CHAPT20B F10 - Chapter 20 ELECTROCHEMISTRY Electrochemistry...

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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
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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 left
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