C1210 lecture 11

C1210 lecture 11 - Oxidation-Reduction Reactions: Among the...

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Oxidation-Reduction Reactions: Among the first reactions studied were the combination of oxygen with metals to form oxides and the reverse reaction to form metals from oxides, called reduction. All oxidation-reduction reactions “ redox reactions ” involve transfer of electrons from one substance to another. We generalize the terms: Definition: oxidation: the loss of electrons. Definition: reduction: the gain of electrons. Na Na + + e - (oxidation) (1) “Sodium is oxidized.” Cl 2 + 2 e - 2Cl - (reduction) (2) “Chlorine is reduced.” Oxidation and reduction always occur together (in solution). To do otherwise would create or destroy electrons and would violate conservation of mass. Corollary: in a redox reaction, the charge on both sides of the reaction is conserved; (1) + (2) 2Na + Cl 2 2Na + +2Cl - 0 + 0 = 2 × (+1) + 2 × (-1) = 0
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More confusing terminology: Definition : A substance that accepts the electrons (and is reduced) is called an oxidizing agent because it caused something else to be oxidized. Definition : A substance that supplies the electron (and is oxidized) is called a reducing agent because it causes something else to be reduced. The reducing agent is the substance that is oxidized. The oxidizing agent is the substance that is reduced. “Redox” reactions are common and fundamental: They are the prototype for simple reactions: atoms or molecules +/- an electron. Electrochemistry involves oxidation-reduction reactions (batteries, fuel cells) The action of household bleach NaOCl( aq ) is an example of a redox reaction.
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Metal + nonmetal = ionic compound Periodic table tells us that the ions are Mg 2+ and O 2- . We write the oxidation and the reduction separately as half-reactions. The “half-reactions” are: Mg Mg 2+ + 2 e - oxidation O 2 + 4 e - 2O 2- reduction Mg loses two electrons, therefore it is oxidized and is the reducing agent. Each O gains two electrons, therefore they are reduced and are oxidizing agents. 2 × eq . (1) + eq . (2) Check balance. Overall reaction: 2Mg + O 2 + 4 e - 2MgO + 4 e - Check oxidation/reduction. Example: Oxidation of magnesium metal, 2Mg + O 2 2MgO Which element is oxidized? Which element is reduced? Which element is the reducing agent? Which element is the oxidizing agent?
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What if we don’t have ionic compounds? S + O 2 SO 2 everything is molecular. We use a bookkeeping system called “oxidation numbers” to pretend that molecular reactions are oxidation-reduction reactions. For a simple monatomic ion, the oxidation number is just the charge on the ion. Do not confuse the charge on the ion with the oxidation number. We write the charge: Na 1+ . We write the oxidation number: Na +1 . We also use the term oxidation state to indicate the oxidation number.
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This note was uploaded on 12/10/2009 for the course CHE 1220 taught by Professor Jespersen during the Spring '09 term at St. John's.

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C1210 lecture 11 - Oxidation-Reduction Reactions: Among the...

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