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CHEMISTRYNinth

Edition GENERAL

Principles and Modern Applications

Petrucci • Harwood • Herring • Madura

Chapter 20: Electrochemistry

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Contents

20-1 Electrode Potentials and Their Measurement

20-2 Standard Electrode Potentials

20-3 Ecell, ΔG, and Keq

20-4 Ecell as a Function of Concentration

20-5 Batteries: Producing Electricity Through Chemical Reactions

20-7 Electrolysis: Causing Non-spontaneous Reactions to Occur

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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-.

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20-2 Standard Electrode Potentials

Cell voltages, the potential differences between electrodes, are among the most precise scientific measurements.

The potential of an individual electrode is difficult to establish.

Arbitrary zero is chosen.

The Standard Hydrogen Electrode (SHE)

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Standard Hydrogen Electrode2 H+(a = 1) + 2 e- H2(g, 1 bar) E° = 0 V

Pt|H2(g, 1 bar)|H+(a = 1)

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Standard Electrode Potential, E°

E° defined by international agreement. The tendency for a reduction process to occur at

an electrode. All ionic species present at a=1 (approximately 1 M). All gases are at 1 bar (approximately 1 atm). Where no metallic substance is indicated, the potential

is established on an inert metallic electrode (ex. Pt).

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Reduction Couples

Cu2+(1M) + 2 e- → Cu(s) E°Cu2+/Cu = ?

Pt|H2(g, 1 bar)|H+(a = 1) || Cu2+(1 M)|Cu(s) E°cell = 0.340 V

Standard cell potential: the potential difference of a cell formed from two standard electrodes.

E°cell = E°cathode - E°anode

cathodeanode

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Standard Cell Potential

Pt|H2(g, 1 bar)|H+(a = 1) || Cu2+(1 M)|Cu(s) E°cell = 0.340 V

E°cell = E°cathode - E°anode

E°cell = E°Cu2+/Cu - E°H+/H2

0.340 V = E°Cu2+/Cu - 0 V

E°Cu2+/Cu = +0.340 V

H2(g, 1 atm) + Cu2+(1 M) → H+(1 M) + Cu(s) E°cell = 0.340 V

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Measuring Standard Reduction Potential

anodeanode cathode cathode

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Standard Reduction Potentials

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