11.1. Galvanic Cells. The Galvanic Cell

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1 Galvanic Cells 11.1 You know that redox reactions involve the transfer of electrons from one reactant to another. You may also recall that an electric current is a flow of electrons in a circuit. These two concepts form the basis of electrochemistry, which is the study of the processes involved in converting chemical energy to electrical energy, and in converting electrical energy to chemical energy. As you learned in Chapter 10, a zinc strip reacts with a solution containing copper(ii) ions, forming zinc ions and metallic copper. The reaction is spontaneous. It releases energy in the form of heat; in other words, this reaction is exothermic. (s) + 2+ (aq) 2+ (aq) + (s) This reaction occurs on the surface of the zinc strip, where electrons are transferred from zinc atoms to copper(ii) ions when these atoms and ions are in direct contact. A common technological invention called a galvanic cell uses redox reactions, such as the one described above, to release energy in the form of electricity. The Galvanic Cell A galvanic cell, also called a voltaic cell, is a device that converts chemical energy to electrical energy. The key to this invention is to prevent the reactants in a redox reaction from coming into direct contact with each other. Instead, electrons flow from one reactant to the other through an external circuit, which is a circuit outside the reaction vessel. This flow of electrons through the external circuit is an electric current. An Example of a Galvanic Cell: The Daniell Cell Figure 11.1 shows one example of a galvanic cell, called the Daniell cell. One half of the cell consists of a piece of zinc placed in a zinc sulfate solution. The other half of the cell consists of a piece of copper placed in a copper(ii) sulfate solution. A porous barrier, sometimes called a semi-permeable membrane, separates these two half-cells. It stops the copper(ii) ions from coming into direct contact with the zinc electrode. Section Preview/ Specific Expectations In this section, you will identify the components in galvanic cells and describe how they work describe the oxidation and reduction half-cells for some galvanic cells determine half-cell reactions, the direction of current flow, electrode polarity, cell potential, and ion movement in some galvanic cells build galvanic cells in the laboratory and investigate galvanic cell potentials communicate your understanding of the following terms: electric current, electrochemistry, galvanic cell, voltaic cell, external circuit, electrodes, electrolytes,,, salt bridge, inert electrode, electric potential, cell voltage, cell potential, dry cell, battery, primary battery, secondary battery Figure 11.1 The Daniell cell is named after its inventor, the English chemist John Frederic Daniell ( ). In the photograph shown here, the zinc sulfate solution is placed inside a porous cup, which is placed in a larger container of copper sulfate solution. The cup acts as the porous barrier. Chapter 11 Cells and Batteries MHR 505

2 Web LINK chemistry12 Galvanic cells are named after the Italian doctor Luigi Galvani ( ), who generated electricity using two metals. These cells are also called voltaic cells, after the Italian physicist Count Alessandro Volta ( ), who built the first chemical batteries. To learn more about scientists who made important discoveries in electrochemistry, such as Galvani, Volta, and Faraday, go to the web site above. Click on Web Links to find out where to go next. In a Daniell cell, the pieces of metallic zinc and copper act as electrical conductors. The conductors that carry electrons into and out of a cell are named electrodes. The zinc sulfate and copper(ii) sulfate act as electrolytes. Electrolytes are substances that conduct electricity when dissolved in water. (The fact that a solution of an electrolyte conducts electricity does not mean that free electrons travel through the solution. An electrolyte solution conducts electricity because of ion movements, and the loss and gain of electrons at the electrodes.) The terms electrode and electrolyte were invented by the leading pioneer of electrochemistry, Michael Faraday ( ). The redox reaction takes place in a galvanic cell when an external circuit, such as a metal wire, connects the electrodes. The oxidation half-reaction occurs in one half-cell, and the reduction half-reaction occurs in the other half-cell. For the Daniell cell: Oxidation (loss of electrons): (s) 2+ (aq) + 2 Reduction (gain of electrons): 2+ (aq) + 2 (s) The electrode at which oxidation occurs is named the. In this example, zinc atoms undergo oxidation at the zinc electrode. Thus, the zinc electrode is the of the Daniell cell. The electrode at which reduction occurs is named the. Here, copper(ii) ions undergo reduction at the copper electrode. Thus, the copper electrode is the of the Daniell cell. Free electrons cannot travel through the solution. Instead, the external circuit conducts electrons from the to the of a galvanic cell. Figure 11.2 gives a diagram of a typical galvanic cell. voltmeter Figure 11.2 A typical galvanic cell, such as the Daniell cell shown here, includes two electrodes, electrolyte solutions, a porous barrier, and an external circuit. Electrons flow through the external circuit from the negative to the positive. porous barrier + (aq) (aq) Figure 11.3 Batteries contain galvanic cells. The + mark labels the positive. If there is a mark, it labels the negative. At the of a galvanic cell, electrons are released by oxidation. For example, at the zinc of the Daniell cell, zinc atoms release electrons to become positive zinc ions. Thus, the of a galvanic cell is negatively charged. Relative to the, the of a galvanic cell is positively charged. In galvanic cells, electrons flow through the external circuit from the negative electrode to the positive electrode. These electrode polarities may already be familiar to you. An example is shown in Figure Each half-cell contains a solution of a neutral compound. In a Daniell cell, these solutions are aqueous zinc sulfate and aqueous copper(ii) sulfate. How can these electrolyte solutions remain neutral when electrons are leaving the of one half-cell and arriving at the of the other half-cell? To maintain electrical neutrality in each half-cell, some ions migrate through the porous barrier, as shown in Figure 11.4, on the next page. Negative ions (anions) migrate toward the, and positive ions (cations) migrate toward the. 506 MHR Unit 5 Electrochemistry

3 porous barrier Figure 11.4 Ion migration in a Daniell cell. Some sulfate ions migrate from the copper(ii) sulfate solution into the zinc sulfate solution. Some zinc ions migrate from the zinc sulfate solution into the copper(ii) sulfate solution. The separator between the half-cells does not need to be a porous barrier. Figure 11.5 shows an alternative device. This device, called a salt bridge, contains an electrolyte solution that does not interfere in the reaction. The open ends of the salt bridge are plugged with a porous material, such as glass wool, to stop the electrolyte from leaking out quickly. The plugs allow ion migration to maintain electrical neutrality. voltmeter NH 4 + Cl salt bridge porous plug Figure 11.5 This Daniell cell includes a salt bridge instead of a porous barrier. In a diagram, the may appear on the left or on the right. (aq) (aq) Suppose the salt bridge of a Daniell cell contains ammonium chloride solution, NH 4 Cl (aq). As positive zinc ions are produced at the, negative chloride ions migrate from the salt bridge into the half-cell that contains the. As positive copper(ii) ions are removed from solution at the, positive ammonium ions migrate from the salt bridge into the half-cell that contains the. Other electrolytes, such as sodium sulfate or potassium nitrate, could be chosen for the salt bridge. Neither of these electrolytes interferes in the cell reaction. Silver nitrate, AgNO 3(aq), would be a poor choice for the salt bridge, however. Positive silver ions would migrate into the half-cell that contains the. Zinc displaces both copper and silver from solution, so both copper(ii) ions and silver ions would be reduced at the. The copper produced would be contaminated with silver. Chapter 11 Cells and Batteries MHR 507

4 Galvanic Cell Notation A convenient shorthand method exists for representing galvanic cells. The shorthand representation of a Daniell cell is as follows The phases or states may be included. (s) 2+ (aq) 2+ (aq) (s) As you saw in Figure 11.4 and Figure 11.5, the may appear on the left or on the right of a diagram. In the shorthand representation, however, the is always shown on the left and the on the right. Each single vertical line,, represents a phase boundary between the electrode and the solution in a half-cell. For example, the first single vertical line shows that the solid zinc and aqueous zinc ions are in different phases or states. The double vertical line,, represents the porous barrier or salt bridge between the half-cells. Spectator ions are usually omitted. Inert Electrodes The zinc and copper of a Daniell cell are both metals, and can act as electrical conductors. However, some redox reactions involve substances that cannot act as electrodes, such as gases or dissolved electrolytes. Galvanic cells that involve such redox reactions use inert electrodes. An inert electrode is an electrode made from a material that is neither a reactant nor a product of the cell reaction. Figure 11.6 shows a cell that contains one inert electrode. The chemical equation, net ionic equation, and half-reactions for this cell are given below. Chemical equation: Pb (s) + 2FeCl 3(aq) 2FeCl 2(aq) + PbCl 2(aq) Net ionic equation: Pb (s) + 2Fe 3+ (aq) 2Fe 2+ (aq) + Pb 2+ (aq) Oxidation half-reaction: Pb (s) Pb 2+ (aq) + 2 Reduction half-reaction: Fe 3+ (aq) + Fe 2+ (aq) The reduction half-reaction does not include a solid conductor of electrons, so an inert platinum electrode is used in this half-cell. The platinum electrode is chemically unchanged, so it does not appear in the chemical equation or half-reactions. However, it is included in the shorthand representation of the cell. Pb Pb 2+ Fe 3+, Fe 2+ Pt A comma separates the formulas Fe 3+ and Fe 2+ for the ions involved in the reduction half-reaction. The formulas are not separated by a vertical line, because there is no phase boundary between these ions. The Fe 3+ and Fe 2+ ions exist in the same aqueous solution. Figure 11.6 This cell uses an inert electrode to conduct electrons. Why do you think that platinum is often chosen as an inert electrode? Another common choice is graphite. Pb salt bridge Pt PbCl 2(aq) FeCl 2(aq) FeCl 3(aq) 508 MHR Unit 5 Electrochemistry

5 Practice Problems 1. (a) If the reaction of zinc with copper(ii) ions is carried out in a test tube, what is the oxidizing agent and what is the reducing agent? (b) In a Daniell cell, what is the oxidizing agent and what is the reducing agent? Explain your answer. 2. Write the oxidation half-reaction, the reduction half-reaction, and the overall cell reaction for each of the following galvanic cells. Identify the and the in each case. In part (b), platinum is present as an inert electrode. (a) Sn (s) Sn 2+ (aq) Tl + (aq) Tl (s) (b) Cd (s) Cd 2+ (aq) H + (aq) H 2(g) Pt (s) 3. A galvanic cell involves the overall reaction of iodide ions with acidified permanganate ions to form manganese(ii) ions and iodine. The salt bridge contains potassium nitrate. (a) Write the half-reactions, and the overall cell reaction. (b) Identify the oxidizing agent and the reducing agent. (c) The inert and are both made of graphite. Solid iodine forms on one of them. Which one? 4. As you saw earlier, pushing a zinc electrode and a copper electrode into a lemon makes a lemon cell. In the following representation of the cell, C 6 H 8 O 7 is the formula of citric acid. Explain why the representation does not include a double vertical line. (s) C 6 H 8 O 7(aq) (s) Introducing Cell Potentials You know that water spontaneously flows from a higher position to a lower position. In other words, water flows from a state of higher gravitational potential energy to a state of lower gravitational potential energy. As water flows downhill, it can do work, such as turning a water wheel or a turbine. The chemical changes that take place in galvanic cells are also accompanied by changes in potential energy. Electrons spontaneously flow from a position of higher potential energy at the to a position of lower potential energy at the. The moving electrons can do work, such as lighting a bulb or turning a motor. The difference between the potential energy at the and the potential energy at the is the electric potential, E, of a cell. The unit used to measure electric potential is called the volt, with symbol V. Because of the name of this unit, electric potential is more commonly known as cell voltage. Another name for it is cell potential. A cell potential can be measured using an electrical device called a voltmeter. A cell potential of 0 V means that the cell has no electric potential, and no electrons will flow. You know that you can generate electricity by connecting a zinc electrode and a copper electrode that have been inserted into a lemon. However, you cannot generate electricity by connecting two copper electrodes that have been inserted into the lemon. The two copper electrodes are the same and are in contact with the same electrolyte. There is no potential difference between the two electrodes. Electric potentials vary from one cell to another, depending on various factors. You will examine some of these factors in the next investigation. The cell voltage is sometimes called the electromotive force, abbreviated emf. However, this term can be misleading. A cell voltage is a potential difference, not a force. The unit of cell voltage, the volt, is not a unit of force. Chapter 11 Cells and Batteries MHR 509

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