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Version 1.0 1994
E
(^) LECTROCHEMISTRY
A S o u r c e B o o k M o d u l e
Funded in part under National Science Foundation Grant No. TPE 88-
ChemSource Project Principal Investigator: Mary Virginia Orna, OSU Department of Chemistry College of New Rochelle New Rochelle, NY 10805 Phone: (914) 654- FAX: (914) 654-
CONTENT IN A NUTSHELL
Topic Overview
Like all of chemistry, electrochemistry is concerned with what electrons do. Electrons in atoms, molecules or ions are bound with a particular energy. When oxidation- reduction (redox) reactions occur, electrons are transferred from the substance being oxidized to the substance being reduced. In the process, energy is either released or absorbed, depending on the electron-binding energy difference between the reacting substances. A battery (a voltaic cell ) is a device that allows the chemical energy released by a spontaneous oxidation-reduction reaction to do electrical work ( e.g. , light a light bulb, power a radio.) An electrolytic cell can reverse this process by using external electrical energy to bring about a nonspontaneous redox reaction ( e.g. , electroplating an automobile bumper.) In both cases, the electron transfer reactions occur at electrodes. The oxidation reaction occurs at the anode, and the reduction reaction occurs at the cathode.
Some substances give up or accept electrons easier than others and can be organized by their relative ability to undergo reduction reactions. A table of standard reduction potentials orders substances by their ability to accept electrons from a common donor (hydrogen gas). Among common substances, fluorine has the highest value (+2.87 V) for its reduction potential. It might be considered the Tyrannosaurus Rex of the Periodic Table because it can gobble electrons from anything. Lithium does not undergo reduction readily and has a very low reduction potential (–3.05 V). The difference between the reacting species in terms of reduction potential is an indication of the driving force for an electron-transfer reaction. In a voltaic cell, this difference defines the cell’s potential and is the same as the electric potential for a standard single-cell battery. In an electrolytic cell, the cell potential is the electrical pressure (electric potential, expressed in volts) needed to drive the oxidation-reduction reaction.
Electrical charge is conducted through a solution by movement of cations and anions. In a voltaic cell, electrons are transferred from the cathode to the species beingreduced. Cations must migrate to the cathode to offset the increase of negative charge near the electrode. Similarly, anions must migrate to the anode to offset the buildup ofpositive charge generated by the oxidation reaction at that electrode (see Transparencies 1 and 2 in the Appendix ). The charge on the electrodes and the direction of movement of ions is just the opposite in electrolytic cells (seeTransparencies 3 and 4). The quantity of work that can be produced by a battery or the quantity of work needed to run an electrolytic cell can be calculated from the electric potential, the current, and the time the cell operates. Thus, electrochemistry is the study of theinterconversion of electrical and chemical energy.
The study of electrochemistry relies heavily on conceptual understanding of redox reactions. In essence, electrochemistry involves practical applications of redox chemistry. When students balance redox reactions by the half-reaction method , they are essentially separating the full reaction into two electrochemical half cells.
The structure and chemical properties of elements and molecular and ionic substances are governed by the chemical activity of their valence electrons. The energy that binds these electrons determines the value of a substance’s standard reduction potential. Consequently, strong ties are apparent between electrochemistry and periodicity.
P LACE IN THE CURRICULUM
Upon completion of their study of electrochemistry, students will be able to:
RELATED SKILLS
P ERFORMANCE
OBJECTIVES
Concept/Skills Development
LABORATORY A CTIVITY: STUDENT V ERSION
Activity 1: A Study of Voltaic Cells
Purpose
To generate an activity series by constructing and comparing several electrochemical cells.
Safety
Procedure
Figure 1. Well plate.
24 Well Plate
Concept/Skills Development
LABORATORY A CTIVITY: TEACHER NOTES
Activity 1: A Study of Voltaic Cells
Introduction
Among several major types of chemical reactions, one of the most important is oxidation-reduction. You may have studied the electron transport system and photosynthesis in a biology course, for example. Both systems are just a series of oxidation-reduction reactions, each involving a species that loses electrons (is oxidized), while another species gains electrons (is reduced).
Even though oxidation cannot occur without reduction and vice versa, it is often useful to consider oxidation-reduction reactions in two parts called half-reactions. Added together, the two half-reactions make up the overall oxidation-reduction reaction.
M → M+^ + e–^ (Oxidation step) N +^ + e –^ → N (Reduction step)
M + N +^ → M+^ + N (Overall reaction)
To decide whether a particular oxidation-reduction reaction will occur, it is helpful to think about the reduction potential. Reduction potential can be considered the driving force for a half-reaction to undergo reduction—that is, to go in the direction of the species gaining an electron. Consider a spontaneous oxidation-reduction reaction. The half-reaction with the larger reduction potential will occur as a reduction reaction. The half-reaction with the smaller reduction potential will run in the reverse direction—as an oxidation reaction in which electrons are lost rather than gained. For a more complete discussion of oxidation-reduction and half-reactions, refer to your textbook.
The standard way to compare half-reactions is in a voltaic cell. A voltaic cell is an electrochemical device that can produce electrical energy from spontaneous oxidation- reduction reactions. All electrochemical cells have two electrodes—a cathode and an anode. (An electrode supplies or accepts electrons from a chemical reaction.) Reduction reactions always occur at the cathode and oxidation reactions always occur at the anode. One easy way to avoid confusion is to remember that “ o xidation” and “ a node” both start with vowels, whereas “ r eduction” and “ c athode” both start with consonants. In voltaic cells, the cathode is charged positively and the anode is charged negatively. The identity of the cathode and anode is determined by the relative reduction potentials of the half-reactions that make up the voltaic cell. The electrode in the half-reaction with the more positive reduction potential is always the cathode in a voltaic cell. The electrode in the half-reaction with the less positive reduction potential is always the anode in a voltaic cell.
Unlike reactions that occur in test-tubes when substances are mixed together, voltaic cells are arranged so that half-reactions are physically separated, often into different containers called half-cells. The half-cells are connected by a conducting wire between the two electrodes. The conducting wire is often called the external circuit. An electrical current passes through the external circuit. Electrical quantities such as current (rate of flow of electrons), electric potential (electric potential energy difference between the two half cells), and resistance can be measured in this external circuit. The unit for measuring electric potential is the volt , thus electric potential is often referred to as voltage. The electricity flowing through the external circuit can be used to provide energy to systems such as flashlight lamps, audio recorders, and science experiments. A system in the external circuit is often called a load.
The half-cells are also connected by an internal circuit, often provided by a salt-bridge. The salt-bridge is composed of positive and negative ions that are free to move from one half-cell to the other but do not participate in the oxidation-reduction reaction. The salt- bridge is needed to keep charge from building up in the two half-cells. When electrons flow from a half-cell through the external circuit, negative ions travel to that half-cell to maintain electrical neutrality. Conversely, when electrons flow into the other half-cell, positive ions travel to that half-cell to ensure electrical neutrality there also.
Figure 2. Voltaic cell.
Major Chemical Concepts
Level
If you decide to have students calculate the half-reaction reduction potentials, this activity is most appropriate for general chemistry students. If students only measure the voltage, the activity may also be used with basic level students.
Expected Student Background
The following concepts are prerequisites for successfully understanding concepts developed in this activity: oxidation and reduction; the general nature of energy including potential energy, chemical energy, and electrical energy; the nature and behavior of charged particles, conductivity in ionic solutions; spontaneity of chemical reactions; arbitrary standards for measurement. Students should have developed fundamental laboratory skills. If you elect to have students calculate reduction potentials of the half-reactions tested, students should have arithmetic and/or calculator skills.
EX
Salt Bridge
Electrode 1 Electrode 2
Half-Cell 1 Half-Cell 2
Mix 2 g potassium nitrate in 10 mL of water. Add 0.1 g agar. Boil the solution for 3 to 5 min. While the agar is cooking, stretch the end of an uncut, unbent polyethylene pipet to reduce the diameter of its tube. Do this by warming in the burner flame and stretching. Cut the end of the pipet so it is even. Remove the agar from the heat. Use the modified pipet to fill each “U” tube with the warm agar solution. Be certain to eliminate all air pockets. After the salt bridge has cooled, trim the ends with the razor blade. Store the salt bridges in a jar containing 2 M potassium nitrate. The salt bridges will last indefinitely if they do not dry out. Make several extras; those used with silver and tin half-reactions will plug up after several uses due to precipitation of silver chloride. This can be avoided by substituting tin(II) nitrate or acetate for tin(II) chloride, but the former are not commonly available in high schools.
If you do not wish to make permanent salt bridges, filter paper strips or pieces of cotton twine soaked in potassium nitrate solution work fine as temporary and easily disposable salt bridges.
Pre-Laboratory Discussion
Pre-laboratory work will take two or three class periods if major concept development is tied to the activity. If your students have studied in advance all concepts to be developed by the activity, use the activity for concept review. The pre-laboratory would thus focus on demonstrating use of the apparatus. We recommend, however, that the activity be used for concept development.
Review prerequisite concepts briefly to ensure student understanding (see Expected Student Background ). Ask certain students to explain each concept, followed by asking other students to elaborate, agree or disagree, correct, and provide examples. Your role is to serve as a “traffic director” for questions and responses, and to record on the board or overhead appropriate ideas to structure the review. You might administer an ungraded pretest quiz a day or so before the pre-laboratory discussion to help focus discussion. One useful way to do this is to provide a list of concept words in the domain being tested, such as oxidation-reduction or energy—and ask students to draw a concept map. (Concept maps are also useful after this discussion to summarize the review.)
Once prerequisite concepts have been reviewed, introduce laboratory equipment to be used. Demonstrate how voltmeter works by testing a small battery. Show students a 24- well plate with two solutions, metal wires or strips, and a salt bridge; connect the voltmeter.
If time permits, develop, via student discussion, the most efficient arrangement of wells for testing the various combinations and the minimum number of tests required for answering the questions. As students provide suggestions, ask them to explain how their arrangement will work and how data from their suggested tests can help answer the questions. Encourage students to propose tests that do not match up the silver and tin half-reactions. Although we recommend that students develop the well arrangement and test sequence, the arrangement in Figure 3 has been found to be very efficient. You can give it to students in advance to save time, but development of thinking skills will be compromised. Figure 3. Well-plate arrangement.
Mg Zn Cu
Cu Sn Ag Mg
Mg Zn
Concept/Skills Development
Teacher-Student Interaction
As students conduct tests, have them identify the cathode and the anode, telling you what observations support their answers. Ask them whether oxidation or reduction is occurring in a particular half-cell, to define whether this is gain or loss of electrons, and to write the predicted half-reaction equation. You may wish to ask only one question of each group as you move from group to group. In that event, you might wish to prepare a checksheet matrix of question type by group so you can ask each group a conceptually different question each time you stop by.
Anticipated Student Results
Once laboratory errors such as placing the wrong metal into a well solution are taken into account, the half-reaction reduction potential order should be the same as that predicted by a standard table of reduction potentials. The actual values in volts for half reactions may differ somewhat from table values, depending on voltmeter impedance, meter reading errors, cleanliness of metal strip and wire surfaces, quality and size of salt bridge, and local concentrations of ions near the electrodes. Deviations from standard conditions under which tabled potentials are expressed should not much affect the results. (To understand why, review applications of the Nernst equation to cell potentials, as developed in college general chemistry textbooks.)
Answers to Questions
Data Analysis and Concept Development
Implications and Applications
Post-Laboratory Discussion
Ask each group to report its results. Even if four of five groups obtain the same result, it is still possible that the four made a common mistake and the one completed the test correctly. Ask students to propose hypotheses to explain any discrepancies.
Concept/Skills Development
Oral Questions
If you wish, you can use questions recommended for teacher/student interactions during the activity as an evaluation tool. If you wish to assess individual student progress, make a student-by-question matrix to ensure that you ask each student the same questions—or at least the same number of questions. Then, as you circulate through the laboratory, ask each student a different question, checking their relative success. By circulating back through the laboratory several times, you can ask each student several questions before the end of the laboratory period, referring to your matrix to see which questions a particular student has been asked. If student performance on questions is to be used for grading, inform students in advance. We recommend that questions asked during the activity not be used for grading purposes.
Pencil and Paper
In the assessments described above, you may elect to allow students to refer to their notes, laboratory reports, and/or textbook.
Other Laboratory Activity Ideas
Give students some bubble solution made of about three parts good quality liquid detergent such as Joy and about one part glycerol, a 9-V battery, some connecting wires, and several pieces of aluminum foil. Have students electrolyze the soap solution with the Al electrodes, holding the electrodes close together so that bubbles form containing a mixture of hydrogen and oxygen gas. Holding a lighted match to the bubbles gives a satisfying but safe explosion. [Edge, 1984]
Demonstration 1: Voltaic Cells
Purpose To demonstrate the ability of chemical reactions to produce electricity. The electric potential produced depends on the nature of the metal/metal ion half cell. Materials Petri dish Filter paper Dropper bottles of 0.1 M solutions of KNO 3 , and Mg 2+^ , Zn2+^ , Sn2+^ , and Cu2+ as chloride or nitrate salt solutions. Zn, Mg, Cu, and Sn strips, 1-2 cm Wires with alligator clips High-impedance voltmeter that will read 1-2 V
DEMONSTRA - TIONS
Safety Use all normal safety precautions. Use accepted procedures for disposing of heavy-metal solutions. Procedure Cut the filter paper in the shape of a cross. Place it in the Petri dish. Add some KNO 3 solution to the center of the cross, allowing it to saturate the paper. Place one drop of each metal ion solution on each of the four corners. Attach two different metal electrodes to alligator clips wired to the voltmeter. Touch the proper electrode to its own metal ion solution. Substitute a different metal on the alligator clip until the electric potential of each of the six combinations is measured. This procedure avoids contamination and the need for a salt bridge, giving instant results.
Demonstration 2: Fruit and Vegetable Batteries
Purpose To show that different combinations of electrodes produce different electric potentials and that some food materials can serve as electrolytes. Materials Metal strip of zinc, copper, aluminum, iron, tin, lead, 1 cm x 3 cm Voltmeter 2 Connecting wires with clips on both ends Firm fruit/vegetables (such as potato, lemon, grapefruit, orange) Safety Do not eat these materials. Wash hands thoroughly when the demonstration is completed. Procedure Cut the fruit sample in half and insert two unlike metal strips into it. Connect the strips to the voltmeter with connecting wires. Read the electric potential in volts. Experiment with placing the metal strips at various distances from each other. Try other fruit/vegetables with the same metals, and other combinations of metal strips. To reinforce concepts, have students predict in each case which of the two metals will be the cathode and which will be the anode.
Demonstration 3: Electrolysis of Water in Color
Purpose To demonstrate electrode half-reactions. Materials Hoffman apparatus (or U-tube fitted with 2-hole stoppers and platinum; Nichrome electrodes; Bent wires in inverted test-tubes will also work.) Bromthymol blue indicator (BTB) 6-V to 9-V DC source, such as a transistor battery 1 M HCl and 1 M NaOH, 5 mL each Wood splints 2 Test-tubes Safety Follow normal laboratory safety precautions.
This system is much less fragile than the one involving two mechanical pencil leads, although it does take more time to make. You can attach the device to a ring stand with a universal clamp on the stopper to avoid holding it during the reaction. You can show the presence of I 2 by squirting a little starch solution near the anode to show the blue-black color created by the starch test. (First demonstrate the starch test to your students with a known iodine sample.) If you use the pencil lead electrolysis apparatus with a solution of SnCl 2 , you can show students Sn needles that “grow” on the cathode.
Demonstration 5: Electroplating Copper
Purpose To demonstrate industrial uses of electrolysis. Materials Copper strip, 1 cm x 5 cm Stainless steel object (spoon, paper clip, knife, etc. ) 18 M Sulfuric acid, H 2 SO 4 , 15 mL Beaker, 250-mL 6 M Nitric acid, HNO 3 , 50 mL 1.5-V Battery 2 Connecting wires with clips Copper(II) sulfate, CuSO 4 , saturated solution, 200 mL Safety Neutralize the acidic copper sulfate solution with an appropriate bicarbonate such as baking soda or a base like soda lime before disposing in accordance with local regulations. Dilute nitric acid can be flushed down the drain with ample water. Be careful to avoid contact with either acid. Copper saltsolutions are toxic. Procedure Clean the copper strip by dipping it into a beaker of dilute nitric acid and washing it thoroughly. Prepare a plating solution by carefully adding 15 mL concentrated sulfuric acid slowly with continuous stirring to about 200 mL saturated copper(II) sulfate solution. Attach the copper strip to the positive battery terminal using a connecting wire. Attach the object to be plated to the negative battery terminal with the other connecting wire. Place the object to be plated and the copper strip in the solution. Rotate frequently for even coating. After 3-5 min, remove the plated object and observe. Reactions Oxidation/anode: Cu(s) → Cu 2+(aq) + 2e– Reduction/cathode: Cu 2+(aq) + 2e –^ → Cu(s) NOTE: An interesting reversal of the reaction can be demonstrated by switching the battery leads.
Demonstration 6: Making a Simple Battery: The Gerber Cell
Purpose To demonstrate the ability of chemical reactions to produce electricity. Materials Large baby food jar Dialysis tubing, 2-cm x 15-cm
Concept/Skills Development
1-Hole rubber stopper to fit jar 1 Strip each of copper and magnesium metal almost as long as the jar is deep and about half as wide as the dialysis tubing Voltmeter 2 Connecting wires with clips on the end 0.5 M Copper(II) sulfate, CuSO 4 , 100 mL (12.5 g CuSO 4 ·5 H 2 O per 100 mL solution) 0.5 M Sodium sulfate, Na 2 SO 4 , 100 mL (16.1 g Na 2 SO 4 ·10 H 2 O per 100 mL solution) 6 M Hydrochloric acid, HCl, 50 mL 6 M Nitric acid, HNO 3 , 50 mL Safety Use normal laboratory safety precautions and disposal procedures. Procedure Clean the Cu strip by dipping it briefly in dilute HNO 3 and then washing well with water. Clean the Mg strip by dipping it briefly in dilute HCl and then washing well with water. Fill the jar about 2/3 to 3/4 full of sodium sulfate solution. Wet the dialysis tubing and tie one end in a knot. Open the other end and fill to a depth slightly less than that of the jar with copper(II) sulfate solution. Place the copper strip in the dialysis tubing. Place the magnesium strip and the dialysis tubing in the jar. Then insert the stopper to hold them in place. Observe any reaction that may occur (see Figuure 4). Figure 4. Simple battery apparatus.
Attach the ends of the Cu and Mg strips to the voltmeter with the connecting wires. Reactions Oxidation/anode: Mg(s) → Mg 2+^ (aq) + 2e– Reduction/cathode: Cu 2+(aq) + 2e –^ → Cu(s) NOTE: If you make six of these cells and hook them in series (anode to cathode), you can produce enough electric potential to operate a small radio that normally uses a 9-V battery. Reference Summerlin and Ealy. (1985). Chemical demonstrations, Vol. 1. Washington, DC: American Chemical Society.
Key Questions
Rubber Stopper
Mg
CuSO 4
Na 2 SO 4
Dialysis Tubing
Cu
GROUP AND DISCUSSION A CTIVITIES
100 000 Gallons
Low rate of flow Few L/min AMPS
Few coulombs Few electrons
Analogous to
Little water Much water
Many coulombs Many electrons
1 gallon
High rate of flow Many L/min
Many electrons/sec High current
Analogous to AMPS Few electrons/sec Low current
low
hig low h
hig h
Little tendency to flow Low pressure
VOLTS
Low P
High electric potential
Great tendency to flow High pressure Analogous to VOLTS
PHigh
Low electric potential
Concept/Skills Development
Figure 5. Electron transport/ water analogies.
height
Other Memory Aids: Mnemonics, etc.
Language of Electrochemistry anode electrode at which an oxidation half-reaction occurs in an electrochemical cell battery voltaic cell used to produce electricity through a chemical reaction cathode electrode at which a reduction half-reaction occurs in an electrochemical cell cell potential potential difference in volts of an electrochemical cell cell voltage synonym for cell potential corrosion oxidation of a metal by the action of air, water, and/or salt solutions electric potential synonym for cell potential electrochemical cell device in which electrons of a redox reaction pass through an electrical circuit electrolytic cell electrochemical cell in which a nonspontaneous reaction is carried out by electrolysis electromotive force (EMF) synonym for cell potential galvanic cell electrochemical cell in which a spontaneous chemical reaction produces electricity half-cell combination of reduced and oxidized forms of a given species upon which a redox equilibrium is established nonspontaneous type of reaction when cell potential is negative salt bridge device used to join two half-cells and containing a salt solution that permits the flow of ions between two half-cells spontaneous type of reaction when cell potential is positive voltaic cell synonym for galvanic cell
Common Student Misconceptions
TIPS FOR THE
TEACHER
