Studio_5_Redox_Fall_ 2006_FINAL

Studio_5_Redox_Fall_ 2006_FINAL - Chem. 25: Studio #5...

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A Survey of Oxidation-Reduction Reactions NAME:___________________________ STUDIO:___________ A Survey of Oxidation-Reduction Reactions Oxidation-reduction (aka. redox) reactions are special. With all other types of reactions, we simply have to keep track of atoms and obey the law of conservation of mass. In redox reactions, electrons are transferred from one species to another, so in addition to balancing atoms, we need to keep track of subatomic particles (electrons) when we deal with this type of process. Redox reactions are worth the trouble. They are of profound importance because they are ubiquitous. Life itself is controlled redox, whether we’re talking about metabolism of food by living organisms or photosynthesis by plants, and all sorts of natural processes in the inanimate part of the world, from corrosion and rusting to the electrochemistry of batteries, fires and explosions, are also oxidation and reduction. The first thing you need to appreciate is that redox is a two-way street. Because it involves the transfer of electrons , two species are required: one that gives up the electrons and a second that accepts them. Giving up electrons is synonymous with being oxidized, hence oxidation ; receiving them is being reduced, hence reduction . We begin to keep track of all this by recognizing substances that tend to give up or accept electrons readily – they’re primed to take part in redox anyway – and by looking for changes in oxidation numbers of species during chemical reactions. Let’s first take a quick look some substances that are frequently involved in redox reactions. The name oxidation itself originally was derived from oxygen, and redox reactions were first defined as those reactions that involved an exchange of oxygen. For example – the reaction of iron ore with carbon monoxide to make metallic iron: Fe 2 O 3 (s) + 3 CO (g) ===> 2 Fe (s) + 3 CO 2 (g) Just look at what happens to the species involved: Fe 2 O 3 loses oxygen in being converted to Fe, while the CO gains oxygen and is converted into CO 2 . The jargon derives from this simple example – the Fe 2 O 3 is reduced in oxygen content, and the CO is oxidized (gains oxygen). Redox reactions have been used for millennia, and the language was developed long before electrons, or even atoms, were defined. Once they realized what goes on here at a more fundamental level, that electrons are being exchanged, chemists acknowledged that oxygen wasn’t the only thing that could participate in a reaction like this, so a more generic method has been developed to describe all this that doesn’t depend on the involvement of oxygen. If we assign a charge (a valence, from now on called an oxidation number ) to iron in Fe 2 O 3 , we’d say that oxygen is -2 (because that’s the charge associated with the oxide ion), so each iron must be +3 for electrical neutrality to be preserved. Iron is in the +3 oxidation state on the reactant side. On the product side, it’s elemental iron, so it has no charge. Elemental
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This note was uploaded on 02/27/2008 for the course CHEM 025 taught by Professor X during the Fall '06 term at Lehigh University .

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Studio_5_Redox_Fall_ 2006_FINAL - Chem. 25: Studio #5...

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