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

2. 2 4.9 Oxidation-Reduction Reactions Oxidation and Reduction lWhen a metal undergoes corrosion it loses electrons to form cations: Ca(s) +2H + (aq)  Ca 2 + (aq) + H 2 (g) lOxidized: atom, molecule, or ion becomes more positively charged. lOxidation is the loss of electrons. lReduced: atom, molecule, or ion becomes less positively charged. lReduction is the gain of electrons.

3. 3 lVanadium compounds having different oxidation numbers or oxidation states Oxidation Numbers

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

5. 5 Common Oxidation Numbers

6. 6 Rules for Assigning Oxidation Numbers ŒThe oxidation number of an atom in an element is 0. Ex: Na (s), O 2 (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. 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 O 2 2 - ion. b. Oxygen has an oxidation number of - 1/2 in superoxides, which contain the O 2 - ion. superoxides, which contain the O 2 - 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. 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 + 1 in their compounds. b. Group IIA (2) elements have oxidation numbers of + 2 in their compounds. c. Group VIIA (17) elements have oxidation of + 2 in their compounds. c. Group VIIA (17) elements have oxidation numbers of - 1 unless combined with oxygen or 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 a halogen closer to the top of the group. d. In binary compounds, Group VIA (16) elements have oxidation numbers of - 2, unless combined have oxidation numbers of - 2, unless combined with oxygen or halogens. with oxygen or halogens.

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

10. 10 Example lH 2 SO 4 lH is +1 lS is -2 unless combined with oxygen or a halogen, so leave this for last lO is -2 lUse summation rule for S: 2(+1) + 1(S) + 4(-2) = 0 lS = 0 - 2 + 8 = +6

11. 11 lDetermine values of the oxidation number of each element in these compounds or ions: H 2 O SO 2 CCl 4 H 2 O 2 NO 3 - MnO 4 - CO 2 NaNO 3 KClO 4 Assignment of Oxidation Numbers

12. 12 lDetermine values of the oxidation number of each element in these compounds or ions: H 2 O O(-2) H(+1)SO 2 CCl 4 H 2 O 2 NO 3 - MnO 4 - CO 2 NaNO 3 KClO 4 Assignment of Oxidation Numbers

13. 13 lDetermine values of the oxidation number of each element in these compounds or ions: H 2 O SO 2 CCl 4 Cl (-1) C (+4)H 2 O 2 NO 3 - MnO 4 - CO 2 NaNO 3 KClO 4 Assignment of Oxidation Numbers

14. 14 lDetermine values of the oxidation number of each element in these compounds or ions: H 2 O SO 2 CCl 4 H 2 O 2 NO 3 - O(-2) N(+5)MnO 4 - CO 2 NaNO 3 KClO 4 Assignment of Oxidation Numbers

15. 15 lDetermine values of the oxidation number of each element in these compounds or ions: H 2 O SO 2 CCl 4 H 2 O 2 NO 3 - MnO 4 - CO 2 O(-2) C(+4)NaNO 3 KClO 4 Assignment of Oxidation Numbers

16. 16 lDetermine values of the oxidation number of each element in these compounds or ions: H 2 O SO 2 CCl 4 H 2 O 2 NO 3 - MnO 4 - CO 2 NaNO 3 KClO 4 O (-2) K (+1) Cl (+7) Assignment of Oxidation Numbers

17. 17 lDetermine values of the oxidation number of each element in these compounds or ions: H 2 O SO 2 O(-2) S(+4) CCl 4 H 2 O 2 NO 3 - MnO 4 - CO 2 NaNO 3 KClO 4 Assignment of Oxidation Numbers

18. 18 lDetermine values of the oxidation number of each element in these compounds or ions: H 2 O SO 2 CCl 4 H 2 O 2 H(+1) O(-1) NO 3 - MnO 4 - CO 2 NaNO 3 KClO 4 Assignment of Oxidation Numbers

19. 19 lDetermine values of the oxidation number of each element in these compounds or ions: H 2 O SO 2 CCl 4 H 2 O 2 NO 3 - MnO 4 - O(-2) Mn(+7) CO 2 NaNO 3 KClO 4 Assignment of Oxidation Numbers

20. 20 lDetermine values of the oxidation number of each element in these compounds or ions: H 2 O SO 2 CCl 4 H 2 O 2 NO 3 - MnO 4 - CO 2 NaNO 3 O(-2) Na(+1) N(+5) KClO 4 Assignment of Oxidation Numbers

21. 21 4.9 Oxidation-Reduction Reactions lOxidation- increase in oxidation state lLoss of electrons lReduction- decrease in oxidation state lGain of electrons lLEO goes GER

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