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4.9 Oxidation-Reduction Reactions

4.9 Oxidation-Reduction Reactions. Redox reactions- reactions in which one or more electrons is transferred. 4.9 Oxidation-Reduction Reactions. Oxidation and Reduction When a metal undergoes corrosion it loses electrons to form cations: Ca(s) +2H + (aq)  Ca 2 + (aq) + H 2 (g)

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4.9 Oxidation-Reduction Reactions

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  1. 4.9 Oxidation-Reduction Reactions • Redox reactions- reactions in which one or more electrons is transferred

  2. 4.9 Oxidation-Reduction Reactions Oxidation and Reduction • When a metal undergoes corrosion it loses electrons to form cations: Ca(s) +2H+(aq)  Ca2+(aq) + H2(g) • Oxidized: atom, molecule, or ion becomes more positively charged. • Oxidation is the loss of electrons. • Reduced: atom, molecule, or ion becomes less positively charged. • Reduction is the gain of electrons.

  3. Oxidation Numbers • Vanadium compounds having different oxidation numbers or oxidation states

  4. Oxidation Numbers • How do maximum and minimum values of the oxidation number correlate with the position in the periodic table?

  5. Common Oxidation Numbers

  6. Rules for Assigning Oxidation Numbers • The oxidation number of an atom in an element is 0. Ex: Na (s), O2 (g), Hg (l) • The oxidation state of a monatomic ion is the same as its charge Ex. Na + = +1 Cl - = -1 • Fluorine has an oxidation number of -1 in its compounds

  7. Rules for Assigning Oxidation Numbers • Oxygen usually has an oxidation number of -2 in its compounds. There are some exceptions: a. Oxygen has an oxidation number of -1 in peroxides, which contain the O22- ion. b. Oxygen has an oxidation number of -1/2 in superoxides, which contain the O2- ion. • Hydrogen has an oxidation number of +1 unless it is combined with metals, in which case it has an oxidation number of -1. • The sum of the oxidation numbers of all atoms in a substance must equal the total charge on the species: 0 for molecules; the ionic charge for ions.

  8. A guide to the Rules The position of the element in the periodic table may be useful:a. Group IA (1) elements have oxidation numbers of +1 in their compounds.b. Group IIA (2) elements have oxidation numbers of +2 in their compounds.c. Group VIIA (17) elements have oxidation numbers of -1 unless combined with oxygen or a halogen closer to the top of the group.d. In binary compounds, Group VIA (16) elements have oxidation numbers of -2, unless combined with oxygen or halogens.

  9. A guide to the Rules General Summary Element: 0 Fluorine: -1 Oxygen: -2 Hydrogen: +1

  10. Example • H2SO4 • H is +1 • S is -2 unless combined with oxygen or a halogen, so leave this for last • O is -2 • Use summation rule for S: 2(+1) + 1(S) + 4(-2) = 0 • S = 0 - 2 + 8 = +6

  11. Assignment of Oxidation Numbers • Determine values of the oxidation number of each element in these compounds or ions: H2O SO2 CCl4 H2O2 NO3- MnO4- CO2 NaNO3 KClO4

  12. Assignment of Oxidation Numbers • Determine values of the oxidation number of each element in these compounds or ions: H2O O(-2) H(+1) SO2 CCl4 H2O2 NO3- MnO4- CO2 NaNO3 KClO4

  13. Assignment of Oxidation Numbers • Determine values of the oxidation number of each element in these compounds or ions: H2O SO2 CCl4Cl (-1) C (+4) H2O2 NO3- MnO4- CO2 NaNO3 KClO4

  14. Assignment of Oxidation Numbers • Determine values of the oxidation number of each element in these compounds or ions: H2O SO2 CCl4 H2O2 NO3-O(-2) N(+5) MnO4- CO2 NaNO3 KClO4

  15. Assignment of Oxidation Numbers • Determine values of the oxidation number of each element in these compounds or ions: H2O SO2 CCl4 H2O2 NO3- MnO4- CO2O(-2) C(+4) NaNO3 KClO4

  16. Assignment of Oxidation Numbers • Determine values of the oxidation number of each element in these compounds or ions: H2O SO2 CCl4 H2O2 NO3- MnO4- CO2 NaNO3 KClO4 O (-2) K (+1) Cl (+7)

  17. Assignment of Oxidation Numbers • Determine values of the oxidation number of each element in these compounds or ions: H2O SO2O(-2) S(+4) CCl4 H2O2 NO3- MnO4- CO2 NaNO3 KClO4

  18. Assignment of Oxidation Numbers • Determine values of the oxidation number of each element in these compounds or ions: H2O SO2 CCl4 H2O2 H(+1) O(-1) NO3- MnO4- CO2 NaNO3 KClO4

  19. Assignment of Oxidation Numbers • Determine values of the oxidation number of each element in these compounds or ions: H2O SO2 CCl4 H2O2 NO3- MnO4-O(-2) Mn(+7) CO2 NaNO3 KClO4

  20. Assignment of Oxidation Numbers • Determine values of the oxidation number of each element in these compounds or ions: H2O SO2 CCl4 H2O2 NO3- MnO4- CO2 NaNO3O(-2) Na(+1) N(+5) KClO4

  21. 4.9 Oxidation-Reduction Reactions • Oxidation- increase in oxidation state • Loss of electrons • Reduction- decrease in oxidation state • Gain of electrons • LEO goes GER

  22. 4.9 Oxidation-Reduction Reactions 2Na(s) + Cl2(g)  2NaCl(s) • What substances are being oxidized and reduced?

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