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Chapter 20: Electrochemistry

Chapter 20: Electrochemistry. Juana Mendenhall, Ph.D. Assistant Professor Lecture 1 March 3. Objectives. Review oxidation-reduction reactions

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Chapter 20: Electrochemistry

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  1. Chapter 20: Electrochemistry Juana Mendenhall, Ph.D. Assistant Professor Lecture 1 March 3 General Chemistry: Chapter 20

  2. Objectives • Review oxidation-reduction reactions • Describe and illustrate how a voltaic cell operates, with the concepts of elelctrodes, salt bridges, half-cell equations, net cell reaction and cell diagram. General Chemistry: Chapter 20

  3. Electrochemistry • Defines the electrochemical processes such as redox (oxidation-reduction) reactions in which the energy released by a spontaneous reaction (DG < 0) is converted to electricity or in which electricity is used to cause a nonspontaneous (DG > 0) chemical reaction to occur. • Electrochemistry provides insight to construction, batteries, electroplating, and corrosion. General Chemistry: Chapter 20

  4. Oxidation-Reduction Reactions • Oxidation: loss of electrons • Reducing agent: donates electrons • Reduction: gain of electrons • Oxidizing agent: accepts electrons • Oxidizing agents are always reduced and reducing agents are always oxidized. • Review oxidation rules and how to calculate OS. • Ex. Zinc and copper sulfate General Chemistry: Chapter 20

  5. Activity Series An easy way to predict whether a metal or hydrogen displacement reaction will actually occur. Metals are arranged according to their ability To displace hydrogen from an acid to water. Li (lithium) is the most reactive metal and Ag (gold) is the least reactive. General Chemistry: Chapter 20

  6. Example • The nickel-cadmium (nicad) battery, a popular rechargeable “dry cell” used in battery-operated tools, uses the following redox reaction to generate electricity: Cd(s) + NiO2(l) Cd(OH)2(s) + Ni(OH)2(s) Identify the substances that are oxidized and reduced and indicated which are oxidizing agents and which are reducing agents General Chemistry: Chapter 20

  7. Balancing Oxidation-Reduction Equations • Oxidation-reduction must take place simultaneously, it’s best to consider them as a separate process Sn2+(aq) + 2Fe3+(aq) Sn4+(aq) + 2Fe2+(aq) Sn2+ (aq) Sn4+(aq) + 2e- 2Fe3+(aq) + 2e- 2Fe2+(aq) Sn2+(aq) + 2Fe3+(aq) Sn4+(aq) + 2Fe2+(aq) General Chemistry: Chapter 20

  8. Cu(s) + 2Ag+(aq) Cu(s) + Zn2+(aq) No reaction Cu2+(aq) + 2 Ag(s) 20-1 Electrode Potentials and Their Measurement General Chemistry: Chapter 20

  9. An Electrochemical Half Cell Anode Cathode General Chemistry: Chapter 20

  10. An Electrochemical Cell General Chemistry: Chapter 20

  11. Terminology • Electromotive force, Ecell. • The cell voltage or cell potential. • Cell diagram. • Shows the components of the cell in a symbolic way. • Anode (where oxidation occurs) on the left. • Cathode (where reduction occurs) on the right. • Boundary between phases shown by |. • Boundary between half cells (usually a salt bridge) shown by ||. General Chemistry: Chapter 20

  12. Terminology Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s) Ecell = 1.103 V General Chemistry: Chapter 20

  13. Terminology • Galvanic cells. • Produce electricity as a result of spontaneous reactions. • Electrolytic cells. • Non-spontaneous chemical change driven by electricity. • Couple, M|Mn+ • A pair of species related by a change in number of e-. General Chemistry: Chapter 20

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