1. 1 Balancing Redox Reactions Chapter 20: Day 2

2. 2 Review of Terminology for Redox Reactions OXIDATION—loss of electron(s) by a species; increase in oxidation number.OXIDATION—loss of electron(s) by a species; increase in oxidation number. REDUCTION—gain of electron(s); decrease in oxidation number.REDUCTION—gain of electron(s); decrease in oxidation number. OXIDIZING AGENT—electron acceptor; species is reduced.OXIDIZING AGENT—electron acceptor; species is reduced. REDUCING AGENT—electron donor; species is oxidized.REDUCING AGENT—electron donor; species is oxidized. OXIDATION—loss of electron(s) by a species; increase in oxidation number.OXIDATION—loss of electron(s) by a species; increase in oxidation number. REDUCTION—gain of electron(s); decrease in oxidation number.REDUCTION—gain of electron(s); decrease in oxidation number. OXIDIZING AGENT—electron acceptor; species is reduced.OXIDIZING AGENT—electron acceptor; species is reduced. REDUCING AGENT—electron donor; species is oxidized.REDUCING AGENT—electron donor; species is oxidized.

3. 20.3 Describing Redox Equations > 3 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Some fruits, including apples, turn brown when you cut them. What is happening on the surface of the fruit? CHEMISTRY & YOU Oxygen in air reacts with chemicals on the surface of the cut fruit. The oxygen oxidizes the chemicals in the fruit, causing a redox reaction and therefore the color change. Why does cut fruit turn brown?

4. 20.3 Describing Redox Equations > 4 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The element whose oxidation number increases is oxidized The element whose oxidation number decreases is reduced If changes in oxidation number occur, the reaction is a redox reaction.

5. 20.3 Describing Redox Equations > 5 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Identifying Redox Reactions Use the change in oxidation number to identify whether each reaction is a redox reaction Cl 2 (g) + 2NaBr(aq) → 2NaCl(aq) + Br 2 (aq) 2NaOH(aq) + H 2 SO 4 (aq) → Na 2 SO 4 (aq) + 2H 2 O(l)

6. 20.3 Describing Redox Equations > 6 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Sample Problem 20.5 a. Assign oxidation numbers. 0 +1 –1 +1 –1 0 Cl 2 (g) + 2NaBr(aq) → 2NaCl(aq) + Br 2 (aq) The chlorine is reduced; The bromide ion is oxidized;

7. 20.3 Describing Redox Equations > 7 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Solve ASSIGN OXIDATION NUMBERS. 2 Sample Problem 20.5 NO change in oxidation number. This is not a redox reaction. +1 –2 +1 +1 +6 –2 +1 +6 –1 +1 –2 2NaOH(aq) + H 2 SO 4 (aq) → Na 2 SO 4 (aq) + 2H 2 O(l) This is an acid-base (neutralization) reaction.

8. 20.3 Describing Redox Equations > 8 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Which of the following are redox reactions? A.NH 3 + HCl → NH 4 Cl B.SO 3 + H 2 O → H 2 SO 4 C.NaOH + HCl → NaCl + H 2 O D.H 2 S + NHO 3 → H 2 SO 4 + NO 2 + H 2 O B.SO 3 + H 2 O → H 2 SO 4 D.H 2 S + NHO 3 → H 2 SO 4 + NO 2 + H 2 O

9. 9 TRANSFER REACTIONS Atom/Group transfer (not) HCl + H 2 O ---> Cl - + H 3 O + Redox: Electron transfer Cu(s) + 2 Ag + (aq) ---> Cu 2+ (aq) + 2 Ag(s)

10. 10 OXIDATION-REDUCTION REACTIONS Cu(s) + 2 Ag + (aq) ---> Cu 2+ (aq) + 2 Ag(s) Why 2?

11. 11 Balancing Equations Need to Balance BOTH mass and CHARGE Cu + Ag + --give--> Cu 2+ + Ag : Step 1: Divide into half-reactions: one for oxidation and the other for reduction. OxCu ---> Cu 2+ Red Ag + ---> Ag

12. 12 Balancing Equations Step 2:Balance each for mass. Already done in this case. Step 3:Balance each half-reaction for charge by adding electrons. Ox Cu ---> Cu 2+ + 2e- Red Ag + + e- ---> Ag

13. 13 Balancing Equations Step 4: Multiply each half-reaction by a factor to have the electrons lost equal to number gained Cu ---> Cu 2+ + 2e- 2 Ag + + 2 e- ---> 2 Ag Step 5:Add to give the overall equation. Cu + 2 Ag + ---> Cu 2+ + 2Ag The equation is now balanced for BOTH charge and mass.

14. 14 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Use the half-reaction method to balance the following redox equation. FeCl 3 + H 2 S → FeCl 2 + HCl + S Oxidation: H 2 S → 2H + + S + 2e – Reduction: 2Fe 3+ + 2e – → 2Fe 2+ 2FeCl 3 + H 2 S → 2FeCl 2 + 2HCl + S

15. 20.3 Describing Redox Equations > 15 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Balancing Redox Equations What are two different methods for balancing a redox equation? Two different methods for balancing redox equations are the oxidation-number-change method and the half-reaction method.

16. 20.3 Describing Redox Equations > 16 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. In the oxidation-number- change method, you balance a redox equation by comparing the increases and decreases in oxidation numbers. Using Oxidation-Number Changes

17. 20.3 Describing Redox Equations > 17 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Using Oxidation-Number Changes Step 1: Assign oxidation numbers to all the atoms in the equation. Write the numbers above the atoms. The oxidation number is stated per atom. Fe 2 O 3 (s) + CO(g) → Fe(s) + CO 2 (g) +3 –2 +2 –2 0 +4 –2

18. 20.3 Describing Redox Equations > 18 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Using Oxidation-Number Changes Step 2: Identify which atoms are oxidized and which are reduced. Iron is reduced. +3 to 0 Carbon is oxidized. +2 to +4 Fe 2 O 3 (s) + CO(g) → Fe(s) + CO 2 (g) +3 –2 +2 –2 0 +4 –2

19. 20.3 Describing Redox Equations > 19 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Using Oxidation-Number Changes Step 3: Use one line to connect the atoms that undergo oxidation and another such line to connect those that undergo reduction. –3 (reduction) Fe 2 O 3 (s) + CO(g) → Fe(s) + CO 2 (g) +3 –2 +2 –2 0 +4 –2 +2 (oxidation) Write the oxidation-number change at the midpoint of each line.

20. 20.3 Describing Redox Equations > 20 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Using Oxidation-Number Changes Step 4: Make the total increase in oxidation number equal to the total decrease in oxidation number by using appropriate coefficients. 2 × (–3) = –6 Fe 2 O 3 (s) + 3CO(g) → 2Fe(s) + 3CO 2 (g) +3 –2 +2 –2 0 +4 –2 3 × (+2) = +6 The oxidation-number increase should be multiplied by 3 and the decrease by 2.

21. 20.3 Describing Redox Equations > 21 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Using Oxidation-Number Changes Step 5: Finally, make sure the equation is balanced for both atoms and charge. Fe 2 O 3 (s) + 3CO(g) → 2Fe(s) + 3CO 2 (g) If necessary, finish balancing the equation by inspection.

22. 20.3 Describing Redox Equations > 22 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Balancing Redox Equations by Oxidation-Number Change Balance this redox equation by using the oxidation-number-change method. Cl 2 (g) + K(s) → KCl(s) 0 0 +1 –1 2 × (-1) = -2 2 × (+1) = +2 22

23. 23 ? Ready for more complex reactions ? YES!

24. 24 Acid / Base Redox Reactions Some redox reactions have equations that must be balanced by special techniques. If reactions occur in the “presence of” acid or base MnO 4 - + 5 Fe 2+ + 8 H + ---> Mn 2+ + 5 Fe 3+ + 4 H 2 O

25. 25 : Step 1: Divide into half-reactions Permanganate and iron(II) ions are reacted in an acidic solution MnO 4 - + Fe 2+ ---> Mn 2+ + Fe 3+ Ox Fe 2+ ---> Fe 3+ Red MnO 4 - ---> Mn 2+ Need to balance MASS

26. 26 Step 2:Balance each for mass. Already done for Iron Fe 2+ ---> Fe 3+ Need to have “O” on both sided: Add water MnO 4 - ---> Mn 2+ + H 2 O Never add O 2, O atoms, or O 2- to balance oxygen. MnO 4 - ---> Mn 2+ + 4H 2 O

27. 27 Step 2:Balance each for mass. MnO 4 - ---> Mn 2+ + 4H 2 O Need to have “H” on both sided. Told in an acidic solution: need H + 8 H + + MnO 4 - ---> Mn 2+ + 4H 2 O Never add H 2 or H atoms to balance hydrogen.

28. 28 Step 3:Balance each half-reaction for charge by adding electrons. 8 H + + 5 e- +MnO 4 - ---> Mn 2+ + 4H 2 O 5Fe 2+ ---> 5Fe 3+ + 5e- Multiply each half-reaction by a factor to have the electrons lost equal to number gained

29. 29 Acid / Base Redox Reactions Step 5:Add to obtain the overall equation MnO 4 - + 5 Fe 2+ + 8 H + ---> Mn 2+ + 5 Fe 3+ + 4 H 2 O Check by adding charges on both sides and by counting atoms The equation is now balanced for BOTH charge and mass.

30. 30 Balancing Equations Never add O 2, O atoms, or O 2- to balance oxygen. Never add H 2 or H atoms to balance hydrogen. Be sure to write the correct charges on all the ions. Check your work at the end to make sure mass and charge are balanced. PRACTICE!

31. 31 Reduction of VO 2 + with Zn

32. 32 Balancing Equations Balance the following in acid solution— VO 2 + + Zn ---> VO 2+ + Zn 2+ Step 1:Write the half-reactions OxZn ---> Zn 2+ RedVO 2 + ---> VO 2+ Step 2:Balance each for mass. VO 2 + ---> VO 2+ 2 H + + Add H 2 O on O-deficient side and add H + on other side for H-balance. + H 2 O

33. 33 Balancing Equations Step 3: Add electrons to half reaction. Zn ---> Zn 2+ + 2e- e- + 2 H + + VO 2 + ---> VO 2+ + H 2 O Step 4:Multiply by an appropriate factor. 2e- + 4 H + + 2 VO 2 + ---> 2 VO 2+ + 2 H 2 O Zn ---> Zn 2+ + 2e-

34. Balancing Equations Step 5:Add balanced half-reactions Zn + 4 H + + 2 VO 2 + ---> Zn 2+ + 2 VO 2+ + 2 H 2 O Check by adding charges on both sides and by counting atoms

35. 20.3 Describing Redox Equations > 35 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Balancing Redox Equations by Half-Reactions Balance this redox equation using the half-reaction method. KMnO 4 (aq) + HCl(l) → MnCl 2 (aq) + Cl 2 (g) + KCl(aq)

36. 20.3 Describing Redox Equations > 36 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Step 1:Write the equation in ionic form. Sample Problem 20.7 K + (aq) + MnO 4 – (aq) + H + (aq) + Cl – (aq) → Mn 2+ (a) + 2Cl – (a) + Cl 2 (g) + H 2 O + K + (a) + Cl – (a)

37. 20.3 Describing Redox Equations > 37 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Step 2:Write half-reactions. Determine the oxidation and reduction process. Oxidation half-reaction: Cl – → Cl 2 Reduction half-reaction: MnO 4 – → Mn 2+ –1 0 +7 +2

38. 20.3 Describing Redox Equations > 38 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Step 3:Balance the atoms in each half-reaction. Oxidation: 2Cl – (aq) → Cl 2 (g) (atoms balanced) Reduction: MnO 4 – (aq) + 8H + (aq) → Mn 2+ (aq) + 4H 2 O(l) (atoms balanced) The solution is acidic, so use H 2 O and H + to balance the oxygen and hydrogen.

39. 20.3 Describing Redox Equations > 39 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Step 4:Balance the charges by adding electrons. Oxidation: 2Cl – (aq) → Cl 2 (g) + 2e – (charges balanced) Reduction: MnO 4 – (aq) + 8H + (aq) + 5e – → Mn 2+ (aq) + 4H 2 O(l) (charges balanced)

40. 20.3 Describing Redox Equations > 40 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Step 5:Make the numbers of electrons equal. Oxidation: 10Cl – (aq) → 5Cl 2 (g) + 10e – Reduction: 2MnO 4 – (aq) + 16H + (aq) + 10e – → 2Mn 2+ (aq) + 8H 2 O(l) Multiply the oxidation half-reaction by 5 and the reduction half-reaction by 2.

41. 20.3 Describing Redox Equations > 41 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Step 6:Add the half-reactions. Then, subtract the terms that appear on both sides. 10Cl – (aq) + 2MnO 4 – (aq) + 16H + (aq) + 10e – → 5Cl 2 (g) + 10e – + 2Mn 2+ (aq) + 8H 2 O(l) Step 7:Add the spectator ions, making sure the charges and atoms are balanced. 10Cl – + 2MnO 4 – + 2K + + 16H + + 6Cl – → 5Cl 2 + 2Mn 2+ + 4Cl – + 8H 2 O + 2K + + 2Cl –

42. 20.3 Describing Redox Equations > 42 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Combine the spectator and nonspectator Cl – on each side. 16Cl – (a) + 2MnO 4 – (a) + 2K + (a) + 16H + (a) → 5Cl 2 (g) + 2Mn 2+ (a) + 6Cl – (a) + 8H 2 O(l) + 2K + (a)

43. 20.3 Describing Redox Equations > 43 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Show the balanced equation for the substances given in the question (rather than for ions). 2KMnO 4 (aq) + 16HCl(aq) → 2MnCl 2 (aq) + 5Cl 2 (g) + 8H 2 O(l) + 2KCl(aq)

44. 20.3 Describing Redox Equations > 44 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Use the half-reaction method to balance the following redox equation. FeCl 3 + H 2 S → FeCl 2 + HCl + S

45. 20.3 Describing Redox Equations > 45 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Glossary Terms oxidation-number-change method: a method of balancing a redox equation by comparing the increases and decreases in oxidation numbers half-reaction: an equation showing either the oxidation or the reduction that takes place in a redox reaction half-reaction method: a method of balancing a redox equation by balancing the oxidation and reduction half-reactions separately before combining them into a balanced redox equation

46. 20.3 Describing Redox Equations > 46 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Redox equations can be balanced by two methods, the oxidation-number-change method and balancing the oxidation and reduction half-reactions. BIG IDEA Reactions