Lecture9 - Electrochemistry: Chemical Change and Electrical...

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1 Chapter 21 Electrochemistry: Chemical Change and Electrical Work Electrochemistry: Chemical Change and Electrical Work 21.1 Half-Reactions and Electrochemical Cells 21.2 Voltaic Cells: Using Spontaneous Reactions to Generate Electrical Energy 21.3 Cell Potential: Output of a Voltaic Cell 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 { Basic Lessons/Concepts • Oxidation Numbers Æ electron book keeping • Identifying Redox reactions (cases where electrons are exchange/electron transfer occurs) Æ Changes in oxidation numbers • Electrochemistry = Redox chemistry at electrodes • The ability to separate chemical reagents and control reactions by virtue of electron conduction through wires = ability to store and control release of chemical potential energy (batteries) • Electrons flow in direction of spontaneous reaction (release stored chemical potential energy)…driving force Æ cell potential • Reduction potentials tabulated relative to a standard reference (hydrogen electrode) Review of Previous Lecture --- Basic Lessons 1. Balancing Redox (reduction/oxidation) reactions Balance atoms in half reactions (separate into oxidation process and reduction process) For aqueous reactions…balance O with H2O, and H atoms by using H+ (acid or neutral) and OH- (basic) Balance charge with # of electrons (equal change in ox.#) Î key distinction from previous balancing of equations….need to know the # of electrons to determine the available work 2. Electrochemical Cells Separate into half reactions of redox process Half reactions connected by virtue of wires….electrons free to move in direction of spontaneous reaction (half reaction with largest reduction potential) Ions in solution are mobile (move to compensate charge from electron transfer….complete the circuit) Æ salt bridge/physical separation but permits ion diffusion Electrons move to lower chemical potential….potential drop = cell voltage Determine reduction half reaction potentials relative to standard hydrogen electrode (SHE) reference Æ powerful tool for predicting whether a reaction will occur or not and maximum work that can be obtained from a reaction
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2 Spontaneity of Electrochemical Reactions Cu 2+ + Zn(s) Æ Cu(s) + Zn 2+ Δ G 0 rxn = -213 J/mole Reaction is spontaneous….electrons are free to flow through wire to connect the “chemical circuit”
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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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Lecture9 - Electrochemistry: Chemical Change and Electrical...

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