2. Objectives Assign oxidation numbers to reactant and product species. Define oxidation and reduction. Explain what an oxidation-reduction reaction (redox reaction) is.

3. 19.1 Oxidation & Reduction Also called REDOX reactions Always involve a TRANSFER of ELECTRONS

4. Redox Reactions: 2 half-reactions occur at the same time 2Na(s) + Cl 2 (g) → 2NaCl(s) LEO – loss of electrons is oxidation Alkali metals: Na → Na + + 1e - electron(s) are product Sodium is oxidized

5. GER – gain of electrons is reduction Halogens: Cl + 1e - → Cl - electron(s) are reactant Chlorine is reduced Remember: “LEO the Lion says GER!”

6. A A B B A A B B Oxidation: A loses 2 electrons Reduction: B gains 2 electrons Reducing agent Oxidizing agent A is oxidized B is reduced

7. Reducing & Oxidizing Agents Reducing agent – causes another to be reduced by donating electrons is oxidized (loses electrons) Oxidizing agent – causes another to be oxidized by accepting electrons is reduced (gains electrons)

8. Oxidation of Zinc Zn loses electrons. Is zinc oxidized or reduced? Is it the oxididizing or reducing agent? Zinc is oxidized. It is a reducing agent Half-reaction: Zn → Zn 2+ + 2e- Copper gains electrons. Is copper oxidized or reduced? Is it the oxididizing or reducing agent? Copper is reduced. It is an oxidizing agent. Half-reaction: Cu 2+ + 2e- → Cu

10. Oxidation States Redox reactions can be identified and understood by assigning oxidation numbers (aka oxidation states) to reactants and products Oxidation numbers/states = artificial “bookkeeping” devices used to keep track of overall electron distribution not physical characteristics of atoms hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic.

11. Transition metals often have more than one oxidation state Ex. Oxidation States of MnOxidation States of Mn In Potassium Permanganate, Mn is in oxidation state +7. This gives a purple salt. In alkaline conditions, and in the presence of a sugar, Mn is slowly reduced to its +6 and +4 oxidation state. (+6 being green, and +4 being yellow)

12. Rules for Assigning Oxidation Numbers 1.Uncombined elements = 0 2.Monoatomic ions = charge 3.More electronegative element in binary compound is assigned number equal to its charge if it were an ion 4.Fluorine in compound is always -1 5.Oxygen is -2 unless combined with F when its +1 or +2, or in a peroxide when its -1

14. 6.Hydrogen is usually +1 unless combined with a metal when it is -1 7.Group 1 & 2 and Aluminum have oxidation numbers of +1, +2, and +3 respectively 8.The sum of the oxidation numbers of all atoms in a neutral compound is zero. 9.The sum of the oxidation numbers of all atoms in a polyatomic ion is equal to the charge on the ion.

15. Sample Problem E p. 233 Assign oxidation numbers to each atom in the following compounds: UF 6 Start with known: F is always -1 Compounds must add up to zero U is +6

16. Sample Problem E p. 233 H 2 SO 4 Oxygen & sulfur more electronegative Hydrogen assigned +1. Oxygen not combined with fluorine or in peroxide. Oxygen assigned -2. Sum must be zero. 2(+1) + 4(-2) + 1(x) = 0 x = +6 Sulfur atom assigned +6

17. ClO - 3 Oxygen is more electronegative and is assigned -2 Sum must be equal to charge of polyatomic ion = -1 3(-2) + 1(x) = -1 x = +5 Chlorine is assigned an oxidation number of +5

18. Homework: “Assigning Oxidation Numbers” worksheet