Oxidation and Reduction

Unit Overview

The study of oxidation and reduction can be treated as an illustration of equilibrium. Students should be able to define oxidation and reduction in terms of electron transfer, and express oxidation-reduction processes in terms of half reactions. Tables of standard reduction (or oxidation) potentials should be used to determine E^o values. Students should also use tables and experimental results to assess the spontaneity of reactions.

The corrosion of metals and metallic deposition are applications which should be given consideration in this unit. Other practical applications of electrochemistry should be explored. These topics would be ideal for case studies, independent research activities, or laboratory investigations.

Connections between this and the Acid-Base unit can be made, by considering the corrosive effects of acids. Qualitatively, students could investigate the effect of pH on the corrosion of metals. An interesting laboratory research investigation would be to assess the corrosive damage to metals due to acid precipitation.

Factors of scientific literacy which should be emphasized

• A3 ..........holistic
• A4 ..........replicable
• A5 ..........empirical
• A9 ..........human/culture related

• B1 ..........change
• B3 ..........orderliness
• B8 ..........quantification
• B9 ..........reproducibility of results
• B11 ..........predictability
• B12 ..........conservation
• B28 ..........equilibrium
• B33 ..........entropy

• C1 ..........classifying
• C3 ..........observing and describing
• C4 ..........working cooperatively
• C5 ..........measuring
• C6 ..........questioning
• C7 ..........using numbers
• C11 ..........controlling variables
• C14 ..........problem solving
• C16 ..........designing experiments
• C17 ..........using mathematics
• C19 ..........consensus making

• D1 ..........science and technology
• D4 ..........science, technology, and the environment
• D5 ..........public understanding gap
• D9 ..........social influence on science and technology
• D10 ..........technology controlled by society

• E3 ..........using equipment safely
• E4 ..........using audiovisual aids
• E5 ..........computer interaction
• E7 ..........manipulative ability
• E11 ..........measuring mass
• E12 ..........using electronic instruments
• F2 ..........questioning
• F4 ..........valuing natural environments
• F6 ..........consideration of consequence

• G5 ..........avocation
• G7 ..........vocation

Foundational Objectives for Chemistry and the Common Essential Learnings

Explore the tendency of elements to participate in electron transfer.

• Define oxidation and reduction in terms of transfer of electrons.
• Develop a reduction potential series based on experimental results.
• Write half reactions and net ionic equations involving oxidation-reduction processes.
• Use a table to compare reduction potentials of half-reactions.
• Describe the processes of corrosion and metallic deposition, using the terms oxidizing agent, reducing agent, oxidized species, and reduced species.
• Identify and investigate means of protecting metals against corrosion.
• Describe the conditions under which automobiles corrode most quickly.

Observe, measure and consider the applications of electron transfer through external circuits.

• Determine the direction of electron flow in an electrochemical cell.
• Measure the voltage in several electrochemical cells.
• Calculate the potential difference in volts of electrochemical cells, using a standard reduction potential table.
• Explain the difference between a standard potential and an observed potential.
• Compare electrochemical and electrolytic cells.
• Examine applications of electrochemistry.

Strengthen students' knowledge and understanding of how to compute, measure, estimate and interpret mathematical data, when to apply these skills and techniques, and why these processses apply within redox chemistry. (NUM)

• Recognize whether a computed answer is sensible.
• Make appropriate use of calculators and computers.
• Verify answers by referring to the problem requirements, by checking the validity of each step in the method of solution, by looking for errors in reasoning or information and wherever appropriate, using an alternative method of solution.
• Distinguish btween quantitative situations where precision is required and those where approximations are acceptable.
• Understand the meaning of precision and determine the most appropriate degree of precision for a needed measurement.

Understand and use the vocabulary, structure and forms of expression which characterize the study of oxidation-reduction reactions. (COM)

• Incorporate the vocabulary of redox chemistry into the writing and talking they do about the topic.
• Use reduction potentials tables to predict spontaneity of reaction and potential difference of reaction.
• Recognize the structure of a reduction potential table and its relationship to experimental eveidence.
• Recognize and correctly use symbols such as E°, mol·L^-1, v, M²⁺_(aq).
• Read diagrams, tables and expository (information-giving) text to enhance understanding of redox chemistry, and explain that understanding both orally and in writing by using analogies, charts, diagrams and descriptive statements.

Suggested activities and ideas for research projects

• Prepare five 5-8 cm long uncoated steel nails by rubbing them with emery cloth or fine sandpaper to remove any surface protection. Make an agar gel by heating 300 mL of water to a boil. Stop heating and stir into the water 2 g of powdered agar. While the gel is still hot, prepare two petri dishes. Place a straight nail and a bent nail in one petri dish.

  Into a test tube, pour some hot gel - about 150 mL - and add 10 mL of 0.1 M KSCN (potassium thiocyanate) solution. Pour this over the iron nails.

  Into a second petri dish, place three iron nails: one with copper wire wound around it, a second with magnesium ribbon wound around it, and a third clean iron nail. Wind the magnesium and the copper tightly so that there is contact between the nail and the metal. Make up another gel solution containing KSCN and pour this into the petri dish.

  Look at these samples on the overhead projector over a period of two or three days.

  A variation of this activity is to add 1 mL of 0.1 M K₃Fe(CN)₆ (potassium ferricyanide) and ten drops of phenolphthalein indicator to the agar, and then pour the mixture over the nails in the petri dishes. After a day or two, the agar gel will turn pink near the regions on the nails where the corrosion is most active. (Note: The ferricyanide compound has low toxicity and is relatively safe. Upon decomposition, however, the fumes are extremely hazardous. Make sure that your stock supply of potassium ferricyanide is stored securely. Ordinary gelatin might work just as well as agar in these activities, since it is just being used as a holding phase. The source of protein in the gel is not important. Agar powder is more expensive than plain gelatin.) (This activity was adapted from CHEM13 NEWS, #81, November 1976, page 22, based on an activity designed by L. Sibley, St. Catharines, ON.)

• (Catalytic oxidation of NH₃. Try this first!) Place 30 mL of concentrated ammonium hydroxide in a 250 mL Erlenmeyer flask. Make a coil of copper gauze or copper wire and support it from the glass rod so that it doesn't touch the liquid when inserted in the flask. Heat the copper, and, when it is red-hot, hang it in the flask. The wire will soon get so hot that it melts and sputters dramatically.

  Methanol instead of ammonium hydroxide in the flask will oxidize to produce methanal (formaldehyde). Use a fume hood. (This activity was adapted from CHEM13 NEWS, #81, November 1976, page 22, based on an activity designed by L. Sibley, St. Catharines, ON.)

• Prepare small metal strips of copper, zinc, magnesium, and iron. Clean each metal surface with sandpaper, and place each sample side by side into a petri dish. Prepare five other petri dishes in the same way.

  Into the first dish, pour copper (II) nitrate, Cu(NO₃)₂, solution, until each piece of metal is covered. In the second dish, use zinc nitrate, Zn(NO₃)₂, solution. Repeat in the other petri dishes, using magnesium nitrate, Mg(NO₃)₂, silver nitrate, AgNO₃, hydrochloric acid, HCl, and iron (II) ammonium sulphate, Fe(NH₄)₂(SO₄)₂·6H₂O.

  Allow the samples to stand for several days. Observe the results. Rank the metals according to how well they acted as reducing agents. Rank the metal ions in solution according to how well they acted as oxidizing agents. Write net ionic equations for all reactions that occurred. Develop a reduction potential series based on the experimental results.

• Tarnished silver is oxidized silver, usually in the form of Ag₂S. Reduce the silver to its metallic state using a reaction between metallic aluminum and the tarnished silver in a medium of hot baking soda (NaHCO₃) solution. Find a pan large enough to place an aluminum foil pie plate or a large piece of aluminum foil on the bottom. Heat, to about 80°C, enough baking soda solution (about 25 g/L) to cover the largest item to be cleaned. Immerse the tarnished silver in the solution and hold it so that it is in contact with the aluminum. Remove, rinse, and dry the silver once it is clean. (Questions for investigation: Why is the solution of baking soda used, rather than just hot water? Why is the solution hot, rather than cold? Will a metal other than aluminum work? Could rust be removed from iron with this method?)

• Electroplating is an interesting, practical application of electrochemistry. One activity is to copper plate a key. An electroplating bath of 200 mL of 1 M copper (II) sulphate, 5 mL of concentrated sulphuric acid, and 10 mL of ethanol per group is required.

  Rub the key being electroplated with steel wool or sandpaper to remove any debris. Attach a thin copper wire to the key. Dip it into a dilute sodium hydroxide solution and then into a dilute nitric acid solution. It is important to avoid touching the key once it has undergone this cleaning process. Oily residue affects the ability of the plating metal to adhere to the surface.

  Hang the key and a copper electrode in the solution so that there is no contact between them. Connect the copper electrode and the copper wire which is suspending the key to a 6 volt battery or a power supply.

  After a few minutes the deposition of copper should be evident on the key. Allow the electrolytic cell to operate for about fifteen minutes. Disconnect the battery or power supply and remove the key. Rinse it thoroughly. Buffing with a mild abrasive, such as chalk dust, may produce a better lustre.

  As a follow up, students could investigate other applications, such as the galvanization of steel with zinc, or chrome-plating car bumpers. Once they understand the chemistry involved in those processes, other electroplating experiments could be designed. Involving students in independent research and the design of experiments may help to give them a much better understanding of chemistry.

• The 'standard' hydrogen half-cell can be produced by connecting two porous carbon electrodes placed on opposite sides of a one litre beaker containing about 600 mL of 6.0 M HCl to a 6 volt power source. If this system runs for several minutes, enough hydrogen gas and chlorine gas will be produced and adsorbed at their respective electrodes to produce a pair of gas electrodes. If the electrodes are connected to a voltmeter when the power source is disconnected, the system will be observed to be functioning as an electrochemical cell. The observed voltage can then be compared with the predicted voltage for a hydrogen-chlorine cell.

• Purchase some 3% hydrogen peroxide solution from a drug store. (If a stronger solution is purchased for dilution, observe strict precautions against its deterioration. Store in a refrigerator, and only for 6 months or less.) Measure from 1 to 3 mL into a 250 mL Erlenmeyer flask, acidify with 10 drops of concentrated sulphuric acid, and add enough water to start a titration. Titrate using 0.01 M potassium permanganate. The end-point will be the faint pink colour of dilute MnO₄^-.

  2MnO₄^- + 6H⁺ + 5H₂O₂ 2Mn²⁺ + 5O₂ + 8H₂O

  Calculate the percentage of H₂O₂ in the original sample.

• Construct carbon electrodes for electrolysis experiments from a 30-40 cm length of NMD7 14-2 electric cable. (This is ordinary house wiring material with two 14 gauge insulated conductors and a ground wire.) Use a pair of pliers to pull the uninsulated ground wire out, leaving the two insulated conductors. Cut from 6 to 8 cm of the outer plastic sheathing from one end of the piece of cable and strip about 1 cm of the insulation from the ends of the exposed conductors. Salvage some carbon rods from some old dry cells and drill a 2mm hole down the centre of the carbon rod to a depth slightly greater than length of the stripped ends of the conductors. Insert the wires in the holes of the rods until the plastic insulation touches the carbon rod. Seal the connection of the conductor to the rod with some silicone. Finally, expose about 1 cm of conductor at the other end of the apparatus to serve as a connection to the power source, and bend the apparatus to give you a configuration appropriate to the container you are using.

  Ordinary pencils can be used as electrodes by sharpening them at both ends, and then carefully using an utility knife to remove the tapered section. This leaves about 2 cm exposed to act as electrical connector and, at the other end, as active electrode. Use these electrodes to decompose various solutions by electrolysis. Several variables such as the concentration of the solutions, the temperature of the solution, the voltage of the power source, and others can be examined.

• Use dialysis tubing (a semipermeable membrane) borrowed from your biology supplies as a substitute for porous cups or salt bridges in producing electrochemical cells. A portable battery can be designed by using an Erlenmeyer flask, a length of soaked dialysis tubing tied off to form a bag about 6 cm long, and whatever combination of metal ion solution/thin metal strips that you have. Some possibilities are aluminum nitrate and aluminum foil, copper nitrate and copper foil or silver nitrate and silver foil. Foil is preferred since the stopper may then be inserted into the flask easily and still allow the electrodes to pass out of the flask.

• Place a folded, moistened filter paper in a funnel. Half fill the paper with degreased iron filings. Pour some 0.1 M copper sulphate solution into the filter and observe the filtrate. Write an equation for the reaction which occurs. (Questions: What metals could be substituted for iron? Which metal ion solutions could be substituted for the copper sulphate?)

• Research and prepare a report to the class on the chemistry of one of the methods of protection of metals from corrosion. Include an explanation of the problems caused by corrosion, and the conditions under which the corrosion is fastest.

• What reactions are involved in the discharging of a lead storage battery (car battery) and in its subsequent recharging?

• Examine a piece of galvanized iron. Describe the pattern on the surface of the metal. Why is iron galvanized? What substances are used in the galvanizing process? What causes the pattern on the surface of the metal?

• Fertilizer storage bins may be either galvanized or epoxy-coated. Why must iron fertilizer bins be protected with some coating? What are the advantages and disadvantages of each method?

• Pipelines, bridges, the steel framework of large building and other structures may be safeguarded from corrosion by the use of cathodic protection. Why is cathodic protection necessary for such structures? How does it work? Design a demonstration to show a piece of metal which is protected in this manner.

• Some brands of paint claim that they can not only prevent rust but can treat rusted surfaces so that they will stop rusting and rust no more. How do these products work?

• Cut a 1 cm by 3 cm strip of copper foil in half to produce two 0.5 cm wide strips. Cut a 0.5 cm length from the end of one strip to serve as an unreacted sample for comparison.

  Place one of the long strips in 5 - 10 mL of dilute HCl(aq) for five minutes. Describe any reaction. Then remove the foil and rinse it. Examine it for evidence of a reaction.

  Wrap the other strip tightly around a 1 cm square of thin zinc sheet, so that there is contact between the two metals. Place it in 5 - 10 mL of dilute HCl_(aq) for five minutes. Describe any reaction. Then remove the foil and rinse it. Examine it for evidence of a reaction.

  Compare the two strips and the 0.5 cm square.

• Carefully dissect several different brands of 1.5 volt 'D' dry cell batteries. Dissect as well a 9 volt dry cell. Use caution and protective clothing since the contents are corrosive and toxic. Compare the similarities and differences of construction and structure. What chemical reactions produce the electricity in these cells?

• Research the "cold fusion" controversy. Is this an example of fusion or of electrochemistry?

Sample ideas for evaluation and for encouraging thinking

• Why is it important to prevent sparking near a car battery when it is being boosted with jumper cables?

• Identify the substance which is oxidized and the substance which is reduced in the following reactions.
  • magnesium metal reacts with steam to form magnesium oxide and hydrogen gas
  • solid carbon reacts with steam to form carbon monoxide gas and hydrogen gas
  • copper(II) sulfide reacts with ammonia to produce copper metal, nitrogen gas and hydrogen sulphide gas.

• Why is an oxidizing agent always reduced during a redox reaction?

• Explain how you can use a standard reduction potentials chart to determine the direction electrons will flow in an electrochemical cell.

• Suppose you wished to plate your house key with chromium. To which terminal of the battery would the key be connected? What would the other electrode be made of? What solution would you use? Sketch a diagram showing a completed setup for this project.

• Suppose you wanted to select the metal which reacts with the greatest number of solutions containing metal ions. Use a standard reduction potential chart to pick this metal. Explain how the chart provided the information necessary for you to make your choice.

• A salt bridge or a porous cup is needed to complete the circuit in an electrochemical cell. Explain how these act to complete the circuit. Why won't an electrochemical cell run without them?

• What would be the result if one tried to electroplate a steel spoon using AC power instead of DC power?

• Suppose a steel spoon was plated with gold. What are some of the advantages of plating the spoon with gold rather than leaving it as pure steel? What are some of the advantages of a steel spoon plated with gold over a spoon of pure gold?

• How can phenolphthalein be used to indicate the polarity of a wire carrying DC current?