1. Redox Reactions Or How Batteries Work

2. REDOX Reactions • The simultaneous transfer of electrons between chemical species. • Actually 2 different reactions occurring at the same time. Oxidation: a loss of electrons Reduction: a gain of electrons

3. ReDox Na +ClNaCl Nao +CloNa+Cl- Which element was Oxidized? Na Which element was Reduced? Cl

4. How do we remember ReDox? mnemonics

5. LEO growls GER Losing Electrons Oxidation Gaining Electrons Reduction

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7. OIL RIG Oxidation Is Losing Reduction Is Gaining

8. How do we know if there is aRedOx Reaction occurring? We first must find the Oxidation states.

9. What is the oxidation state of each of the following species? • Cl in Cl2 • 0 • Mg in MgO • +2 • Na in NaBr • +1 • F in SrF2 • -1 • Sr in SrF2 • +2 • O in O2 • 0

10. Regents Question Which particles are gained and lost during a redox reaction? Electrons (2) Protons (3) Neutrons (4) Positrons

11. Regents Question As a Ca atom undergoes oxidation to Ca2+, the number of neutrons in its nucleus Decreases (2) Increases (3) Remains the same

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13. Rules for Determining Oxidation Numbers • Free Elements have an oxidation number of zero. Ex: Na, S8, H2 • All Metals in group 1 have a +1 Oxidation # in compounds. Ex: Na, K, Li • All Metals in group 2 have a +2 Oxidation # in compounds. Ex: Mg, Ca, Ba • Oxygen has a –2 Oxidation Number • Unless it is in a Peroxide like H2O2

14. Continued Rules • Hydrogen has a +1 oxidation #. • Halogens have a -1 oxidation #. • For any neutral compound, the sum of the oxidation numbers of the atoms in the compound must equal zero. For ex: H2SO4 • For a polyatomic ion, the sum of the oxidation #’s must equal the ionic charge of the ion. Ex: SO42-

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17. Now we will take a deeper look into Oxidation. • Can be defined as: • The loss of electrons • The gain of oxygen • The loss of hydrogen

18. Sodium loses electron to chlorine - + Na + Cl Na + Cl OXIDATION • The loss of electrons Na(s) Na+ + e- The sodium ion has been oxidized to the sodium cation.

19. OXIDATION • Gain of oxygen • Combustion reactions are classic examples C(s) + O2(g) CO2(g) (burning of coal) 2Fe(s) + 3O2(g)  2Fe2O3(s) (rusting of iron)

20. OXIDATION • Loss of hydrogen • Oxidation can sometimes be best seen as the loss of hydrogen CH3OH(l) CH2O(l) + H2(g) Methyl alcohol has been oxidized into formaldehyde

21. Which one of the following elements were oxidized? • Cl in Cl2 • 0 → NO • Mg in MgO • +2 → YES • Na in NaBr • +1 → YES • F in SrF2 • -1 → NO • Sr in SrF2 • +2 → YES • O in O2 • 0 → NO

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23. Now we will take a deeper look into Reduction. • 3 Definitions • The gain of electrons • The loss of oxygen • The gain of hydrogen

24. REDUCTION Gain of electrons • The process of silver electroplating Ag+ + e- Ag Silver cation has gained an electron and has been reduced to silver metal.

25. REDUCTION Loss of oxygen • Reduction can also be seen as the loss of oxygen in going from reactant to product. Fe2O2 (s) + 3 CO (g) 2 Fe(s) + 3 CO2(g) Iron ore is reduced to iron metal in a blast furnace with carbon monoxide

26. REDUCTION • Gain of hydrogen A reduction can also be described as the gain of hydrogen atoms going from reactant to product. CO(g) + 2 H2(g) CH3OH(l) Carbon monoxide has been reduced to methyl alcohol

27. One’s loss is another’s gain • Neither oxidation or reduction can take place without the other. • When those electrons are lost something else has to gain them. When trying to remember which is which think of LEO the lion goes GER OIL RIG Lose Electrons Oxidation Oxidation is Losing Gain Electrons Reduction Reduction is Gaining

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29. Use a chart to determineOxidation States.

30. First we will do K2PtCl6

31. Now do the following: • What is the oxidation state of S in MgSO4? • What is the oxidation state of N in NaNO3? • What is the oxidation state of Cl in KClO3?

32. Now do the following: • What is the oxidation state of C in CO3- • What is the oxidation state of Cl in ClO2- • What is the oxidation state of Cr in CrO42- • What is the oxidation state of Cr in CrO72-

33. Packet Page 10-11

34. Review of Terms • oxidation-reduction (redox) reaction: • involves a transfer of electrons from the reducing agent to the oxidizing agent. • oxidation: loss of electrons • reduction: gain of electrons • Reducing Agent get oxidized • Oxidizing agent get reduced

35. Packet Page 12-13

36. An oxidation-reduction, (redox), reaction involves the transfer of electrons. Sodium transfers its electrons to chlorine The oxidation numbers of the atoms will change…. one goes up (oxidation) and one goes down (reduction)

37. Reduction is the gain of electrons. Nonmetals gain electrons to form – ions The oxidation number goes down (reduces)

38. A half-reaction can be written to represent reduction. Cl0+ 1e- Cl-1 In reduction half reactions, electrons are written on the left because electrons are gained

39. Oxidation is the loss of electrons. Metal atoms lose electrons to become + ions The oxidation numbers go up (increases)

40. A half-reaction can be written to represent oxidation. Na0 Na1+ + 1e- In oxidation half reactions, electrons are written on the right because electrons are lost

41. Half-Reactions • Show either the oxidation or reduction portion of a redox reaction and the electrons gained or lost.

42. Steps for Writing Half-Reactions Cu0+ Ag+1N+5O-23 Cu+2 (N+5O-23)2 + Ag0 • Assign an oxidation number to each element. • Write a partial half-reaction to show the change in oxidation state: Oxidation: Cu0 Cu+2 Reduction: Ag+1 Ag0

43. Steps Continued… 3. Show the number of electrons needed to explain how the oxidation number changed. Oxidation: Cu0 Cu+2 + 2e- Reduction: Ag+1 +1e- Ag0 • 4. Last, but not least, achieve conservation of charge: 2Ag+1 +2e- 2Ag0

44. Practice Problems • Mg + Cl2 MgCl2 • Cu + 2Ag+ Cu2+ + 2Ag • Al0 + Cr3+ Al3+ + Cr0

45. Mg + Cl2 MgCl2

46. Cu + 2Ag+ Cu2+ + 2Ag

47. Al0 + Cr3+ Al3+ + Cr0

48. Regents Question: Given the reaction: Mg(s) + 2H+(aq) + 2Cl–(aq) Mg2+(aq) + 2Cl–(aq) + H2(g) Which species undergoes oxidation? (1) Mg(s) (2) H+(aq) (3) Cl–(aq) (4) H2(g) þ LEO growls GER LEO

49. Regents Question: Given the equation: C(s) + H2O(g) CO(g) + H2 (g) Which species undergoes reduction? (1) C(s) (2) H + (3) C2+ (4) H2 (g) þ GER LEO growls GER

50. Regents Question: In any redox reaction, the substance that undergoes reduction will (1) lose electrons and have a decrease in oxidation number (2) lose electrons and have an increase in oxidation number (3) gain electrons and have a decrease in oxidation number (4) gain electrons and have an increase in oxidation number þ