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Oxidation

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Oxidation-reduction reactions (or redox) reactions, are a type of reaction in aqueous solutions that involves a transfer of electrons between two species. An oxidation-reduction reaction is any chemical reaction in which the oxidation number of a molecule, atom, or ion changes by gaining or losing an e-. They occur every day and are vital to some of the basic functions of life. Some examples include photosynthesis (within plants), respiration (within animals), combustion, and corrosion or rusting. To understand these types of reactions, you must first understand oxidation numbers or states.
Rules for Assigning Oxidation States
The oxidation number corresponds to the number of electrons, e-, that an atom loses, gains, or appears to use when joining with other atoms in compounds. When determining the Oxidation State of an atom there are seven guidelines to follow: 1. The Oxidation State of an individual atom is 0. 2. The total Oxidation State of all atoms in: a neutral species is 0 and in an ion is equal to the ion charge. 3. Group 1 metals have an Oxidation State of +1 and group 2 an Oxidation State of +2 4. The Oxidation State of fluorine is -1, when in compounds 5. Hydrogen generally has an Oxidation State of +1 in compounds 6. Oxygen generally has an Oxidation State of -2 in compounds 7. In binary metal compounds, group 17 elements have an Oxidation State of -1, group 16 of -2, and group 15 of -3.
(Note: The sum of the oxidation states is equal to zero for neutral compounds and equal to the charge for polyatomic ion species.)
Sample Problems: Determine the oxidation states: 1. Fe(s) + O2(g) → Fe2O3(g) 2. Fe2+ 3. Ag(s) + H2S → Ag2S(g) + H2(g)
Solutions
1. Fe and O2 are free elements, therefore they have an O.S. of "0" according to Rule #1. The product has a total O.S. equal to "0" and following Rule #6, O3 has an O.S.

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