Lec-29-Chap18-1-bare

Lec-29-Chap18-1-bare - Lec-29-Chap18-1-bare 11/9/2011...

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Lec-29-Chap18-1-bare 11/9/2011 Redox Reactions. Half reaction balance equation method. Voltaic cells. 1 Lecture 29 Electrochemistry and its Applications Chapter 18 1 Redox Reactions Oxidation-reduction (redox) reactions involve the transfer of electrons from an atom, molecule, or ion, to another atom, molecule, or ion. 2 in which Magnesium is oxidized and hydrogen is reduced.         22 Mg s 2HCl aq MgCl aq H g An example is the reaction         00 1 1 2 1 2 2 Mg s 2HCl aq MgCl aq H g In terms of oxidation numbers
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Lec-29-Chap18-1-bare 11/9/2011 Redox Reactions. Half reaction balance equation method. Voltaic cells. 2 Redox Reactions As a reminder of oxidation numbers: Oxidation number ( ON ): The charge that an atom would have if bonding electrons are assigned to the more electronegative atom.         00 1 1 2 1 2 2 Mg s 2HCl aq MgCl aq H g We say that magnesium is oxidized : it went from   Mg: 0 2 lost electrons ON We say that hydrogen is reduced : it went from   H: 1 0 gained electrons  3 Redox Reactions         1 1 2 1 2 2 Mg s MgCl aq H g Magnesium is oxidized magnesium is the reducing agent Oxidation and reduction always occur together. The oxidizing agent is reduced (gains electrons), while the reducing agent is oxidized (loses electrons). 4 Hydrogen is reduced hydrogen is the oxidizing agent
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Lec-29-Chap18-1-bare 11/9/2011 Redox Reactions. Half reaction balance equation method. Voltaic cells. 3 5 Rules for assigning oxidation numbers An atom in its elemental state has ON = 0 ( Na, H 2 , Br 2 , S ). An atom in a monoatomic ion has ON = ionic charge       3 1 Na Al Cl 31 ON       An atom in a polyatomic ion or molecule has usually the same ON it would have if it were a monoatomic ion: 3 1 2 1 23 1 2 H O OH NH    ON (H) can be either +1 or ‒1: 1 1 2 1 1 1 2 1 22 Na H Ca H HCl S H   6 Rules for assigning oxidation numbers Halogens usually have ON = ‒1. The major exception is when Cl, Br, or I, when the halogen is bonded to oxygen 1 2 1 2 1 2 Cl O HOBr   The sum of the ON is zero for a neutral compound, and is equal to the net charge for a polyatomic ion 6 7 3 1 2 2 1 24 44 H SO ClO  Oxygen usually has ON = ‒2. The major exception are the peroxides where ON = ‒1. 1 2 1 1 2 H O H O
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Lec-29-Chap18-1-bare 11/9/2011 Redox Reactions. Half reaction balance equation method. Voltaic cells. 4 Redox Reactions in terms of Half-Reactions We can gain insight on a redox reaction when we view it in terms of the ionic species that participate.
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Lec-29-Chap18-1-bare - Lec-29-Chap18-1-bare 11/9/2011...

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