Chapter 17: ELECTROCHEMISTRY

Transcription

1 Chapter 17: ELECTROCHEMISTRY ELECTROCHEMISTRY = study of interchange of energy 1. Generation of from spontaneous 2. Use of to produce 17.1 GALVANIC CELLS OXIDATION = REDUCTION = GALVANIC CELL = a device in which from a spontaneous rxn is changed to that can be used to do work ANODE = CATHODE = CELL POTENTIAL or ELECTROMOTIVE FORCE (E cell ) = the pull of from to ; measured in (V) 1 V = 1 Joule of work per coulomb of charge transferred What purpose does salt bridge/porous disk serve? 1

2 VOLTMETER = device used to measure ( ) in a galvanic cell STANDARD HYDROGEN ELECTRODE = electrode in contact w/ and bathed in gas at 1 atm; assigned a half-reaction potential of All other half-reaction potentials are based on assignment of zero volts to the process 2H + + 2e - H 2 STANDARD REDUCTION POTENTIALS = potentials of half-rxn s are given as processes 2

3 Combining 2 half-rxn s to obtain balanced redox rxn. usually requires 2 manipulations: 1. One of the reduction half-rxn s must be reversed (since redox rxn s must involve a substance being oxidized and a substance being reduced), which means that the sign of the potential for the reversed half-rxn must also be reversed. 2. Since the number of e - lost must equal the number gained, the halfrxn s must be multiplied by integers as necessary to achieve the balanced equation. However, the value of E º is not changed when a half-rxn is multiplied by an integer. Since a standard reduction potential is an intensive property (it does not depend on how many times the rxn occurs), the potential is not multiplied by the integer required to balance the cell rxn 3

4 LINE NOTATION components listed on left, components on right (alphabetical), separated by (represents SALT BRIDGE/POROUS DISK) Ex: Al 3+ (aq) + Mg(s) Al(s) + Mg 2+ (aq) Complete description of a galvanic cell includes 4 items: 1. (always for galvanic cell) and the balanced cell rxn. 2. Direction of (in direction that gives pos. cell potential) 3. Designation of and 4. Nature of each and present in each compartment ( ( ) is used as electrode in absence of conducting solid) 17.3 Cell Potential, Electrical Work, and Free Energy emf = Therefore, So the maximum work in a cell would be expressed as follows: In addition, q is equal to the # of moles of e - times the charge per mole of e -, which is called the : 4

5 Then, the actual work is expressed as follows: And the final units for work will be. We can relate the potential of a galvanic cell to free energy. For any process carried out at constant temp. and press., the change in free energy equals the maximum useful work obtainable from that process: For a galvanic cell, Since we have From now on, we will omit the subscript on E max to give For standard conditions, This equation states that the maximum cell potential is directly related to the free energy difference between the reactants and the products in the cell. This relationship is important because: 1. It provides an experimental means to obtain G. 2. It confirms that a galvanic cell will run in the direction that gives a value for E cell ; a E cell corresponds to a G value, which is the condition for. 5

6 Use Sample Exercise 17.3 on p. 839 to answer #37 on p. 869 Use Sample Exercise 17.3 on p. 840 to answer #45 on p. 869 Use Sample Exercise 17.5 on p. 841 to answer #51 on p. 869 Use Sample Exercise 17.6 on p. 842 to answer #52 on p. 870 THE NERNST EQUATION The dependence of the cell potential on concentration results directly from the dependence of free energy on concentration. Recall from Chapter 16 that the equation where Q is the reaction quotient, was used to clac. The effect of conc. on G. Since G = -nf E and Gº = -nf Eº, the equation becomes Dividing each side of the equation by nf gives The Nernst equation is often given in a form that is valid at 25ºC: Use Sample Exercise 17.7 on p. 844 to answer #55, 57 on p. 870 Use Sample Exercise 17.8 on p. 846 to answer #63, 65 on p

7 17.5 Batteries a. A battery is: a or a of galvanic cells connected in where the potentials of the individual cells add to give the a source of an essential source of LEAD STORAGE BATTERY Played a major role in the rise of the Can function for several years under Typical lead battery has connected in series Anode rxn: Cathode rxn: Cell rxn: _ is consumed as battery discharges, lowering of electrolyte soln (this is how battery condition is monitored) Batteries by forcing current through in direction to (alternators continually do this) OTHER BATTERIES The common dry cell battery used in portable radios, watches, calculators, etc. Acid version: Anode rxn: Cathode rxn: Alkaline version: Anode rxn: Cathode rxn: 7

8 *Which type of dry cell battery do you think lasts longer and why? Nickel-cadmium version: Anode rxn: Cathode rxn: *Like lead storage battery, products adhere to electrodes, so nickel-cadmium battery can be recharged an indefinite number of times by forcing current through in opposite direction and reversing the rxn FUEL CELLS Fuel cell = Fuel Cell Animation 17.6 Corrosion Corrosion = involves of the metal often causes metals to lose their and attractiveness Explain why metals oxidize so easily in terms of the following: 1. oxygen 2. standard reduction potentials 3. Eº value 8

9 What is preventing the Statue of Liberty from corroding completely? Corrosion of iron (steel) requires an instead of direct oxidation Prevention of corrosion (chromium and tin usually used w/ steel) 3. = zinc is used to coat steel and forms a mixed oxide-carbonate coating; the Zn undergoes oxidation instead of the Fe ( sacrificial coating) 4. = using a mixture of metals to promote better oxide coating which prevents further corrosion 5. = an active metal (such as Mg) is connected by a wire to the buried pipeline or tank to be protected Mg is better reducing agent than Fe, so the Mg furnishes the electrons, keeping the iron from being oxidized (pictured on next page) 9

10 17.7 Electrolysis ELECTROLYSIS = use of to force electrons to flow through a cell to produce the nonspontaneous rxn: Note the opposite directions of e - flow and reversed in electrolytic cell RXN is also reversed What happens to ion flow through porous disk? 10

11 Stoichiometry of electrolytic process how much chem. change occurs with the flow of a given current for a specified time: We need the following steps: (This means that mol e - flowed into Cu 2+ soln) Use Sample Exercise 17.9 on p. 856 to answer #73, 75 on p. 871 Electrolysis of Water H 2 and O 2 combine spontaneously to form water decrease in G can be used to run a fuel cell to produce electricity Reverse process (nonspontaneous) can be forced by electrolysis: Anode rxn: Cathode rxn: Net rxn: _ or 11

12 Electrolysis of Mixtures of Ions Consider a soln in an electrolytic cell containing Cu 2+, Ag +, and Zn 2+. If the voltage is initially very low and gradually turned up, in which order will the metals be plated out onto the cathode? (Hint: Look at the Eº for each of the ions) The more + the Eº value, the more the rxn has a tendency to proceed in the direction indicated: Therefore, will plate out first, followed by, and finally Use Sample Exercise on p. 858 to answer #85 on p. 872 Read Section 17.8 Commercial Electrolytic Processes 12