1. Oxidation Reduction Reactions

2. Oxidation Reduction Reactions… are chemical changes that occur when electrons are transferred between reactants

3. What is oxidation? The terms oxidation and reduction have been known to chemists for centuries. One of the earliest definitions tells us that… oxidation means combining an element with oxygen such as… Mg + O 2  2MgO and …

4. What is reduction? reduction means losing oxygen from a compound such as… 2KClO 3  2KCl + 3O 2

5. Today… chemists define oxidation-reduction in terms of the transfer or shift of electrons. Oxidation is the complete or partial loss of electrons. Reduction is complete or partial gain of electrons.

6. Oxidation and reduction occur simultaneously. Electrons are transferred from one reactant to the other.

7. “LEO the lion goes GER!” LEO – Loss of Electrons is Oxidation GER – Gain of Electrons is Reduction

8. “OIL RIG” Oxidation Is a Loss and Reduction Is a Gain of electrons

9. Mg + S  Mg 2+ + S 2- In the formation of magnesium sulfide… Mg atoms become Mg 2+ ions in the compound. Oxidation: Mg  Mg 2+ + 2 e - (loss of electrons) S atoms become S 2- ions in the compound. Reduction: S + 2 e -  S 2- (gain of electrons)

10. Mg + S  Mg 2+ + S 2- The substance in a redox reaction that donates electrons is a reducing agent. In the above example, magnesium reduces sulfur. Magnesium is the reducing agent. The substance in a redox reaction that accepts electrons is an oxidizing agent. By accepting electrons, sulfur oxidizes magnesium. Sulfur is the oxidizing agent.

11. Identify what is oxidized and reduced: 2Na + S  Na 2 S Na is oxidized, S is reduced 2K + Cl 2  2KCl K is oxidized, Cl 2 is reduced 3O 2 + 4Al  2Al 2 O 3 Al is oxidized, O 2 is reduced

12. With compounds of carbon: The addition of oxygen OR the removal of hydrogen is always oxidation. The removal of oxygen OR addition of hydrogen is reduction.

13. Assigning oxidation numbers A helpful tool in the study of electron transfer is assigning oxidation numbers. To identify oxidation-reductions, look for a change in oxidation states. The oxidation number of an element in a compound is defined as the charge an atom has, or appears to have.

14. Oxidation numbers help us keep track of electrons transferred in redox reactions.

15. Guidelines for determining oxidation numbers: 1.Free, uncombined elements have an oxidation number of zero (as in Fe, H 2, O 3 ). 0 0 NaBr 2 2.For a single, monatomic ion, the oxidation number is equal to the charge on the ion. +1 -3 K + N 3-

16. Guidelines… 3.The oxidation number of H is +1 in most compounds (except in hydrides, -1) and the oxidation number of O is -2 in most compounds (except in peroxides, -1).

17. Guidelines… 4.The sum of the oxidation numbers in a neutral compound must be zero. ? -2 N2O4N2O4 Totals ? + -8 = 0 N must carry a total of +8, so each N = +4 +4 -2 N2O4N2O4

18. Guidelines… The sum of the oxidation numbers in a polyatomic ion must be equal to the charge on the ion. ? -2 PO 4 3- Totals ? + -8 = -3 P must carry a total of +5 +5 -2 PO 4 3-

19. Extra details: In compounds and polyatomic ions, the more electronegative atom will have an oxidation number equal to the charge, as if it was an ion. Oxidation numbers are written with the sign of the charge first (such as +3), while ion charges are written with the number first (such as 3+).

20. Determine the oxidation number of sulfur in the following ions or compounds: SO 2 S = +4 Al 2 (SO 4 ) 3 S = +6 Na 2 S 2 O 3 S = +2 K 2 SO 3 S = +4 S 8 S = 0

21. Using oxidation number changes to identify oxidation and reduction

22. The oxidation number of an element increases when it is oxidized, and decreases when it is reduced.

23. Identify what is oxidized and reduced, then identify the oxidizing and reducing agents: Cl 2 + 2HBr  2HCl + Br 2 Answer: the element chlorine is reduced (0  -1) and the element bromine is oxidized (-1  0)

24. Identifying Redox Reactions All oxidation-reduction reactions involve a change in oxidation numbers. If there is no change in oxidation state, it is not a redox reaction.

25. Identifying Redox Reactions Use the change in oxidation number to identify which of these reactions are redox reactions: A) N 2 O 4  2NO 2 B) Cl 2 + 2NaBr  2NaCl + Br 2 Answer: A is not redox, while B is redox. Bromine is oxidized and chlorine is reduced. Bromine is the reducing agent and chlorine is the oxidizing agent