Prentice-Hall © 2007General Chemistry: Chapter 20Slide 1 of 54

Juana Mendenhall, Ph.D.

Assistant Professor

Lecture 1 March 3

Chapter 20: Electrochemistry

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Objectives

1. Review oxidation-reduction reactions

2. Describe and illustrate how a voltaic cell operates, with the concepts of elelctrodes, salt bridges, half-cell equations, net cell reaction and cell diagram.

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Electrochemistry

Defines the electrochemical processes such as redox (oxidation-reduction) reactions in which the energy released by a spontaneous reaction (G < 0) is converted to electricity or in which electricity is used to cause a nonspontaneous (G > 0) chemical reaction to occur.

Electrochemistry provides insight to construction, batteries, electroplating, and corrosion.

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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

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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.

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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

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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)

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20-1 Electrode Potentials and Their Measurement

Cu(s) + 2Ag+(aq)

Cu2+(aq) + 2 Ag(s)

Cu(s) + Zn2+(aq)

No reaction

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An Electrochemical Half Cell

Anode

Cathode

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An Electrochemical Cell

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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 ||.

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Terminology

Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)

Ecell = 1.103 V

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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-.