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Redox Reactions

Redox Reactions. Chapter 20. Section 20.1. Oxidation And Reduction. Redox Reactions. A chemical rxn in which electrons are transferred from one atom to another is called an oxidation-reduction reaction , or redox rxn .

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Redox Reactions

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  1. Redox Reactions Chapter 20

  2. Section 20.1 Oxidation And Reduction

  3. Redox Reactions • A chemical rxn in which electrons are transferred from one atom to another is called an oxidation-reduction reaction, or redox rxn. • In a redox reaction, the loss of electrons from atoms of a substance is called oxidation. • The gain of electrons is called reduction.

  4. Changes in Oxidation Number • You already learned that the oxidation number of an atom is equal to the number of electrons gained or lost in an ionic compound. • Oxidation increases an atom’s oxidation number; reduction decreases the oxidation number.

  5. Complementary Processes • Oxidation and reduction are complementary processes that always occur together. • The substance that is reduced in a redox reaction is the oxidizing agent. • The substance that is oxidized in a redox rxn is the reducing agent.

  6. Example • Redox rxn are not limited to reactions in which atoms change to ions or vice versa. • For example in the rxn, identify the oxidizing and reducing agent H2 (g) + Cl2 (g)  2HCl (g)

  7. Practice Problems • Identify what is oxidized and what is reduced. Also identify the oxidizing agent and the reducing agent. • Zn + Ni2+ Ni + Zn2+ • 2I- + Br2  I2 + 2Br- • 2NO  N2 + O2 • 2H2 + S2  2H2S

  8. Determining Oxidation Numbers • Use these rules to determine the oxidation numbers in redox reactions. • The oxidation number of an uncombined atom is zero. • The oxidation number of a monatomic ion is equal to the charge on the ion. • The oxidation number of the more electronegative atom in a molecule or complex ion is the same as the charge it would have if it were an ion. • The most oxidation number of oxygen in compounds is always -2, except in peroxides, where it is -1. When bonded to fluorine the oxidation number of oxygen is 2+.

  9. Example • Determine the oxidation number of each element in the following compound and ion. a. SrCO3 b. Cr2O72-

  10. Practice Problems • Determine the oxidation number of the boldface element in each of these compounds. • Li2SiO3 • Al4C3 • CaH2 • BeSeO4 • K2GeF6 • Al(ClO3)3 • Determine the oxidation number of the boldface element in each of these ions. • PO43- • Hg22+ • HSO4- • PtCl62- • PuO2+ • TeO32-

  11. Reactions as RedoxRxn • Not all chemical rxns are redox rxns. • Most double-displacement rxns are not redox rxns, because there is not transfer of electrons between atoms. • Combustion and single-replacement rxns are always redox rxns. • Many synthesis and decomposition rxns are redox rxns as well.

  12. Section 20.3 Half-Reactions

  13. Balancing Redox Reaction • When balancing redox rxns in addition to balancing the # of each atom, the number of electrons gained or lost must also be balanced. • The method used is balancing half-reactions. • A half-reaction shows the oxidation half of the rxn, and the reduction half of the rxn in each half-rxn. • One half-rxn shows the oxidation and the other shows the reduction.

  14. Balancing Half-Reactions • The steps to balance a redox half-reaction are as follows: • Write the net ionic equation for the rxn, omitting spectator ions. • Write the oxidation and reduction half-reactions for the net ionic equation. • Balance the atoms and charges in each half-reaction. • Adjust the coefficients so that the # of electrons lost in the oxidation equals the # of electrons gained in reduction. • Add the balanced half-reactions and return spectator ions.

  15. Example • Use the half-reaction method to balance this equation. K2Cr2O7 (aq) + HCl (aq)  CrCl3 (aq) + KCl (aq) + Cl2 (g)

  16. Practice • Use the half-reaction method to balance. • I2 (s) + H2SO3 (aq)  I- (aq) + HSO4- (aq) • Fe2+ (aq) + MnO4- (aq)  Fe3+ (aq) + Mn2+ (aq) • Zn (s) + Cr2O72- (aq)  Zn2+ (aq) + Cr3+ (aq) • IO3- (aq) + I- (aq)  I2 (s)

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