1. Aqueous Reactions CHAPTER 20 “Oxidation-Reduction Reactions” LEO SAYS GER

2. Aqueous Reactions Oxidation-Reduction Reactions An oxidation occurs when an atom or ion loses electrons. A reduction occurs when an atom or ion gains electrons.

3. Aqueous Reactions Oxidation and Reduction (Redox) Redox currently says that electrons are transferred between reactants Mg + S → Mg 2+ + S 2- The magnesium atom (which has zero charge) changes to a magnesium ion by losing 2 electrons, and is oxidized to Mg 2+ The sulfur atom (which has no charge) is changed to a sulfide ion by gaining 2 electrons, and is reduced to S 2- (MgS)

4. Aqueous Reactions Oxidation-Reduction Reactions One cannot occur without the other.

5. Aqueous Reactions Problems 4.45 a,b

6. Aqueous Reactions Oxidation Numbers To determine if an oxidation-reduction reaction has occurred, we assign an oxidation number to each element in a neutral compound or charged entity.

7. Aqueous Reactions Oxidation Numbers Elements in their elemental form have an oxidation number of 0. ( Al) The oxidation number of an ion is the same as its charge. B +2

8. Aqueous Reactions Oxidation Numbers Nonmetals have negative oxidation numbers based on the ions they form, although some are positive in certain compounds or ions.  Oxygen has an oxidation number of −2, except in the peroxide ion in which it has an oxidation number of −1.  Hydrogen is −1 when bonded to a metal, +1 when bonded to a nonmetal.

9. Aqueous Reactions Oxidation Numbers Nonmetals tend to have negative oxidation numbers, Fluorine always has an oxidation number of −1.  The other halogens have an oxidation number of −1 when they are negative; they can have positive oxidation numbers, however, most notably in oxyanions.

10. Aqueous Reactions Oxidation Numbers The sum of the oxidation numbers in a neutral compound is 0. The sum of the oxidation numbers in a polyatomic ion is the charge on the ion.

11. Aqueous Reactions Problems 4.49

12. Aqueous Reactions Oxidation and Reduction (Redox) Each sodium atom loses one electron: Each chlorine atom gains one electron:

13. Aqueous Reactions LEO says GER : LEO says GER : Lose Electrons = Oxidation Sodium is oxidized Gain Electrons = Reduction Chlorine is reduced

14. Aqueous Reactions LEO says GER : LEO says GER : - Losing electrons is oxidation, and the substance that loses the electrons is called the reducing agent. - Gaining electrons is reduction, and the substance that gains the electrons is called the oxidizing agent. Mg (s) + S (s) → MgS (s) Mg is oxidized: loses e -, becomes a Mg 2+ ion S is reduced: gains e - = S 2- ion Mg is the reducing agent S is the oxidizing agent

15. Aqueous Reactions Problems 4.51 a,b,c,d

16. Aqueous Reactions Displacement Reactions In this reaction, silver ions oxidize copper metal. Cu (s) + 2 Ag + (aq)  Cu 2+ (aq) + 2 Ag (s)

17. Aqueous Reactions Displacement Reactions The reverse reaction, however, does not occur. Cu 2+ (aq) + 2 Ag (s)  Cu (s) + 2 Ag + (aq) x

18. Aqueous Reactions Activity Series

19. Aqueous Reactions Using Molarities in Stoichiometric Calculations

20. Aqueous Reactions Titration The analytical technique in which one can calculate the concentration of a solute in a solution.

21. Aqueous Reactions Titration