5.7 Galvanic Cells. Electrochemical Gizmos
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1 5.7 Galvanic Cells Have you ever accidentally bitten into a piece of aluminum foil? If you have silver amalgam fillings, you may have experienced a bit of a jolt (Figure 1). The aluminium, in contact with a metal filling and your saliva, creates a minibattery in your mouth, sending electricity into the nerves of your tooth. Believe it or not, this mini-battery works on the same principle as the little button battery in your calculator and the 20-kg lead acid battery that starts a car. How does a battery work? You ll find out in the next three sections. Figure 1 A redox reaction is responsible for the electric shock you may feel if aluminum foil comes into contact with a metal filling in your mouth. TRY THIS activity Electrochemical Gizmos Potatoes, lemons, bananas, and limes have all been used to power novelty clocks, such as the potato clock in Figure 2. The fruits and vegetables really aren t necessary, however! In this activity, you will use simpler materials to power a clock. Figure 2 The energy that is needed to power this digital clock comes from the electrons that are travelling along the wires. Materials: eye protection, 2 copper strips, magnesium strip, sandpaper, connecting wires with alligator clips, LCD clock (battery removed), plastic cup, distilled water, tap water 1. Clean the metals with sandpaper to remove any surface oxides. 2. Attach one alligator clip to a copper strip. Attach another alligator clip to the magnesium strip. 3. Attach the other end of each wire to the battery connectors on the LCD clock. 4. Dip both strips into a cup that is half-full of distilled water. Make sure that the metal strips do not touch inside the cup. Does the clock turn on? 5. Replace the distilled water with tap water, and try again. 6. Replace the magnesium strip with another copper strip. Place both copper strips in a cup of tap water. (a) Which combination of strips and water made the clock turn on? (b) Why was one combination of strips more successful at running the clock than another combination of strips? (c) How are distilled water and tap water different? Suggest an explanation for the difference in your observations when using distilled water and tap water. Converting Chemical Energy to Electrical Energy In section 5.1, you looked at the oxidation of zinc by the copper(ii) ion, Cu 2 : Zn(s) Cu2 Cu (s) Zn2 394 Unit 5 NEL
2 Section 5.7 In this reaction, copper ions remove two electrons from zinc atoms. The following redox changes occur: Zn becomes Zn 2, a loss of 2 e (oxidation). Cu 2 becomes Cu, a gain of 2 e (reduction). By separating the zinc metal and the solution containing copper ions (Figure 3) and by placing a metal conductor (wire) between them, electrons that are lost by zinc are forced to travel through the metal conductor to reach the copper ions. Moving electrons have energy. This energy can be used to power an electrical device, such as a radio or a clock. The apparatus, called a galvanic cell, converts chemical energy from a redox reaction into electrical energy. If you think that a galvanic cell sounds like a battery, you re correct! Batteries contain galvanic cells. By learning how the galvanic cell in Figure 3 works, you will better understand how batteries produce electricity (section 5.9). The reactions that occur in a galvanic cell are spontaneous reactions: they require no outside assistance or energy input to make them occur. In Figure 3, the oxidation of zinc and the reduction of copper ions occur in separate compartments, called half-cells. Each half-cell contains a solid conductor called an electrode. In each half-cell, electron transfers occur between atoms on the surface of the electrode and ions in the solution. The electrode where oxidation occurs is called the anode. The electrode where reduction occurs is called the cathode. Metals or inert (non-reactive) conductors, such as graphite (the black material inside your pencil), are common electrodes used in galvanic cells. galvanic cell a device that converts chemical energy from redox reactions into electrical energy LEARNING TIP Galvanic or Galvanic cells can also be called electric, electrochemical, or voltaic cells. To avoid confusion, galvanic cell is used throughout this textbook. spontaneous reaction a reaction that proceeds on its own without outside assistance half-cell one of the two compartments of a galvanic cell, composed of an electrode and an electrolyte solution electrode a solid electrical conductor where, in a galvanic cell, the electron transfers occur anode the electrode at which oxidation occurs cathode the electrode at which reduction occurs wire wire Na salt bridge zinc electrode (anode) copper electrode (cathode) zinc nitrate electrolyte solution (1.0 mol/l) zinc half-cell copper half-cell copper(ii) nitrate electrolyte solution (1.0 mol/l) Figure 3 This galvanic cell consists of two half-cells, connected by a metal conductor and a salt bridge. NEL Electrochemistry 395
3 Mg (s) wire Cu (s) In Figure 3, each electrode is immersed in an electrolyte solution that contains ions of the same metal as the electrode. The galvanic cell has a zinc half-cell consisting of a strip of zinc in a zinc nitrate solution; and a copper half-cell consisting of a strip of copper in a copper(ii) nitrate solution. The half-cells are connected to each other using a wire (for electron flow) and a salt bridge (for ion flow). Both connections are necessary for the cell to produce electrical energy. A salt bridge is a tube that contains a concentrated solution of an electrolyte, such as sodium nitrate, Na. The electrolyte that is chosen for the salt bridge should not react with the other chemicals in the cell. You will find out more about salt bridges after the next subsection. Figure 4 shows a simpler galvanic cell design. tap water Figure 4 This simple galvanic cell design produces electricity using two different electrodes and one electrolyte solution. salt bridge a container of electrolyte solution that connects the two solutions of a galvanic cell LEARNING TIP Memory Aid anode oxidation (Both start with vowels.) cathode reduction (Both start with consonants.) DID YOU KNOW? Electrode Charges The anode and cathode of a galvanic cell are sometimes labelled ( ) and ( ), respectively. You can think of the anode as being electron rich and the cathode as being electron poor. Cell Reactions You can predict that zinc is oxidized in the zinc/copper cell because zinc is higher than copper in the activity series. Therefore, copper(ii) ions, Cu 2,are reduced. The chemical equation for this reaction can be divided into two parts, called half-reactions. The two half-reactions for the zinc/copper cell are given below. Cathode half-reaction: Notice the following: Zn (s) Zn 2 2e (oxidation) Cu 2 2e Cu (s) (reduction) Atoms from the zinc electrode lose electrons and dissolve as Zn 2 ions in the zinc half-cell. Cu 2 ions in the copper half-cell gain electrons and become neutral copper atoms. Therefore, as the cell operates, the mass of the zinc electrode decreases and the mass of the copper electrode increases. Furthermore, the blue colour of the copper solution fades as more and more of the Cu 2 ions become copper atoms at the copper electrode. An overall cell reaction for the zinc/copper cell may be obtained by adding the half-reaction of each half-cell: Zn (s) Zn 2+ 2e Cathode half-reaction: Cu 2 2e Cu (s) Overall cell reaction: Cu 2 Zn (s) Cu (s) Zn2+ In summary, electrons that are lost at the anode (where oxidation occurs) flow through the wire to the cathode (where reduction occurs). As with any redox reaction, the electrons that are lost by one reactant are gained by the other reactant. 396 Unit 5 NEL
4 Section 5.7 SAMPLE problem Writing Chemical Equations for Cell Reactions Write the anode, cathode, and overall cell reactions that occur when each pair of half-cells is combined to form a galvanic cell. (a) a copper strip in a solution of copper(ii) nitrate, Cu( ) 2, and a tin strip in a solution of tin(ii) chloride, SnCl 2 Step 1: Establish Elements Oxidized and Reduced Tin is higher than copper in the activity series. Therefore, tin atoms are oxidized and copper(ii) ions, Cu 2, are reduced. Sn (s) Sn 2 2e Cathode half-reaction: Cu 2 2e Cu (s) Overall cell reaction: Sn (s) Cu 2 Sn2 Cu (s) (b) an aluminum strip in a solution of aluminum nitrate, Al( ) 3, and a silver strip in a solution of silver nitrate, Ag Step 1: Establish Elements Oxidized and Reduced Aluminum is higher than silver in the activity series. Therefore, aluminum atoms are oxidized and silver ions, Ag, are reduced. Al (s) Al 3 3 e Cathode half-reaction: Ag e Ag (s) Notice that the number of electrons lost by each aluminum atom (3 e ) is not equal to the number of electrons gained by each silver ion (1 e ). DID YOU KNOW? It s All Greek The word ion is a Greek word meaning traveller. It was first used by the Swedish chemist Jons Berzelius ( ). Step 2: Balance Charges Multiply both sides of the cathode reaction by 3 so that the number of electrons lost is equal to the number of electrons gained. Al (s) Al 3 3 e Cathode half-reaction: 3 Ag 3 e 3 Ag (s) Overall cell reaction: Al (s) 3 Ag Al3 3 Ag (s) Practice Understanding Concepts Write the chemical equations for the anode, cathode, and overall cell reactions that occur when a galvanic cell is made using each pair of the following halfcells: (a) a lead strip in a solution of lead(ii) nitrate, Pb( ) 2, and a zinc strip in a solution of zinc nitrate, Zn( ) 2 (b) a silver strip in a solution of silver nitrate, Ag, and a magnesium strip in a solution of magnesium nitrate, Mg( ) 2 NEL Electrochemistry 397
5 LEARNING TIP Ion Migration Positive ions are sometimes called cations because they migrate to the cathode. Negative ions are sometimes called anions because they migrate to the anode. The Purpose of the Salt Bridge The salt bridge provides ions to prevent charge buildup from occurring. In Figure 5, for example, as Cu 2 ions are removed from the copper nitrate solution on the right, Na ions from the salt bridge move into the cathode half-cell. Similarly, negative nitrate ions flow into the anode half-cell from the salt bridge. As a result, the solution around each electrode remains neutral, even though the concentrations of zinc ions and copper ions are changing. What happens if you remove the salt bridge? The production of zinc ions, Zn 2, results in a buildup of positive charge around the zinc electrode (the anode). This buildup prevents further production of zinc ions. Similarly, the loss of copper ions, Cu 2, at the cathode leaves the solution negatively charged, which prevents electrons in the copper electrode from being transferred to other copper ions. The end result is that the electrode reactions stop occurring. Thus, removing the salt bridge has the same effect as disconnecting the electrodes. e e Na + zinc electrode (anode) e Zn Cu e copper electrode (cathode) Na + anode half-cell cathode half-cell Zn (s) Zn + 2 e Cu + 2 e Cu (s) Figure 5 Notice how the electrons flow through the wire (to the right) from anode to cathode. The circuit is completed by negative nitrate ions flowing to the anode through the salt bridge. At the same time, positive sodium ions flow in the opposite direction through the salt bridge. 398 Unit 5 NEL
6 Section 5.7 Cell Potential The cell potential (also called the voltage) of the cell in Figure 5 is 1.20 V (using 1.0-mol/L solutions measured at 25 C). Let s use a water analogy to understand what this measurement means. Consider the water behind the gates in the lock in Figure 6.In Figure 6(a), a kilogram of surface water on the right side of the gate is higher and, therefore, has more gravitational potential energy than a kilogram of surface water on the left side of the gate. Once the water outlet is opened, gravitational potential energy is converted to the kinetic energy of the water flowing from the higher side to the lower side. As the water level drops (Figure 6(b)), the difference in gravitational potential energy decreases. Water stops flowing when the difference in gravitational potential energy between the two sides is zero (Figure 6(c)). Similarly, the anode is the high side of a galvanic cell, and the cathode is the low side. Once a connection is made, electrons flow downhill from anode to cathode. The cell potential is a measure of the electrical potential energy difference (or potential difference) across the two half-cell electrodes. It is measured using a voltmeter. The units of potential difference are volts (V). As the cell operates, the potential difference gradually decreases until it becomes zero. At this point, electron flow has stopped and the cell is dead. Learning about galvanic cell design is useful for understanding how galvanic cells produce electricity. In section 5.9, you will apply your understanding of galvanic cells to the operation of batteries. cell potential (voltage) a measure of the potential difference across the electrodes (a) (b) (c) SUMMARY Galvanic Cells The reaction that occurs within a galvanic cell is spontaneous. Every galvanic cell consists of two different electrodes and at least one electrolyte. Oxidation occurs at the anode. Reduction occurs at the cathode. Two connections are needed to complete the circuit in a galvanic cell: a wire (for electron flow) and a salt bridge (for ion flow). Ions from the salt bridge help to keep each half-cell electrically neutral. Positive ions (cations) flow to the cathode. Negative ions (anions) flow to the anode. The cell potential (or voltage) measures the potential difference across the electrodes. Figure 6 (a) The water on the right side of the gate has more gravitational potential energy than the water on the left side. (b) The difference in potential energy decreases as water from the high side flows to the low side through underground pipes. (c) The flow of water between sides stops when the water levels become equal. LEARNING TIP Electron Affinities Think of the anode as pushing the electrons away, while the cathode is pulling the electrons toward it. NEL Electrochemistry 399
7 Section 5.7 Questions Understanding Concepts 1. What energy change occurs during the operation of a galvanic cell? 2. How are the anode and cathode similar, and how are they different? 3. What are the two connections that have to be made before a galvanic cell can produce electricity? Extension 10. Many ph meters contain electrodes that sense the hydrogen ion concentration of a solution and then convert this information into a ph reading. Research and report on how a ph meter works. GO 4. A galvanic cell is made using Mg (s) in Mg( ) 2 and Zn (s) in Zn( ) 2. Write equations for the anode half-cell reaction, the cathode half-cell reaction, and the overall cell reaction. 5. What does cell potential measure? 6. Explain why the cell potential drops to zero when the salt bridge is removed. 7. (a) Identify the anode and cathode in Figure 7. (b) Write the anode and cathode half-cell reactions. (c) Write the overall cell reaction. (d) In which direction do electrons flow? (e) In which direction do ions flow in the salt (f) bridge? Predict which electrode increases in mass and which electrode decreases in mass. Justify your predictions. (g) Why is sodium nitrate a more suitable electrolyte for the salt bridge than sodium chloride? (Hint: Examine the solubility rules you learned about in Unit 1.) Cu (s) Na + Ag (s) Applying Inquiry Skills 8. (a) Draw and label a diagram of a galvanic cell that can be constructed using the following materials: beakers, connecting wires, glass U-tube, cotton, distilled water, strip of lead, solid lead(ii) nitrate, solid sodium nitrate, strip of nickel, solid nickel(ii) nitrate. (b) Indicate the direction of electron flow, and identify the anode and cathode in the cell. Cu 2+ Ag + Making Connections 9. In the late 1780s, the Italian anatomist Luigi Galvani (after whom the galvanic cell is named) reported a startling discovery. While dissecting the leg muscle of a dead frog, Galvani noticed that the leg suddenly twitched! This effect happened only when the leg muscle came in contact with two different metals. Research and summarize how Galvani interpreted his discovery. Was he correct? copper(ii) nitrate solution silver nitrate solution Figure 7 The electrons flow from the copper electrode to the silver electrode. GO Unit 5 NEL
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