Chemical Dominoes Activity 6 Electrochemical Cells GALS In this activity you will: Determine if a substance will conduct electricity when dissolved in water. Construct a galvanic cell and explain the function of its components. Describe the effect of adding cells in series. What Do You Think? Portable CD players, electronic video games, and watches are just a few of the items that are powered by batteries. ow does a battery work? Why does a battery eventually die? Record your ideas about these questions in your log. Be prepared to discuss your responses with your small group and the class. Investigate In this activity, you will learn how to build batteries that generate voltage in different amounts. To be able to control the amount of voltage produced, you will have to understand how electricity is produced. Part A: Solutions That Conduct Electricity 1. In Activity 4, you learned how electricity flows through metal wires and magnesium strip to produce a complete circuit. Now you are going to investigate how electricity can flow through solutions. To do this, you are going to build a circuit to test the conductivity of solutions. Construct a conductivity tester using a wooden tongue depressor, two wires, a 9-V battery, a resistor, and the red LED, as shown in the diagram. 308
Activity 6 Electrochemical Cells Touch the two exposed wires together briefly to make sure the LED will light. Your conductivity tester is now ready to use. If the two exposed wires are placed in a solution that conducts electricity, the LED will light. 2. All of the solutions you will test are made of different substances dissolved in distilled water. Add some distilled or de-ionized water to a beaker. Dip the exposed wires of your conductivity tester into the water. Make sure that the exposed parts of both wires are in contact with the water. a) Does the LED light? Record what you did and what you saw in your log. b) What is the purpose of testing distilled water before beginning the tests of substances dissolved in distilled water? 3. btain five clean 100-mL beakers. Label the beakers: distilled water 2 (l) (control) sodium chloride solution NaCl(aq) potassium nitrate solution KN 3 (aq) sucrose solution C 12 22 11 (aq) fructose solution C 6 12 6 (aq) 4. Fill each beaker with 25 ml of the appropriate solution. a) In your log, record the appearance of each solution. b) Look at the formula for each substance that is dissolved in distilled water. Based on what you learned in Activity 4, predict which substances are likely to create charged ions when dissolved in water. Explain how you arrived at these predictions. 5. Test each solution to see if it conducts electricity. In between each test, wash off the exposed wires and dry them. Then test the distilled water again to make sure there is nothing left on the wires that would contaminate solutions and give you false results for some of your tests. a) rganize your observations in a data table in your log. 6. You used distilled water in this investigation as the control. Why didn t you use tap water? Test the conductivity of tap water and explain why it can t be used as the control. a) Based on your results, draw pictures in your log to represent the particles in distilled water and the particles in tap water. 7. Dispose of the materials as directed by your teacher. Clean up your workstation. Part B: Making a Battery 1. Construct a battery. Begin by cleaning a piece of zinc metal and a piece of copper with steel wool. This should get rid of any oxides that have built up on the surface. These pieces of metal are called electrodes. 309
Chemical Dominoes 2. Add 25 ml of 1.0 M copper (II) nitrate solution to a 50-mL beaker. Label the beaker. Place the copper electrode into the beaker. This beaker is called a half-cell. a) What happens to the copper electrode? Record your observations in your log. 3. Add 25 ml of 1.0 M zinc nitrate solution to another 50-mL beaker. Label the beaker. Place the zinc electrode into this beaker. This beaker is also a half-cell, specifically a zinc half-cell. a) What happens to the zinc electrode? Record your observations in your Active Chemistry log. b) Based on your observations, why can t you place the copper in the zinc solution or the zinc in the copper solution in order to construct half-cells to make a battery? 4. In this system, each beaker contains a metal and its salt, forming a half-cell. A half-reaction will occur between zinc atoms and zinc ions in one beaker. In the other beaker, a half-reaction will occur between copper atoms and copper ions. a) Write the two half-reactions that show how zinc atoms and copper atoms lose electrons. Refer to Activity 4 for help. b) Combine the two half-reactions into a single oxidation-reduction equation. Use your Metal Activity Series to be sure the equation you have written represents a reaction that will actually occur. 5. Place the copper half-cell next to the zinc half-cell. olding a U-shaped tube U-side up, fill the tube with 1.0 M sodium nitrate solution to within 1 cm of the open ends of the tube. Place one piece of glass wool into each end to seal it. This tube is called a salt bridge. 6. Place the U-shaped tube into the half-cells so that the U is upside down. ne end of the U will be in the copper half-cell and one end in the zinc half-cell. (See the diagram.) a) Does any change appear to occur? 7. Attach one wire to each metal electrode. a) Draw what your cell looks like in your log. Make sure to label all of the parts. 8. You now have a battery. Place a voltmeter across the anode and cathode. a) Record the voltage of the battery and which half-cell is the anode ( ) and which half-cell is the cathode (). 9. It is likely that your battery does not produce enough voltage to light the LED on its own. Add a commercial D-cell battery in series (in line) with your copper/zinc battery to increase the amount of voltage that flows through the LED. 310
Activity 6 Electrochemical Cells LED direction that electrons are moving When the LED lights, the electrodes on your battery have the charge shown here because the two batteries are connected in the same direction. zinc anode + copper cathode + D-cell battery direction that electrons are moving your battery First, connect the longer leg of the LED to the () terminal of the D- cell and the shorter leg of the LED to the ( ) terminal of the D-cell. Then open the ( ) connection from the D-cell battery and connect it to zinc anode. Add a lead from the ( ) terminal of the battery to the copper cathode. After adding the copper/ zinc battery, the LED should light. 10. nce the LED is connected to your battery and producing light, allow your cell to run for 5 10 min. a) After this time period, look at the copper and the zinc electrodes. Record your observations of the electrodes. Where does the copper that appears on the copper electrode come from? Where does the zinc go as the zinc electrode is disappearing? 11. While the LED is lit, remove the salt bridge from the circuit. a) Does the LED remain lit? b) Put the salt bridge back. Does the LED light up again? 12. Remove the salt bridge from your cell again. old the bridge so the U is right side up. Remove the glass wool and empty the potassium nitrate solution into a beaker. Rinse the salt bridge three times with distilled water. Fill the salt bridge with distilled water and seal the ends with two new glass wool plugs. Predict if the LED will light up when the salt bridge is put back. Why or why not? a) Record your answer and your reasoning in your Active Chemistry log. b) Put the salt bridge back. Does the LED light up? 13. Dispose of the materials as directed by your teacher. Clean up your workstation. Wash your hands and arms thoroughly after the activity. 311
Chemical Dominoes Chem Words electrolyte: a solute that forms ions in an aqueous (water) solution. ion: an atom or molecule that has acquired a charge by either gaining (anion) or losing (cation) electron(s). Sodium Chloride Solution Cl Na + Cl Cl Na + SLUTINS TAT CNDUCT ELECTRICITY Substances that dissolve in water to make solutions that conduct electricity are called electrolytes. In this activity, you used a conductivity tester to determine if a solution conducted electricity. For any solution to be able to conduct electricity, it must contain charged particles that are able to move. All of the solutions you tested were made of substances that were dissolved in distilled water. But only the substances that broke into charged particles when dissolved were electrolytes. For a solution to conduct electricity, there are two conditions that must be met. Charged particles must be present and the charged particles must be able to move around. Some compounds dissolve in water to form charged particles called ions. Usually, these compounds are made of a positively charged metal ion and a negatively charged ion to balance the charge. For example, regular table salt, sodium chloride (NaCl), is made of sodium ions (Na ) with charges of 1, and chloride ions (Cl ) with charges of 1. When some NaCl crystals dissolve in water, sodium ions and chloride ions separate from the crystal and are surrounded by water molecules. Na + Sugar Solution Na + Cl sodium ions chloride ions water molecules sugar molecules water molecules Since the ions are surrounded by water, they can move about in the water. Since there are charged particles that can move, a solution of sodium chloride is able to conduct electricity. Molecular compounds do not break up into ions when they dissolve. For example, sugar remains as sugar molecules when it dissolves. Since molecules do not form charged particles in solution, solutions made of molecules dissolved in water do not conduct electricity. 312
Activity 6 Electrochemical Cells The Path of Electricity in a Battery In a battery, part of the path of the electricity runs through the solutions. First, electrons are produced by the oxidation half-reaction that occurs. Zn Zn 2 2e While the battery is operating, the zinc metal electrode is slowly undergoing a reaction. Neutral zinc metal atoms (Zn) make two products. Zinc ions (Zn 2 ) enter the zinc nitrate solution and electrons (e ) are produced. These electrons flow through the wire, through anything that s connected to the operating battery, and into the copper metal electrode. When the electrons reach the copper electrode, they enter into the reduction half-reaction: Cu 2 2e Cu In this half-reaction, copper ions from the solution combine with electrons to make neutral copper metal atoms. So, the copper metal electrode increases in mass because copper metal is slowly attaching to it. This was the reddish brown sludge you observed forming on the copper electrode. The blue color of the copper (II) nitrate solution will diminish as the copper ions are used up. The copper ions are not the only ions in the solution. The solution was originally prepared by dissolving Cu(N 3 ) 2 crystals in water. When this compound dissolves in water, Cu 2 ions and N 3 ions (twice as many of the latter) are both surrounded by water molecules. As the positively charged copper ions (Cu 2 ) in the solution get used up, negatively charged nitrate (N 3 ) ions get kicked out of the solution, because the solution must remain neutral. The only place for the N 3 ions to go is through the salt bridge. n the other side of the bridge, they enter the zinc half-cell. The Zn 2 ions are being created in the zinc half-cell as the battery operates. So, there is a need for more negative ions to balance the charge in that solution. ow Does This Relate to the Metal Activity Series? ne very important point here is that the electricity only runs spontaneously in one direction in a battery. This is due to the relative activities of the two metal electrodes used in the half-reactions. In this case, the half-reactions were converting from Zn metal atoms to Zn 2 ions in one of the cells, and from Cu 2 ions to Cu metal 313
Chemical Dominoes Checking Up 1. Why can a solution containing salt, NaCl, conduct electricity but one containing sucrose, C 12 22 11, cannot? 2. What is the difference between an electrolyte and a non-electrolyte? 3. Describe where the electrons in the zinccopper battery come from and where they go to. 4. What is the anode of a battery and what happens there? 5. What is the cathode of a battery and what happens there? atoms in the other cell (see halfreactions on the previous page). If the opposite conversions were happening, then the electricity would run in the other direction. So, why does it happen spontaneously in one direction and not the other? The Metal Activity Series provides an answer. Zinc (Zn) metal is more reactive than copper (Cu) metal. Therefore, Zn will react spontaneously with Cu 2, and Cu will not react spontaneously with Zn 2. Recall also from Activity 4 that not only does the activity series provide predictions of which reactions will and won t occur spontaneously, but also, the farther apart two metals are from each other within the activity series, the more powerful the metal/other metal ion reaction will be. What Do You Think Now? At the beginning of this activity you were asked: ow does a battery work? Why does a battery eventually die? Now that you have investigated batteries and electricity flow, how would you answer these questions? Why do electronic devices not work if you insert the battery backwards? 314
Activity 6 Electrochemical Cells What does it mean? Chemistry explains a macroscopic phenomenon (what you observe) with a description of what happens at the nanoscopic level (atoms and molecules) using symbolic structures as a way to communicate. Complete the chart below in your log. ow do you know? The battery in your activity does not resemble the battery you use in your CD or MP3 player. ow do you know that it is a battery? Why do you believe? MACR NAN SYMBLIC Describe how you constructed a battery using electrodes, solutions, and wires. List five items that use battery power that you use on a regular basis. Why should you care? ow do electrons flow through the zinc/copper battery when it is connected in series with the LED? Suppose you decide to build a battery to light the LED in your Chemical Dominoes apparatus. List three ways you could change the design of a battery system to make it produce more voltage. Explain why you think these changes increase the voltage of the system. Draw a labeled picture of the battery you built. Show how you would connect the LED to this circuit. Draw arrows on your picture to show the flow of electrons through the battery. Reflecting on the Activity and the Challenge In this activity, you built a conductivity tester and tested whether particular solutions conducted electricity. You may want to use the conductivity tester as part of your Chemical Dominoes apparatus. Think about how you could lower it into a solution that would cause the LED to light. r, if the conductivity tester were already sitting in a container of pure water, perhaps you could add a substance to the water that would make the LED light. In the activity, you also built a galvanic cell, or battery. You experimented with two ways of creating more voltage. If you want to use a battery to light the LED in your dominoes apparatus, you will have to be able to produce enough voltage. This may require you to use the activity series to choose a certain combination of metals. You might also need to connect several batteries in series and use certain concentrations of the solutions in the apparatus. 315
Chemical Dominoes 1. In Part A of the activity, you tested the conductivities of four solutions. Identify which of the solutions were electrolytes and explain how you could tell. 2. Which compounds listed below would you expect to form solutions that conduct electricity when you dissolve the compounds in water? Explain your reasoning. a) KF b) C 6 6 c) CCl 4 d) Na 2 S 4 3. All hair dryers, shavers, and other electrical appliances that are often used in the bathroom come with labels, mandated by the government, saying not to operate the appliance while taking a bath. a) Based on what you learned in Part A of the activity, explain why it would be dangerous to use an appliance while bathing. b) You observed in the experiment that distilled water does not conduct electricity. If you were to take a bath in distilled water, why would it still not be safe to use an electrical appliance while bathing? Explain. 4. You may have seen sports drink advertisements talk about the electrolytes in the sports drink. When you sweat, you lose electrolytes. (Sweat tastes salty, right?) Electrolytes are important in the body because your nerves require electrolytic solutions to transfer electrical impulses. a) Would you expect a sports drink to conduct electricity? b) Design an experiment to determine which sports drink on the market has the highest concentration of electrolytes. List the equipment and other materials you would need to conduct the experiment, and briefly describe the procedure you would follow. c) Would you need a control in this experiment? If so, what would you use and what would you expect the results from testing the control to be? 5. Predict a metal half-cell that you could replace the zinc half-cell with in your battery and produce more voltage. Assume you would use 1.0 M metal ion solutions in both half-cells. Explain the reasoning behind your prediction. 6. A student was studying how the voltage generated by a battery can be changed. She prepared five different cells and changed something about each. The changes she tested are shown below. For each change, state if the voltage would increase, decrease, or remain the same. Explain your reasoning in each case. a) The salt bridge was removed. b) The contents of the salt bridge were replaced with distilled water. c) The wire at the anode was disconnected. d) The battery was allowed to run for one hour. 316
Activity 6 Electrochemical Cells 7. Given the reaction: Ba() 2 (aq) 2 S 4 (aq) BaS 4 (s) 2 2 (l) energy As the barium hydroxide solution is added to the solution of sulfuric acid, the electrical conductivity of the acid solution decreases because the: a) volume of the reaction mixture increases b) temperature of the reaction mixture decreases c) concentration of ions increases d) concentration of ions decreases 8. Which half-reaction correctly represents reduction? a) Ag Ag e b) F 2 2F 2e c) Au 3 3e Au d) Fe 2 Fe 3 e 9. In a redox reaction, how does the total number of electrons lost by the oxidized substance compare to the total number of electrons gained by the reduced substance? a) The number lost is always greater than the number gained. b) The number lost is always equal to the number gained. c) The number lost is sometimes equal to the number gained. d) The number lost is sometimes less than the number gained. 10. Preparing for the Chapter Challenge Decide with your group on a way that you could use either a battery you make or the conductivity tester (or both) to light the LED in your Chemical Dominoes apparatus. From other activities, you already have several other components you may use in the sequence. Describe how you will light the LED using your battery and/or conductivity tester. Include a labeled drawing in your description. Inquiring Further Factors affecting voltage generated Use your knowledge from this activity and the activity series to design an experiment to determine which of the following factors most affects voltage generated: 1. Identity of electrodes 2. Number of cells in series 3. Concentration of cathode and anode cells used After your teacher approves your procedure, carry out your experiment under supervision. 317