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Oxidation-Reduction Reactions

Oxidation-reduction reactions involve the transfer of electrons between reactants, crucial in chemical processes. Oxidation refers to the loss of electrons (often seen in metals), leading to an increase in oxidation number and a positive charge. Conversely, reduction signifies the gain of electrons (common in nonmetals), resulting in a decrease in oxidation number and a negative charge. Key rules help determine oxidation numbers (e.g., each uncombined atom has an oxidation number of zero, and for monoatomic ions, it equals their charge). Engaging examples clarify these principles.

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Oxidation-Reduction Reactions

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  1. Oxidation-Reduction Reactions

  2. What are they? • Reactions with a loss and a gain of electrons among the reactants • Oxidation • Loss of electrons • Most metals are oxidized • Reduction • Gain of electrons • Most nonmetals are reduced

  3. Oxidizing and Reducing Agents • What is oxidized acts as a reducing agent • With the loss of electrons, the oxidation number increases • This gives a positive charge • What is reduced acts as an oxidizing agent • With the gaining of electrons, the oxidation number decreases • This results in a negative charge

  4. Electronegativity Review • Defined as: an element’s tendency to gain electrons

  5. Rules for Determining Oxidation Numbers • Rule 1: The oxidation number of an uncombined atom is zero • Examples: Ca, S, H2, N2, O2 all would have oxidation numbers of zero (n = 0)

  6. Rules for Determining Oxidation Numbers • Rule 2: The oxidation number of a monoatomicion is equal to the charge of the ion • Examples: • Calcium ion, Ca+2 has an oxidation number, n = +2 • Bromide ion, Br-1 has an oxidation number, n = -1

  7. Rules for Determining Oxidation Numbers • Rule 3: The oxidation number of the more electronegative atom in a molecule is the same as the charge it would have if it were an ion • Examples: • N in NH3 has a charge of -3 • O in NO2 has a charge of -2

  8. Rules for determining oxidation numbers • Rule 4: The oxidation numbers of group 1 and 2 metals and aluminum are positive and equal to their number of valence electrons • Examples: • Na has an oxidation number of +1 • Mg has an oxidation number of +2 • Al has an oxidation number of +3

  9. Rules for determining oxidation numbers • Rule 5: The oxidation number of oxygen is always -2 except in peroxides, such as hydrogen peroxide (H2O2), where it is -1

  10. Rules for determining oxidation numbers • Rule 6: The sum of the oxidation numbers in a neutral compound is always zero • Example: • CaBr2 (+2) + 2 (-1) = 0

  11. Rules for determining oxidation numbers • Rule 7: The sum of the oxidation numbers of the atoms in a polyatomic ion or compound with a charge is equal to the charge • Example: • NO3- = (+5) + 3(-2) = -1

  12. Sample Problems • Determine the oxidation number for each of the elements in the following: • KClO3 • K: n = • Cl: n = • O: n = • SO42- • S: n = • O: n =

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