lecnotes25 - MIT OpenCourseWare http/ocw.mit.edu 5.111...

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MIT OpenCourseWare http://ocw.mit.edu 5.111 Principles of Chemical Science Fall 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .
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25.1 5.111 Lecture Summary #25 Oxidation/Reduction Lecture 2 Topics:Electrochemical Cells, Faraday's Law, and the Relationship between Gibbs Free Energy and Cell Potential Chapter 12 OXIDATION/REDUCTION REACTIONS Guidelines for assigning oxidation numbers 1) In free elements, each atom has an oxidation number of zero. Example H 2 2) For ions composed of only one atom the oxidation number is equal to the charge on the ion. Thus Li +1 has an oxidation number of +1. Group 1 and group 2 metals have oxidation numbers of +1 and +2, respectively. Aluminum has an oxidation number of +3 in all its compounds. 3) The oxidation number of oxygen in most compounds is -2. However, in peroxides such as H 2 O 2 and O 2 -2 , oxygen has an oxidation state of -1. 4) The oxidation number of hydrogen is +1, except when it is bonded to metals in binary compounds, such as LiH, NaH, CaH 2 . In these cases, its oxidation number is -1. 5) F has an oxidation number of -1 in all its compounds. Other halogens (Cl, Br, and I) have negative oxidation numbers when they occur as halide ions in compounds (Ex. NaCl). However, when combined with oxygen (oxoacids), they have positive oxidation numbers (Ex. ClO - ). 6) In a neutral molecule, the sum of the oxidation numbers of all the atoms must be zero. In a polyatomic ion, the sum of oxidation numbers of all the elements in the ion must be equal to the net charge of the ion. For example NH 4 + H is +1 N is _-3 Sum is +1 7) Oxidation numbers do not have to be integers. For example, the oxidation number of oxygen in superoxide O 2 -1 is _ - ½ ___ Examples Li 2 O PCl 5 HNO 3 N 2 O Definitions Oxidation - Reduction - Oxidizing agent - Reducing agent –
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25.2 Disproportionation Reaction A reactant element in one oxidation state is both oxidized and reduced. NaClO NaClO 3 + NaCl in basic solution Write the half reactions and determine the changes in oxidation state. Na + is a spectator ion so: - ClO - ClO 3 ClO - Cl - Balancing Redox Reactions (Ch12.2) A. BALANCE IN ACIDIC SOLUTION Fe 2+ + Cr 2 O 7 2- Cr 3+ + Fe 3+ (1) Write two unbalanced half reactions for oxidized and reduced species. Cr
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lecnotes25 - MIT OpenCourseWare http/ocw.mit.edu 5.111...

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