Instructors Guide: Introduction to Voltaic Cells Standards Connections: Connections to NSTA Standards for Science Teacher Preparation C.3.a.8 Oxidation reduction chemistry. Connections to the National Science Education Standards: Properties and Changes of Properties in Matter Substances react chemically in characteristic ways with other substances to form new substances (compounds) with different characteristic properties. In chemical reactions, the total mass is conserved. Chemical Reactions A large number of important reactions involve the transfer of electrons (oxidation/reduction reactions) between reacting ions, molecules or atoms. Connections to New York Chemistry Core Curriculum: Write and balance half-reactions for oxidation and reduction of free elements and their monatomic ions (3.2f, 3.2h) An oxidation-reduction (redox) reaction involves the transfer of electrons (e - ) (3.2d) Reduction is the gain of electrons (3.2e) Oxidation is the loss of electrons (3.2g) Oxidation numbers (states) can be assigned to atoms and ions. Changes in oxidation numbers indicate that oxidation and reduction have occurred (3.2i) An electrochemical cell can be either voltaic or electrolytic. In an electrochemical cell, oxidation occurs at the anode and reduction at the cathode (3.2j) A voltaic cell spontaneously convert chemical energy to electrical energy (3.2k) POGIL 2005, 2006 1/7
Hints for facilitation: Key Questions #8 and 9: Students will probably have problems with these questions; they may require teacher intervention. Problem #2 and 3: Students will probably have problems with these questions; they may require teacher intervention. Answer to Problem Diagram Suggestions for appropriate demonstrations, manipulatives, etc., that will enhance the activity. Demonstration / Experiment: (Practice this before doing for the class!) Lemon battery: Creating a battery from a lemon is a common project in many science textbooks. Successfully creating one of these devices is not easy. Batteries consist of two different metals suspended in an acidic solution. Copper and Zinc work well as the metals and the citric acid content of a lemon will provide the acidic solution. Batteries like this will not be able to run a motor or energize most light bulbs. It is possible to produce a dim glow from an LED. The lemon: A large, fresh, "juicy" lemon works best. The nail: Galvanized nails are coated in zinc. I used a 2" galvanized common nail. The penny: Any copper coin will work. (Pennies from 1960-2001 all worked) POGIL 2005, 2006 2/7
Creating the battery: Insert a penny into a cut on one side of the lemon. Push a galvanized nail into the other side of the lemon. The nail and penny must not touch. The zinc nail and the copper penny are called electrodes. The lemon juice is called electrolyte. All batteries have a "+" and "-" terminal. Electric current is a flow of atomic particles called electrons. Certain materials, called conductors, allow electrons to flow through them. Most metals (copper, iron) are good conductors of electricity. Electrons will flow from the "-" electrode of a battery, through a conductor, towards the "+" electrode of a battery. Volts (voltage) are a measure of the force moving the electrons. (High voltage is dangerous!) You can connect a voltmeter to the single cell lemon battery. POGIL 2005, 2006 3/7
Why? Introduction to Voltaic Cells Voltaic cells, also known as batteries, are used to store energy and provide power on demand. In a voltaic cell there is a flow of ions and a flow of electrons. Because batteries are so common you should understand how batteries function. Learning Objectives Recognize the parts of a voltaic cell. Understand how a voltaic cell functions. Success Criteria Identify and label the parts of a voltaic cell, identifying the anode, cathode, and salt bridge, given the reaction equation. Identify the direction of electron flow in the external circuit and the direction of ion diffusion through the salt bridge, given the reaction equation. Identify each half-cell as the site of oxidation or reduction by interpreting the reaction equation. Explain the function of each cell component. Prerequisites Rules for assigning oxidation numbers Ability to write oxidation and reduction half-reactions Requirements Computer and internet access for the following sites: http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf Vocabulary Reduction Oxidation Half-reaction Half-cell Information A voltaic cell consists of a cathode, and anode, a salt bridge. When the voltaic cell is operating, electrons flow through an external circuit, and ions diffuse through the salt bridge. POGIL 2005, 2006 4/7
Model 1: The Zn/Cu Cell Go to the Internet site: http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf [Be patient, it may take a moment or two for all elements of the diagram to appear.] Look carefully at the working model; explore each of the buttons at the bottom of the cell to see a close-up view of each electrode and the salt bridge opening in each beaker. Answer the following questions about the model shown on the web site. Key Questions 1. In the model shown on the web site, which label (anode or cathode) is attached to the zinc metal and to the copper metal? Anode = Zinc Cathode = Copper 2. Which way do electrons flow through the wire, from the anode to the cathode or from the cathode to the anode? From the anode to the cathode 3. What is happening to the zinc atoms in the zinc half-cell? Zinc ions? The amount of zinc atoms are decreasing The amount of zinc ions are increasing 4. What is happening to the copper atoms in the copper half-cell? Copper ions? The amount of copper atoms are increasing The amount of copper ions are decreasing 5. Is the reaction occurring at the anode oxidation or reduction? Oxidation 6. Is the reaction at the cathode oxidation or reduction? Reduction 7. Which ions move through the salt bridge? Na + and NO 3-8. Why do you think positive ions move through the salt bridge from the anode compartment to the cathode compartment? POGIL 2005, 2006 5/7
The Cu +2 ions are reacting with the electrons and are being removed from the solution. The Na + ions are sent from the salt bridge to keep the solution electrically neutral 9. Why do you think negative ions move through the salt bridge from the cathode compartment to the anode compartment? The Zn +2 ions are being produced and are therefore adding to the positive charge in the zinc cell. The NO - 3 ions are sent to keep the solution electrically neutral. Exercises 1. Write the half-reaction to show the change in zinc as the cell is running. Zn 0 Zn +2 + 2e - 2. Write the half-reaction to show the change in copper as the cell is running. Cu +2 + 2e - Cu 0 3. List the ions that flow into and out of the salt bridge in each half-cell. Zinc Half Cell: Zn +2 - ions going in, NO 3 ions going out Copper Half Cell: NO - 3 ions going in, Na + ions going out Key Questions 1. What would happen if the salt bridge were to be removed from the set-up? Explain your answer. The reaction would stop. The salt bridge allows for the flow of ions, this balance of neutrality is what keeps the reaction continuing. 2. What characteristic of NaNO 3 (aq) makes it useful as the solution in the salt bridge? NaNO 3 is a water-soluble ionic compound and has a positive ion and negative ion to aid in the balancing of charges. POGIL 2005, 2006 6/7
Problem Two half-cells are prepared by a student in the laboratory and are connected as shown in the diagram below: Half-cell 1 contains a tin electrode in a solution of Sn(NO 3 ) 2 (aq). Half-cell 2 contains an aluminum electrode in a solution of Al(NO 3 ) 3 (aq). The salt bridge contains a solution of NaNO 3 (aq). When the switch is closed the following reaction occurs: 2 Al 0 + 3 Sn 2+ 2 Al 3+ + 3 Sn 0 1. Label the electrode and solution in each of the half-cells. In teacher section. 2. Write the oxidation half-reaction as shown in the equation above. 2 Al 0 2 Al +3 + 6e - OR Al 0 Al +3 + 3e - 3. Write the reduction half-reaction as shown in the equation above. 3 Sn +2 + 6e - 3 Sn 0 OR Sn +2 + 2e - Sn 0 4. Based on your answers to previous questions, decide which electrode is the anode and which electrode is the cathode. Place the appropriate label in the diagram. Diagram labeled in teacher section. 5. When the switch is closed the circuit will be completed. Use an arrow to mark the direction of electron flow in the cell. Diagram labeled in teacher section. POGIL 2005, 2006 7/7