Chapter_20[1] - Electrochemical cells redox rxns Involve...

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Electrochemistry   1 Electrochemical cells – redox rxns Involve transfer of electrons between reactants Oxidation numbers Keep track of electrons Reduction = when atom’s oxidation number decreases E.g. gains electrons ( Zn + 2 e Zn ) Oxidation = when atom’s oxidation number increases reducing agent oxidizing agent electrons e - e - reduction takes place here oxidation takes place here Oxidant = the oxidizing agent Reductant = the reducing agent
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Electrochemistry   2 Rules for oxidation numbers Atoms in elemental form : ON = 0 E.g. each H atom in H 2 (g) has ON = 0 Monoatomic ions: ON = ion charge Na + has ON = +1 Ca 2+ has ON = +2 O 2- has ON = -2 In ionic compounds: Group IA elements ON = +1 Group IIA elements ON = +2
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Electrochemistry   3 Rules for oxidation numbers Nonmetals usually have negative ON with some exceptions Oxygen: usually has ON = -2 Except when exists in peroxide ion, O 2 2- , then ON = -1 Hydrogen: ON = +1 when bonded to nonmetals ON = -1 when bonded to metals Halogens: ON = -1 Except ON = +1 when combined with oxygen in an oxyanion Fluorine always has ON = -1
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Electrochemistry   4 Rules for oxidation numbers For neutral compounds (no charge) the sum of the oxidation numbers of all atoms must equal 0 EXAMPLES: SO 3 , C 2 H 6 O, H 2 O, H 2 O 2 For polyatomic ions , the sum of the oxidation numbers of all atoms in the ion must equal the ion charge EXAMPLES: (NH 4 ) 2 SO 4 , C 2 H 3 O 2 -
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Electrochemistry   5 Balancing redox reactions Balancing the overall reaction: 1. Write half reactions 1. Balance numbers of atoms in half reactions (conservation of mass) 1. Add H 2 O for needed O atoms 2. Add H + for H atoms 1. NOTE in basic solution add OH - to convert H + to H 2 O 1. Balance charge by adding electrons (conservation of charge) 1. Reduction half reaction: e - are reactants 2. Oxidation half reaction: e - are products 1. Add half reactions together 1. Adjusting coefficients so that all electrons cancel 1. Check to make sure atoms and charge are balanced
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Electrochemistry   6 Voltaic (galvanic) cells Produces electricity from a spontaneous redox reaction Half reactions occur in separate location Cell compartments ( half-cells ) Electrodes (Zn & Cu) provide path for electrons Anode = electrode where oxidation takes place
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