip over the rim of the beakerso that the copper strip is secured in a verticalposition. This copper strip will serve as the anode.

3. Wrap the copper wire around a pencil tomake a closely spaced coil. Leave 10 cm ofthe wire unwrapped. Measure and record themass of the wire. Dip the coil in the nitricacid, and rinse the coil with water. Use the 10 cm of uncoiled wire to secure the coil onthe opposite side of the beaker from theanode, as shown in the diagram. This copperwire will serve as the cathode.

Chapter 11 Cells and Batteries • MHR 543

4. Pour 120 mL of the acidified CuSO4 solutioninto the beaker. Attach the lead from the negative terminal of the power supply to the cathode. Attach the positive terminal to the anode.

5. Turn on the power supply and set the currentto 1 A. Maintain this current for 20 min byadjusting the variable current knob as needed.

6. After 20 min, turn off the power. Remove the cathode and rinse it very gently withdeionized water. Place the cathode in a drying oven for 20 min. Alternatively, rinse the cathode gently with acetone, and let theacetone evaporate in the fume hood for 5 min.

7. Measure and record the new mass of the cathode.

8. Dispose of all materials as instructed by your teacher.

Analysis1. Write a balanced equation for the half-reaction

that occurs at the cathode.

2. Use the measured current and the time forwhich the current passed to calculate thequantity of electricity used.

3. Use your answers to questions 1 and 2 to calculate the mass of copper plated onto the cathode.

4. Compare the calculated mass from question 3with the measured increase in mass of thecathode. Give possible reasons for any difference between the two values.

Conclusion5. How did the mass of copper electroplated

onto the cathode of the electrolytic cell compare with the mass calculated usingFaraday’s law? Compare your answer withyour prediction from the beginning of thisinvestigation.

Applications6. Suppose you repeated this investigation using

iron electrodes, and 0.5 mol/L iron(II) sulfatesolution as the electrolyte. If you used thesame current for the same time, would youexpect the increase in mass of the cathode to be greater than, less than, or equal to theincrease in mass that you measured? Explainyour answer.

7. Suppose you repeated the investigation with the copper(II) sulfate solution, but youpassed the current for only half as long asbefore. How would the masses of copper plated onto the cathode compare in the twoinvestigations? Explain your answer.

8. Could you build a galvanic cell withoutchanging the electrodes or the electrolytesolution you used in this investigation?Explain your answer.

D.C.power source

copper wire(cathode)

copper strip(anode)

acidified CuSO4(aq)

(−) (+)

544 MHR • Unit 5 Electrochemistry

The alloy used to make pop cans contains about97% aluminum, by mass. The otherelements in the alloy are magnesium, manganese, iron, silicon, and copper.

Figure 11.24

Industrial Extraction and Refining of MetalsMany metals, and their alloys, are widely used in modern society. Theenormous variety of metal objects ranges from large vehicles, such as carsand aircraft, to small items, such as the pop cans shown in Figure 11.24.

Extraction is a process by which a metal is obtained from an ore.Some metals are extracted in electrolytic cells. In section 11.3, you sawthe extraction of sodium from molten sodium chloride in a Downs cell.Other reactive metals, including lithium, beryllium, magnesium, calcium,and radium, are also extracted industrially by the electrolysis of theirmolten chlorides.

One of the most important electrolytic processes is the extraction of aluminum from an ore called bauxite. This ore is mainly composed of hydrated aluminum oxide, Al2O3 • xH2O. (The “x” in the formula indicates that the number of water molecules per formula unit is variable.)In industry, the scale of production of metals is huge. The electrolytic production of aluminum is over two million tonnes per year in Canadaalone. As you know from Faraday’s law, the amount of a metal producedby electrolysis is directly proportional to the quantity of electricity used.Therefore, the industrial extraction of aluminum and other metals by electrolysis requires vast quantities of electricity. The availability and cost of electricity greatly influence the location of industrial plants.

In industry, the process of purifying a material is known as refining.After the extraction stage, some metals are refined in electrolytic cells. For example, copper is about 99% pure after extraction. This copper is pure enough for some uses, such as the manufacture of copper pipes for plumbing. However, the copper is not pure enough for one of its principal uses, electrical wiring. Therefore, some of the impure copper isrefined electrolytically, as shown in Figure 11.25. Nickel can be refinedelectrolytically in a similar way. You refined copper on a small scale inInvestigation 11-C.

This electrolytic cell is used to refine copper. The anode is impure copper,and the cathode is pure copper. During electrolysis, the impure copper anode dissolves, andpure copper is plated onto the cathode. The resulting cathode is 99.99% pure metal. Mostimpurities that were present in the anode either remain in solution or fall to the bottom of the cell as a sludge.

Figure 11.25

Chapter 11 Cells and Batteries • MHR 545

Section SummaryIn this section, you learned how stoichiometry relates the quantities of reactants and products to the quantity of electricity consumed in anelectrolytic cell. You used Faraday’s law to solve problems relating to electrolysis. You also learned that the extraction and refining of some metals are carried out electrolytically. In the next section, you will see several other important applications of electrochemistry to modern society.

In Section 10.3, you learned about a redox reaction used in the production of compact discs. In another step of this productionprocess, nickel is electroplated onto the silver-coated master disc. The nickel layer is removed and used to make pressings of the CD onto plastic discs. The plastic pressings are then coated with aluminum to make the finished CDs.

(a) When nickel is plated onto the silver master disc, is the master disc the anode or the cathode of the cell? Explain.

(b) Calculate the quantity of electricity needed to plate each gram ofnickel onto the master disc. Assume that the plating processinvolves the reduction of nickel(II) ions.

Most industrial reactions take place on a much larger scale thanreactions in a laboratory or classroom. The voltage used in a Downscell for the industrial electrolysis of molten sodium chloride is notvery high, about 7 V to 8 V. However, the current used is 25 000 A to 40 000 A. Assuming a current of 3.0 × 104 A, determine the mass of sodium and the mass of chlorine made in 24 h in one Downs cell.Express your answers in kilograms.

An industrial cell that purifies copper by electrolysis operates at2.00 × 102 A. Calculate the mass, in tonnes, of pure copper produced if the cell is supplied with raw materials whenever necessary, and if it works continuously for a year that is not a leap year.

Canada is a major producer of aluminum by the electrolysis ofbauxite. However, there are no bauxite mines in Canada, and all theore must be imported. Explain why aluminum is produced in Canada.

Research the extraction of aluminum by the electrolysis of bauxite. Write a report on your findings. Include a description of the electrolytic cell and how it operates. Indicate where aluminum is produced in Canada. Also include any environmental concerns associated with aluminum production by electrolysis.

Nickel and copper are both very important to the Ontario economy. Before they can be refined by electrolysis, they must beextracted from their ores. Both metals can be extracted from a sulfideore, NiS or Cu2S. The sulfide is roasted to form an oxide, and then theoxide is reduced to the metal. Research the extraction processes forboth nickel and copper, and write balanced equations for the redoxreactions involved. One product of each extraction process is sulfurdioxide. Research the environmental effects of this compound.Describe any steps taken to decrease these effects.

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Section Review

Electroplating is a complexprocess. If you wish to workon your Unit 5 Investigationnow, do some research onelectroplating. What are themost important factors in this process? How can youimprove the quality of the final product?

Unit Investigation Prep