Chapter 20: Oxidation-Reduction

1. Table of Contents Chapter 20: Oxidation-Reduction 20.1: Oxidation and Reduction 20.2: Balancing Redox Equations 20.3: Half-Reactions

2. Table of Contents Chapter 20: Oxidation-Reduction 20.1: Oxidation and Reduction

3. Oxidation-Reduction: Basic Concepts Electron Transfer and Redox Reactions

4. Oxidation-Reduction: Basic Concepts Electron Transfer and Redox Reactions • A reaction in which electrons are transferred from one atom to another is called an oxidation–reduction reaction. • Also called redox reactions Click box to view movie clip.

5. Oxidation-Reduction: Basic Concepts Electron Transfer and Redox Reactions • Single-replacement and combustion reactions always involve the transfer of electrons from one atom to another. • So do many, but not all, synthesis and decomposition reactions.

6. Oxidation-Reduction: Basic Concepts Electron Transfer and Redox Reactions

7. Oxidation-Reduction: Basic Concepts Changes in oxidation number • The oxidation numberof an atom in an ionic compound is the number of electrons lost or gained by the atom when it forms ions. • Oxidation numbers are tools that scientists use in written chemical equations to help them keep track of the movement of electrons in a redox reaction.

8. Oxidation-Reduction: Basic Concepts Rules for determining oxidation numbers 1. All atoms in elemental form have an oxidation number of 0. • This is true for elements that exist as polyatomic molecules such as O2, Cl2, H2, N2, S8.

9. Oxidation-Reduction: Basic Concepts Rules for determining oxidation numbers 2. Each element in a compound gets the number it would have if it were an ion. • Split ionic compounds into known ions. E.g. NH4NO3 becomes NH4+ & NO3− , Ca2+ = +2, Br– = –1. • Si = +4. • N = -3. • NH3 • SiCl4 • Cl = -1. • H = +1.

10. Oxidation-Reduction: Basic Concepts Rules for determining oxidation numbers 3. Nonmetals usually have negative oxidation numbers. • Hydrogen is +1 when bonded to nonmetals and -1 when bonded to metals. • Double check the oxidation number of any element on the 2nd period and lower

11. Oxidation-Reduction: Basic Concepts Rules for determining oxidation numbers 4. The sum of the oxidations numbers in a neutral compound is zero. • (+1) + (-1) = 0 (+2) + 2(-1) = 0 • NaCl CaBr2 • 2(+1) + (+6) + 4(-2) = 0 • Na2SO4

12. Oxidation-Reduction: Basic Concepts Rules for determining oxidation numbers 5. The sum of the oxidation numbers of the atoms in a polyatomic ion is the charge on the ion. • (-3) + 4(+1) = +1 (+4) + 3(-2) = -2 • NH4+ SO32-

13. Basic Assessment Questions Try it out! Determine the oxidation number of the boldface element in each of these compounds. Li2SiO3 Al4C3 Al = +3 C = – 4 O = -2 Li = +1 Si = + 4

14. Table of Contents Chapter 20: Oxidation-Reduction 20.2: Balancing Redox Equations

15. Review • Define a “redox reaction” • Why do we use oxidation numbers? • What are the oxidation numbers for reach of the following elements in the following compound: PO43- Cu3N2 • P=+5 O=-2 • N=-3 Cu=+2

16. Oxidation-Reduction: Basic Concepts How do oxidation and reduction differ? • Oxidationis the loss of electrons from atoms of a substance. • If a substance loses electrons another substance must take those electrons.

17. Oxidation-Reduction: Basic Concepts How do oxidation and reduction differ? • Reductionis the gain of electrons by atoms of a substance. • Half-reaction: an equation showing either the oxidation or the reduction half of a redox reaction (always includes the electrons)

18. Electrochemistry: Basic Concepts Identify which ½ rxns are oxidation and which are reduction • Fe → Fe2+ + 2e- oxidation • Te2+ + 2e- → Te • IO4- + 2e- → IO6- • Al → Al3+ + 3e- • I2 +2e- → 2I- • reduction • reduction • oxidation • reduction

19. Oxidation-Reduction: Basic Concepts How do oxidation and reduction differ? • Can oxidation occur without reduction? • Oxidation and reduction are complementary processes; oxidation cannot occur unless reduction also occurs.

20. Oxidation-Reduction: Basic Concepts How do oxidation and reduction differ? • LEO the lion says GER (LEO GER) • Loss of Electrons is Oxidation • Gain of Electrons is Reduction. • OIL RIG • Oxidation Is Loss • Reduction Is Gain

21. Oxidation-Reduction: Basic Concepts Oxidation Number in Redox Reactions spectator ion

22. Oxidation-Reduction: Basic Concepts Oxidizing and Reducing Agents Oxidized • Loses electrons • Oxidation number increases • Reducing agent: it is the substance that reduces another substance by giving its electrons.

23. Oxidation-Reduction: Basic Concepts Oxidizing and Reducing Agents Reduced • Gains electrons • Oxidation number decreases • Oxidixing agent: it is the substance that oxidizes another substance by accepting its electrons.

24. Oxidation-Reduction: Basic Concepts TRY IT OUT! Oxidizing and Reducing Agents 0 0 +1 -1 • Reducing agent = potassium (K) • Oxidizing agent = bromine (Br2) • (always reactants)

25. Oxidation-Reduction: Basic Concepts Identifying Oxidation–Reduction Reactions • The following equation represents the redox reaction of aluminum and iron. • Identify what is oxidized and what is reduced in this reaction. • Identify the oxidizing agent and the reducing agent.

26. Oxidation-Reduction: Basic Concepts Identifying Oxidation–Reduction Reactions • Oxidized: Al • Reduced: Fe3+ • Reducing agent: Al • Oxidizing agent: Fe3+ The iron ion accepts the three electrons lost from aluminum in the reduction process.

27. Basic Assessment Questions Try it out! For each of the following reactions, identify what is oxidized, what is reduced, the oxidizing agent & the reducing agent. Ox: I– Red: Br2 Ox ag: Br2 Red ag: I– Zn + NiSO4 → Ni + ZnSO4 Ox: Zn Red: Ni2+ Ox ag: Ni2+ Red ag: Zn

28. Table of Contents Chapter 20: Oxidation-Reduction 20.3: Half-Reactions

29. Writing half-reactions 1. Net ionic equation 2. Split into groups, add electrons 2Al → 2Al3+ + 6e- 2Fe3+ + 6e- → 2Fe 3. Reduce • Oxidized: Al • Red. Agent: Al • Reduced: Fe3+ • Ox. agent: Fe3+

30. Oxidation-Reduction: Additional Concepts • Writing half-reactions 0 0 +3 -1 3Br2 + 2 Al → 2 AlBr3 Net ionic eq: 3Br2 + 2Al → 2Al3+ + 6Br- 3Br2 + 6e- → 6Br- 2Al → 2Al3+ + 6e- Br2 + 2e- → 2Br- Al → Al3+ + 3e- Oxidized Red agent Reduced Ox agent

31. Oxidation-Reduction: Basic Concepts Oxidizing and Reducing Agents • A common application of redox chemistry is to remove tarnish from metal objects, such as a silver cup. Click box to view movie clip.

32. Oxidation-Reduction: Basic Concepts Oxidizing and Reducing Agents • Several oxidizing and reducing agents play significant roles in your daily life. • For example, when you add chlorine bleach to your laundry to whiten clothes, you are using an aqueous solution of sodium hypochlorite (NaClO), an oxidizing agent.

33. Oxidation-Reduction: Basic Concepts • Hydrogen peroxide (H2O2) can be used as an antiseptic because it oxidizes some of the vital biomolecules of germs, or as an agent to lighten hair because it oxidizes the dark pigment of the hair. Oxidizing and Reducing Agents

34. It’s your time! • Start bringing scientific calculators to class

35. End of Topic Summary File

36. Oxidation-Reduction: Basic Concepts Redox and Electronegativity • The chemistry of oxidation–reduction reactions is not limited to atoms of an element changing to ions or the reverse. Click box to view movie clip.

37. Oxidation-Reduction: Additional Concepts Half-Reactions • The oxidation half-reaction shows the loss of electrons by aluminum. • The reduction half-reaction shows the gain of electrons by bromine.

38. Oxidation-Reduction: Additional Concepts Half-Reactions • Balance half-reactions by multiplying their equations so that the electrons cancel 2 2 6 Al → Al3+ + 3 e- 6 3 6 Br2 + 2e- → 2Br- 3Br2 + 2 Al → 2 Al3+ + 6 Br- 3Br2 + 2 Al → 2 AlBr3