Redox Reactions. key terms: oxidizing agent reducing agent oxidation number

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Dominick Black
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1 Redox Reactions key terms: oxidizing agent reducing agent oxidation number

2 Oxidation Numbers In order to keep track of what loses electrons and what gains them, we assign oxidation numbers. Electrochemistry 2012 Pearson Education, Inc.

3 0 1+ Zn (s) + 2HCl (aq) Zn Cl - H + H + Cl - Cl - H + H + Cl -

4 0 1+ Zn (s) + 2HCl (aq) Zn Cl - H + H + Cl - Cl - H + H + Cl -

0 1+ 0 2+ Zn (s) + 2HCl (aq) H 2 (g) + ZnCl 2

5 Zn (s) + 2HCl (aq) H 2 (g) + ZnCl 2 (aq) H 2 H 2 Zn Cl - Zn 2+ H + Cl - Cl - Zn 2+ Cl -

6 Using the activity series provided, write the net ionic chemical equations for the following reaction. If no reaction occurs, simply write NR 2+ 0 a) Cu 2+ (aq) + Mg (s)?

7 best reducing agents worst oxidizing agents worst reducing agents best oxidizing agents

8 best reducing agents best oxidizing agents

9 Using the activity series provided, write the net ionic chemical equations for the following reactions. If no reaction occurs, simply write NR reducing agent a) 2+ oxidized Cu 2+ (aq) + reduced oxidizing agent 0 Mg (s) 0 Cu (s) + 2+ Mg 2+ (aq)

10 oxidation-reduction reactions In studying a redox reaction we often think of it as two half reactions. Cu 2+ (aq) + Mg(s) Cu (s) + Mg 2+ (aq) oxidation Mg(s) Mg 2+ (aq) + 2e - reduction Cu 2+ (aq) + 2e - Cu (s)

11 oxidation-reduction reactions Cu 2+ (aq) + Mg(s) Cu (s) + Mg 2+ (aq) oxidation Mg(s) Mg 2+ (aq) + 2e - reduction Cu 2+ (aq) + 2e - Cu (s)

12 Balancing Oxidation-Reduction Equations

13 Using the activity series provided, write the net ionic chemical equations for the following reactions. If no reaction occurs, simply write NR 2+ 0 e) Co 2+ (aq) + Al(s)

14 best reducing agents best oxidizing agents

15 Using the activity series provided, write the net ionic chemical equations for the following reactions. If no reaction occurs, simply write NR 3 x 2+ 0 e) 3 Co 2+ (aq) + gaining 6 electrons 2Al(s) 0 2 x 3+ 3Co (s) + 2Al 3+ (aq) losing 6 electrons

16 Half-Reaction Method in Acid 1. Write the unbalanced equation in ionic form. 2. Separate the equation into two half-reactions. 3. Balance each half reaction (except for O and H). 4. In acid solution, balance O by adding H 2 O and H by adding H + 5. Balance the charges by adding electrons. 6. Add the half reactions 7. Check to make sure atoms and charges are balanced

17 Example Balance the following equation for the reaction in acid solution. * MnO Fe 2+ Mn 2+ + Fe 3+ *All species are (aq)

18 1. Write the unbalanced equation in ionic form. MnO Fe 2+ Mn 2+ + Fe Separate the equation into two half-reactions Fe 2+ Fe 3+ MnO 4 - Mn 2+

19 3. Balance each half reaction (except for O and H). 4. In acid solution, balance O by adding H 2 O and H by adding H + Fe 2+ Fe 3+ MnO H + Mn H 2 O

20 5. Balance the charges by adding electrons. You need the same number of electrons on both sides of the equation. 5 x ( Fe 2+ Fe e - ) 1 x( MnO H + + 5e - Mn H 2 O)

21 5. Balance the charges by adding electrons. 5Fe 2+ 5Fe e - MnO H + + 5e - Mn H 2 O

22 6. Add the half reactions 5Fe 2+ 5Fe e - MnO H + + 5e - Mn H 2 O MnO H + + 5Fe 2+ Mn H 2 O + 5Fe 3+ This is the balanced equation

23 Half-Reaction Method in Base 1. Use the half-reaction method as specified for acidic solutions to obtain the final balanced equation as if H + ions were present. 2. Add the number of OH - ions to both sides of the equation to turn the remaining H + ions to H 2 O 3. Eliminate waters that appear on both sides of the equation.

24 Example Balance the following equation for the reaction in basic solution. * HS - + NO 3 - S + NO 2 - HS - S + H + + 2e - 2e - + 2H + + NO 3 - NO H 2 O *All species are (aq)

25 Example Balance the following equation for the reaction in basic solution. * HS - + NO 3 - S + NO 2 - HS - S H + + NO 3 - NO H 2 O OH - + HS - + H + + NO - 3 NO S + H 2 O + OH -

26 Example Balance the following equation for the reaction in basic solution. * HS - + NO 3 - S + NO 2 - HS - S H + + NO 3 - NO H 2 O OH - + HS - + H + + NO - 3 NO S + H 2 O + OH -

27 Example Balance the following equation for the reaction in basic solution. * HS - + NO 3 - S + NO 2 - HS - S H + + NO 3 - NO H 2 O H 2 O + HS - + NO - 3 NO S + H 2 O + OH -

28 Example Balance the following equation for the reaction in basic solution. * HS - + NO 3 - S + NO 2 - HS - + NO - 3 NO S + OH -

29 a) Complete and balance the following equations, and identify the oxidizing and reducing agents: acidic solution Cr2O7 2- (aq) + I - (aq) Cr 3+ (aq) + IO3 - (aq) 6 e - + 3H2O + I - IO3-14H + + Cr2O7 2-2 Cr H + + 6e H2O 8H + + I - + Cr2O IO3 + 2Cr H2O

30 Complete and balance the following equations, and identify the oxidizing and reducing agents: b) acidic solution I2 (s) + OCl - (aq) IO3 - (aq) + Cl - (aq) 6H2O + I2 2 IO H e ( 2 e - + 2H + + OCl - Cl - + H2O ) H2O + I2 + 5OCl - - 2IO3 + 5Cl - + 2H +

31 Complete and balance the following equations, and identify the oxidizing and reducing agents: c) acidic solution As2O3 (s) + NO3 - (aq) H3 AsO4 (aq) + N2O3 (aq) 5H2O + As2O3 2 H3 AsO4 + 4H + + 4e - 4 e - + 6H NO3 - N2O3 + 3H2O 2H + + 2H2O + 2NO3 - + As2O3 2H3 AsO4 + N2O3

32 Half-Reaction Method in Base 1. Use the half-reaction method as specified for acidic solutions to obtain the final balanced equation as if H + ions were present. 2. Add the number of OH - ions to both sides of the equation to turn the remaining H + ions to H 2 O 3. Eliminate waters that appear on both sides of the equation.

33 Complete and balance the following equations, and identify the oxidizing and reducing agents: d) basic solution MnO4 - (s) + Br - (aq) MnO2 (s) + BrO3 - (aq) ( 3 e - + 4H + + MnO4 - MnO2 + 2H2O ) 3H2O + Br - BrO H + + 6e - 2OH - + 2H + + Br - + 2MnO4 - BrO3 - + H2O + Br - + 2MnO4-2 MnO2 + H2O + 2OH - BrO MnO2 + 2OH -

34 Complete and balance the following equations, and identify the oxidizing and reducing agents: e) acidic solution BrO3 - (s) + N2H4 (g) Br - (aq) + N2 (g) ( N2H4 N2 + 4H + + 4e - ) ( 6 e - + 6H + + BrO3 - Br - + 3H2O ) 3N2H4 + 2BrO3-2Br - + 3N2 + 6H2O

35 Complete and balance the following equations, and identify the oxidizing and reducing agents: f) basic solution NO2 - (aq) + Al (s) NH4 + (aq) + AlO2 - (g) 6 e - + 8H + + NO2 - NH H2O ( 2H2O + Al AlO H + + 3e - ) 2H2O + 2BrO Al AlO2 - + NH4 +

36 36

37 Complete and balance the following equations, and identify the oxidizing and reducing agents: acid solution MnO4 - (s) + Br - (aq) Mn 2+ (aq) + Br2 (aq) ( 5 e - + 8H + + MnO4 - Mn H2O ) ( 2 Br - Br2 + 2e - ) 16H Br MnO4-5 Br2 + 2 Mn H2O

38 38

39 Half-Reaction Method in Acid 1. Write the unbalanced equation in ionic form. 2. Separate the equation into two half-reactions. 3. Balance each half reaction (except for O and H). 4. In acid solution, balance O by adding H 2 O 5. In acid solution, balance H by adding H + 6. Balance the charge in the half reaction by adding electrons. 7. multiply by half reactions by an integer to make the electrons lost equal to the electrons gained 8. Add the half reactions (canceling anything that is redundant) 9 Check to make sure atoms and charges are balanced Half-Reaction Method in base 1. Use the acid method, then add the number of OH - ions to both sides of the equation to turn the remaining H + ions to H 2 O 2. Eliminate waters that appear on both sides of the equation.

40 problems refer to the chemical reaction below NH4NO3 (s) + Zn (s) ZnO (s) + 2H2O (g) + N2 (g) which atom is acting as the oxidizing agent? What is the oxidation state of zinc ion at the end of the reaction? Which element is reduced during the reaction reaction?

41 Complete and balance the following equations, and identify the oxidizing and reducing agents: acidic solution a) Cr2O7 2- (aq) + I - (aq) Cr 3+ (aq) + IO3 - (aq) acidic solution b) I2 (s) + OCl - (aq) IO3 - (aq) + Cl - (aq) acidic solution c) As2O3 (s) + NO3 - (aq) H3 AsO4 (aq) + N2O3 (aq)

42 basic solution d) MnO4 - (s) + Br - (aq) MnO2 (s) + BrO3 - (aq) acidic solution e) BrO3 - (s) + N2H4 (g) Br - (aq) + N2 (g) basic solution f) NO2 - (aq) + Al (s) NH4 + (aq) + AlO2 - (g)

43 Example Balance the following equation for the reaction in acid solution. * HNO 3(aq) + H 3 PO 3 (aq) NO (g ) + H 3 PO 4 aq) + H 2 O(l)

44 1. Write the unbalanced equation in ionic form. H + (aq) NO 3 - (aq) + H 3 PO 3 (aq) NO (g ) + H 3 PO 4 aq) + H 2 O(l) 2. Separate the equation into two half-reactions NO 3 - NO H 3 PO 3 H 3 PO 4

45 3. Balance each half reaction (except for O and H). 4. In acid solution, balance O by adding H 2 O and H by adding H + 4H + + NO 3 - NO + 2H 2 O H 2 O + H 3 PO 3 H 3 PO 4 + 2H +

46 5. Balance the charges by adding electrons. 3e - + 4H + + NO 3 - NO + 2H 2 O H 2 O + H 3 PO 3 H 3 PO 4 + 2H + + 2e -

47 5. Balance the charges by adding electrons. 2 x ( 3e - + 4H + + NO 3 - NO + 2H 2 O ) 3 x ( H 2 O + H 3 PO 3 H 3 PO 4 + 2H + + 2e - )

48 5. Balance the charges by adding electrons. 6e - + 8H + + 2NO- 3 2NO + 4H 2 O 3H 2 O + 3H 3 PO 3 3H 3 PO 4 + 6H + + 6e -

49 6. Add the half reactions 6e - + 8H + + 2NO- 3 2NO + 4H 2 O 3H 2 O + 3H 3 PO 3 3H 3 PO 4 + 6H + + 6e -

50 6. Add the half reactions 6e - + 2H + + 2NO- 3 2NO + 4H 2 O 3H 2 O + 3H 3 PO 3 3H 3 PO 4 + 6H + + 6e -

51 6. Add the half reactions 6e - + 2H + + 2NO- 3 2NO + H 2 O 3H 2 O + 3H 3 PO 3 3H 3 PO 4 + 6H + + 6e - 2H + + 2NO H 3 PO 3 2NO + H 2 O + 3H 3 PO 4

52 6. Add the half reactions 2H + + 2NO H 3 PO 3 2NO + H 2 O + 3H 3 PO 4 This is the balanced equation

53 Example Balance the following equation for the reaction in acid solution. * Fe(s) + HCl(aq) HFeCl 4 (aq) + H 2 (aq ) 3 2e - + ( 2 H + H ) 2 2 ( H + + 4Cl - + Fe HFeCl 4 + 3e - )

54 Example Balance the following equation for the reaction in acid solution. * Fe(s) + HCl(aq) HFeCl 4 (aq) + H 2 (aq ) 6e H + 3H 2 2H + + 8Cl - + 2Fe 2HFeCl 4 + 6e -

55 Example 6e H + 3H 2 2H + + 8Cl - + 2Fe 2HFeCl 4 + 6e - 8H + + 8Cl - + 8HCl + 2Fe 2HFeCl 4 or 2Fe 2HFeCl 4 + 3H 2 + 3H 2

Balancing Redox Equations in Acidic Solutions. 6 2Cr H 2 O. 6 2Cr 3+ Example 1: Balance the following equation for the reaction of Cr 2 O 7

Balancing Redox Equations in Acidic Solutions Example 1: Balance the following equation for the reaction of Cr 2 O 7 2 with Fe 2+ in an acidic solution. Cr 2 O 7 2 + Fe 2+ 6 Cr 3+ + Fe 3+ Step 1: Divide