Allen, Donna L. Bowen High School

Objectives: One form of energy may be transformed into another. How can electricity be changed into chemical energy? How can chemical energy be transformed into electricity? Apparatus needed: Strips of copper, zinc, lead, magnesium 0.5 M solutions of soluble salts of copper, zinc, lead and magnesium (acetates or chlorides) 0.05 M copper (II) sulfate plastic cups (preferably clear) index cards (support strips) voltmeters (milliammeters, galvanometers as available) insulated wires one lemon Recommended strategy: I. Changing electricity into chemical energy A. electrolysis of water B. electrolysis of saturated sodium chloride solution C. electroplating II. Changing Chemical energy into electricity A. A zinc strip is put into a 0.5 M solution of copper(II) sulfate and allowed to stand for at least two hours. Another cup of the same solution without the zinc strip should be prepared at the same time for comparison. Record observations. Write equation. A reaction has occurred spontaneously with the liberation of a small amount of heat energy. Could that energy be harnessed in the form of electricity? B. Stick strips of copper and zinc into a lemon and connect to a meter. Is electricity being generated? C. Moisten a small piece of paper towel in your mouth and put it between a penny and a dime. Touch the penny to one lead to the meter and the other lead to the dime. Is electricity being generated? This is a Voltaic cell. D. In order to cause electrons to pass through an external circuit instead of reacting directly with the liberation of heat energy, the two electrodes must be separated. However the electrons must have a pathway or the circuit will not be complete. This can be accomplished with a semipermeable membrane or a salt bridge. The salt bridge can be a U shaped glass tube filled with a conducting solution, but we will use a strip of porous paper which has been dampened with a saturated solution of sodium or potassium chloride. Fill one cup with the solution of zinc acetate and mount a strip of zinc metal through a slit in an index card in this solution. Fill another cup with a solution of copper acetate and mount a strip of copper metal in this cup. Stand the two cups side by side and hang a damp strip of paper over the sides so that one end is in the zinc acetate solution and the other end is in the copper(II) acetate solution. Now connect the two electrodes by copper wires to the meter. Does the meter give evidence that an electric current is generated? Record whatever quantity and units the meter shows. Repeat, using all the possible combinations of the four metals. Note which metal is at the positive electrode. Does current flow if the metals are reversed. Twelve observations. - +Cu Pb Mg Zn Cu XX Pb XX Mg XX Zn XX Write equations: Can we rank the metals in order activity? Arrange on a line. III. Standard Reduction Potentials - The potential difference across a Voltaic cell is easily measured, but it is impossible to measure an individual electrode potential. Therefore the hydrogen electrode which consists of a platinum electrode immersed in a 1.0 M solution of hydrogen ions is assigned a value of zero and all other half cells are measured with respect to this. Here are some standard reduction potentials: Cu ---> Cu+2 + 2e -0.337 v Pb ---> Pb+2 + 2e 0.126 v Mg ---> Mg+2 + 2e 2.370 v Zn ---> Zn+2 + 2e 0.763 v Oxidation potential equals reduction potential with the sign changed. The predicted potential difference is the algebraic sum of the oxidation and reduction potentials. Example: Zn + Cu = 0.763 -(-0.337) = +1.10 v The observed voltage is always less than the predicted voltage. Calculate the predicted voltage for each of these cells, having first written the equation and identified the element which is oxidized. How do these values compare with differences taken from the number line above? IV. Discuss drawings of: A. Flash light dry cell B. Alkaline dry cell C. Automobile battery
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