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Unformatted text preview: Chapter 20 ELECTROCHEMISTRY Electrochemistry - the study of the interchange of chemical and electrical energy Review - oxidation/reduction (Chapter 4) Oxidation-reduction reaction (redox reaction) a chemical reaction in which the oxidation state of one or more substances changes. One or more electrons are transferred. oxidation - loss of e- (increase in oxidation state) reduction - gain of e- (decrease in oxidation state) reducing agent is oxidized (loses e- ) oxidizing agent is reduced (gains e- ) Board discussion of Zn (s) + Cu 2+ (aq) ------> Zn 2+ (aq) + Cu (s) Board discussion of oxidation numbers (See rules page 137) Balancing oxidation-reduction (redox) reactions : 1. Separate the unbalanced equation into half-reactions 2. Balance the elements that are not O or H 3. Balance O by adding H 2 O (l) 4. Balance H by adding H + (aq) 5. Balance the charge by adding electrons to the side with the highest positive charge 6. Equalize the electrons transferred in the two half-reactions 7. Add the half-reactions together 8. Simplify In acid: Example problem 1 1. In base, balance as if it were in acid. 2. Neutralize the acid at the end by doing a “paper titration” adding OH- (aq) to form H 2 O (l) In base: Example problem 2 Electrochemical cell – system consisting of electrodes that dip into an electrolyte and in which a chemical reaction either uses or generates an electric current Voltaic (galvanic) cell – Produces an electric current from a spontaneous chemical reaction. Changes chemical energy to electrical energy. (See Figure 20.5 on page 852.) Electric current is a stream of moving electrons. Electrons always travel in a set J. Hilderbrandt, Chem. 12 1 direction and at a set speed. The energy to make the electrons move will come from the chemical energy supplied by the spontaneous chemical reaction. Description and construction of a voltaic cell (See Figure 20.4 on page 852) 1. Consists of two electrically connected half-cells 2 . Half-cell – cell where a half reaction is taking place 3 . Salt bridge – connects the two half cells to allow for the flow of ions 4 . Anode – electrode where oxidation takes place; has a negative sign 5 . Cathode – electrode where reduction takes place; has a positive sign 6. By convention, oxidation is on the left6....
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This note was uploaded on 03/04/2012 for the course PUBH 2113 taught by Professor Edberg during the Fall '11 term at GWU.
- Fall '11