Lecture8 - Electrochemistry: Chemical Change and Electrical...

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1 CHM151 – University of Toronto Chapter 21 Electrochemistry: Chemical Change and Electrical Work CHM151 – University of Toronto Electrochemistry: Chemical Change and Electrical Work 21.1 Half-Reactions and Electrochemical Cells 21.2 Voltaic Cells: Using Spontaneous Reactions to Generate Electrical Energy 21.4 Free Energy and Electrical Work 21.5 Electrochemical Processes in Batteries 21.6 Corrosion: A Case of Environmental Electrochemistry 21.7 Electrolytic Cells: Using Electrical Energy to Drive a Nonspontaneous Reaction { 21.3 Cell Potential: Output of a Voltaic Cell CHM151 – University of Toronto Review of Previous Lecture --- Basic Lessons 1. Re: Oxidation Numbers Electrons go to strongest “electron magnet” (atoms with largest electronegativities) Keeps track of electrons to help balance equations (mass conserved in chemical reactions including electrons/charge)…basically electron “book keeping” 2) Electrochemical Cells Electrons are mobile --- not fixed…free to move to largest “electron magnet” Ions in solution are mobile (move to compensate charge from electron transfer….complete the circuit) CHM151 – University of Toronto Outline: Basic Lessons • Review of Oxidation Numbers --- electron book keeping • Identifying Redox reactions (cases where electrons are exchange/electron transfer occurs) • If electron transfer occurs….can also occur if reagents hooked up with wires …permit electrons to transfer Î electrochemistry • Energy can be stored as separated reagents…chemical potential energy Î electrode potentials
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2 CHM151 – University of Toronto Recognizing Redox Reactions First assign oxidation numbers --- using O(-2) and H(+1) to start and then consider relative electronegativities of atoms and octet rule Change in oxidation number of an atom electron exchange must occur ….electron transfer process AND A Redox Reaction CHM151 – University of Toronto Examples Assign Oxidation Numbers to: KMnO 4 SiCl 4 S 8 LiAlH 4 (ionic) Ox # Notes: KMnO4….know O and K • SiCl4 ….know Cl • S8 ….elemental ? • LiAlH4 …know electroneg. of Li < H …Li will be +1 CHM151 – University of Toronto Examples Assign Oxidation Numbers to: KMnO 4 SiCl 4 S 8 LiAlH 4 (ionic) +1 -2 -1 0 +1 ?? Charge -7 - 4 0 +1 Need +7 +4 0 To balance charge Notes: KMnO4….know O and K SiCl4 ….know Cl S8 ….elemental ? LiAlH4 …know electroneg. of Li < H …Li will be +1 and Al < H; H can only accept 1 additional electron to complete valence shell (re: He)_ CHM151 – University of Toronto Electronegativities of the elements. Wil iamson and Moore
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3 CHM151 – University of Toronto Examples Assign Oxidation Numbers to: KMnO 4 SiCl 4 S 8 LiAlH 4 (ionic) +1 +7 -2 +4 -1 0 +1 ? -1 Charge -7 -4 0 +1 -4 Need +7 +4 0 +3 To balance charge Notes: LiAlH4 ….Al has the +3 oxidation state (preferred for Al); H as with other metals takes on -1 oxidation number. CHM151 – University of Toronto
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This note was uploaded on 04/18/2011 for the course CHM 151 taught by Professor Dong during the Winter '08 term at University of Toronto- Toronto.

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Lecture8 - Electrochemistry: Chemical Change and Electrical...

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