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Chapter+21 - Chapter 21 Electrochemistry Chemical Change...

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Click to edit Master subtitle style 6/6/11 Dr. Shamindri M. Arachchige [email protected] Davidson 11 1 Office Hours: Location: Davidson 11 Monday 1:45-5:00 PM Wednesday 1:45-4:15 PM Other hours by appointment 11 Chapter 21 Electrochemistry: Chemical Change and Electrical Work
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6/6/11 Chapter 21 Electrochemistry: Chemical Change and Electrical Work In this chapter, we will look at electron transfer reactions See Chapter 4 pages 158-164 for introduction to electron transfer reactions, oxidation-reduction reactions or redox reactions. Example: Cu(s) + 2Ag + (aq) Cu 2+ (aq) + 2Ag(s) Write as two half-reactions : chemical equations describing the individual oxidation and reduction processes 22
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6/6/11 Chapter 21 Electrochemistry: Chemical Change and Electrical Work 33 Cu(s) + 2Ag + (aq) Cu 2+ (aq) + 2Ag(s) Reduction Oxidation Ag + (aq) + e Ag(s) Cu(s) Cu 2+ (aq) + 2e Gain of electrons (write electron/s on the reactant side) Loss of electrons (write electron/s on the product side) Oxidation number decreases, becomes less positive Oxidation number increases, becomes more positive Oxidizing agent: The reactant that is reduced (undergoes reduction). The reactant that can accept electrons from another atom, so that atom can be oxidized. Reducing agent: The reactant that is oxidized (undergoes oxidation). The reactant that gives electrons to another atom, so that atom can be reduced.
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6/6/11 Chapter 21 Electrochemistry: Chemical Change and Electrical Work Oxidation Numbers To keep track of electrons lost and gained, oxidation numbers are assigned to every atom in the reaction. Rules for Assigning Oxidation Numbers The oxidation number of an atom in the free (uncombined) element is 0 H 2 , O 2 , Cl 2 , Mg(s), P 4 In their compounds, Group 1 metals always have an oxidation number of +1. In their compounds, Group 2 metals always have an oxidation number of +2 In its compounds, fluorine is always -1 In its compounds, hydrogen is usually +1 (H is –1 in some compounds with metals) In its compounds, oxygen is usually –2 (can also be –1 in O 2 2- or –0.5 in O 2 - ) In their binary (two-element) compounds with metals, Group 15 is –3, Group 16 is –2 and Group 17 is –1. The total of the oxidation numbers of all the atoms in a molecule is 0. For an ion this total is equal to the charge on the ion. 44
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6/6/11 Chapter 21 Electrochemistry: Chemical Change and Electrical Work KMnO 4 C 2 O 4 2 55
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6/6/11 Chapter 21 Electrochemistry: Chemical Change and Electrical Work Steps to Balancing Redox Reactions Oxidation and reduction half reactions combine so the (1) atoms balance, (2) charge balances, and (3) electrons cancel. 1. Assign all oxidation numbers and determine what elements are being oxidized and reduced. 1. Write separate half reactions for oxidation and reduction.
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