Chemistry: Electrochemistry and Galvanic Cells Objectives: Students will be able to: • Define oxidation and reduction in terms of loss or gain of electrons • Describe the operation of a galvanic cell (using such terms as anode, cathode, electron flow, salt bridge, and ions) • Interpret the activity series in terms of elements that are more or less easily oxidized • Relate cell potential to the activity series • Build simple galvanic cells and measure cell potential California Content Standards: Physics: Conservation of Energy and Momentum: 2.h. Students know how to solve problems involving conservation of energy in simple systems with various sources of potential energy, such as capacitors and springs). Electric and Magnetic Phenomena: 5.e. Students know charged particles are sources of electric fields and are subject to the forces of the electric fields from other charges. Chemistry: Conservation of Energy and Stoichiometry: 2.a. Students know how to describe chemical reactions by writing balanced equations. Acids and Bases: 5.a. Students know the observable properties of acids, bases, and salt solutions. 5.a. Students know acids are hydrogen-­‐ion-­‐donating and bases are hydrogen-­‐ion-­‐accepting substances. Solutions: 6.a. Students know the definitions of solute and solvent. 6.d. Students know how to calculate the concentration of a solute in terms of grams per liter, molarity, parts per million, and percent composition. Earth Sciences: Investigation & Experimentation: 1.a. Students will select and use appropriate tools and technology to perform tests, collect data, analyze relationships, and display data 1.l. Students will analyze situations and solve problems that require combining and applying concepts from more than one area of science. Reference: Institute for Chemical Education (Chemistry Fundamentals), Mt. San Antonio College, Walnut CA, 1995. Before you begin, you may want to watch the DSSC videos to prepare the lab: http://www.youtube.com/caltech and click on “Resources for Teachers” on the right. For technical assistance please contact a scientist at Caltech at JuiceFromJuice@caltech.edu High School Lesson Plan 1 Chemistry: Electrochemistry and Galvanic Cells Background: One of the most important types of chemical reactions is the oxidation–reduction (redox) reaction. An oxidation–reduction reaction involves the transfer of one or more electrons from one species to another. The substance that loses electrons is oxidized; the substance that gains electrons is reduced. All reduction reactions must occur with a corresponding oxidation (i.e. the electrons must come from somewhere). It is often useful to consider redox reactions in two parts called half reactions. Added together, these two half reactions make up the overall oxidation–reduction redox reaction: A+ + e-­‐ à A + -­‐
B à B + e B + A+ à B+ + A (reduction) (oxidation) (overall reaction) Single replacements are common examples of redox reactions. For these types of reactions the activity series of metals is used to determine whether a particular reaction occurs. In such reactions, a metal in a salt is replaced by an elemental metal that is higher on the series or “more active.” “More active” in this case means more readily oxidized. A galvanic cell is an electrochemical device that can produce electrical energy from spontaneous single-­‐
replacement redox reactions. All electrochemical cells have two electrodes, a cathode and an anode. Solution species are always reduced at the cathode, and oxidized at the anode. In galvanic cells the cathode is then charged positive and the anode negative. When two different metals are used, the identity of which is the anode and which is the cathode is determined by their relative position in the activity series. The electrode that is higher in the series (more easily oxidized) is the anode. In this experiment you will construct a series of galvanic cells using metals and metal salt solutions. Each cell will consist of two half cells, each containing a metal electrode and its corresponding ion in solution (e.g. a piece of copper in a Cu2+ solution). Pairs of half cells will be connected together by a salt bridge which will supply inert cations and anions to each of the half cells, providing a pathway for ion flow (see diagram below). By examining your results you will be able to place three metals on a list, which should correspond to the activity series. Materials: Per Class • 2 Multimeters with probes* • 4 Alligator clips* Per group of 2 students • 1 twelve-­‐well reaction plate* • Many 2 cm strips of Whatman® #1 filter paper* • 1 beaker for KCl (~1M as 1.03g/16mL)* solution# • 1 Plastic Tweezer* • Many 1 – 2 cm strips of Zn, Cu, Sn, and Mg metal* • 15 drops of each of the following solutions: ZnSO4 (~0.1M as 0.46g/16mL)* CuSO4 (~0.1M as 0.40g/16mL)* SnCl2 (~0.1M as 0.36g/16mL)* MgSO4 (~0.1M as 0.39g/16mL)* *Provided in the Juice from Juice kit #
• 1 piece of sandpaper (fine)# Fewer can be used as groups can share For technical assistance please contact a scientist at Caltech at JuiceFromJuice@caltech.edu High School Lesson Plan 2 Chemistry: Electrochemistry and Galvanic Cells STUDENT LABORATORY PROCEDURE SAFETY NOTE: Prudent lab safety practices are required in performing this lab. The solutions contain heavy metal ions and care should be taken in their handling. Put on your goggles! 1. Split the students up into groups of two, for up to 30 students in total. 2. Fill the wells you will use with 15 drops of the appropriate metal ion solution, in the recommended arrangement below: 1 2 3 Mg Zn Cu 5 6 7 8 9 10 11 12 Cu Sn Mg 4 3. Identify each of the four metals you will use in the experiment. 4. Clean each of the metal strips with sandpaper and place them on a piece of paper. Label the paper near each strip with the metal’s name so that you know which metal is which. 5. Make a salt bridge by soaking a 2 cm strip of filter paper in the KCl solution. Use the tweezers. Make separate salt bridges for each cell. 6. Select the two wells to be tested (see student data table below), and place the salt bridge so that it is immersed in both solutions. 7. Attach the alligator clips from the multimeter to the metal strips of the corresponding solutions. 8. Immerse the metal electrodes into their metal ion solutions and record the voltage. (Note: Be sure to use the correct metal electrode for each solution.) 9. When finished, test whether the electrode area or time immersed affects the cell voltage or current. 10. When finished, dilute the well with water and test whether the electrolyte concentration affects the cell voltage or current. For technical assistance please contact a scientist at Caltech at JuiceFromJuice@caltech.edu High School Lesson Plan 3 Chemistry: Electrochemistry and Galvanic Cells Fill out the following tables. Potentials (V) of all possible combinations black / red Cu Mg Sn Zn Cu Mg Sn Zn Anode (Black -­‐) Cathode (Red +) Potential (Volts) Cell A Mg Zn B Zn Cu C Mg Cu D Zn Sn E Mg Sn F Sn Zn G Sn Cu H Zn Mg For technical assistance please contact a scientist at Caltech at JuiceFromJuice@caltech.edu High School Lesson Plan 4 Chemistry: Electrochemistry and Galvanic Cells Checking for Understanding 1. Which cell produced the highest (positive) voltage? Lowest (positive) voltage? How does this relate to the difference in “activity?” (Refer to the activity series.) Deduce from your data the next easiest to reduce, the next, and so on. In this way you can build the activity series of single replacement reactions for these four metals/cations. 2. Examine your data. Was there a metal which was oxidized by each of the others (i.e. an anode in a cell producing a positive voltage with all other metals)? Compare the voltages for the cells above and arrange the metals in order, from most easily oxidized (most “active”) to least. 3. Why is the salt bridge necessary? 4. Explain cells F and H. 5. Draw a diagram for cell C. Write the anode and cathode reactions. 6. Describe, write, and balance anode and cathode half reactions for cells A, B, and F. 7. Wreckage of the iron (Fe) ship Monitor was discovered in 1987, and scientists attached Zn anodes to protect it from corrosion. Why would this protect it? For technical assistance please contact a scientist at Caltech at JuiceFromJuice@caltech.edu 5 High School Lesson Plan