# Chapter 18. Oxidation Reduction Reactions and Electrochemistry

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1 Chapter 18 Oxidation Reduction Reactions and Electrochemistry

2 Chapter 18 Table of Contents 18.1 Oxidation Reduction Reactions 18.2 Oxidation States 18.3 Oxidation Reduction Reactions Between Nonmetals 18.4 Balancing Oxidation Reduction Reactions by the Half-Reaction Method 18.5 Electrochemistry: An Introduction 18.6 Batteries 18.7 Corrosion 18.8 Electrolysis Copyright Cengage Learning. All rights reserved 2

3 Section 18.1 Oxidation Reduction Equations Oxidation reduction reaction (redox reaction) a chemical reaction involving the transfer of electrons. Oxidation loss of electrons Reduction gain of electrons Copyright Cengage Learning. All rights reserved 3

4 Section 18.1 Oxidation Reduction Equations Exercise In the reaction below Sn(II). Sn Fe 3+ Sn Fe 2+ a) gains electrons b) is reduced c) is oxidized d) is neither oxidized nor reduced Copyright Cengage Learning. All rights reserved 4

5 Section 18.2 Oxidation States Allow us to keep track of electrons in oxidation reduction reactions by assigning charges to the various atoms in a compound. Oxidation States for the Transition Metals Copyright Cengage Learning. All rights reserved 5

6 Section 18.2 Oxidation States Rules for Assigning Oxidation States 1. Oxidation state of an atom in an element = 0 2. Oxidation state of monatomic ion = charge of the ion 3. Oxygen = 2 in covalent compounds (except in peroxides where it = 1) 4. Hydrogen = +1 in covalent compounds 5. Fluorine = 1 in compounds 6. Sum of oxidation states = 0 in compounds 7. Sum of oxidation states = charge of the ion in ions Copyright Cengage Learning. All rights reserved 6

7 Section 18.2 Oxidation States Exercise Find the oxidation states for each of the elements in each of the following compounds: K 2 Cr 2 O 7 CO 3 MnO 2 PCl 5 SF 4 K = +1; Cr = +6; O = 2 C = +4; O = 2 Mn = +4; O = 2 P = +5; Cl = 1 S = +4; F = 1 Copyright Cengage Learning. All rights reserved 7

8 Section 18.2 Oxidation States What are the Oxidation Numbers for each element in the following? H 2 O N 2 KMnO 4 CO 2 CH 4 CHCl 3 He Cu Na 2 Cr 2 O 7 +1 for H, -2 for O Zero for N, elemental state +1 for K, -2 for O, +7 for Mn -2 for O, +4 for C +1 for H, -4 for C +1 for H, -1 for Cl, +2 for C Zero for He, elemental state Zero for Cu, elemental state +1 for Na, -2 for O, +6 for Cr 1(+1 K)=+1 4(-2 O)= (+1 H)=+1 3(-1 Cl)= (+1 Na)=+2 7(-2 O)= Copyright Cengage Learning. All rights reserved 8

10 Section 18.3 Oxidation Reduction Reactions Between Nonmetals 2Na(s) + Cl 2 (g) 2NaCl(s) Na oxidized Na is also called the reducing agent (electron donor). Cl 2 reduced Cl 2 is also called the oxidizing agent (electron acceptor). Copyright Cengage Learning. All rights reserved 10

11 Section 18.3 Oxidation Reduction Reactions Between Nonmetals CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) C oxidized CH 4 is the reducing agent. O 2 reduced O 2 is the oxidizing agent. Copyright Cengage Learning. All rights reserved 11

12 Section 18.3 Oxidation Reduction Reactions Between Nonmetals Redox Characteristics Transfer of electrons Transfer may occur to form ions Oxidation increase in oxidation state (loss of electrons); reducing agent Reduction decrease in oxidation state (gain of electrons); oxidizing agent Oxidation Zn(s) + CuCl 2 (aq) ZnCl 2 (aq) + Cu(s) Reduction Copyright Cengage Learning. All rights reserved 12

13 Section 18.3 Oxidation Reduction Reactions Between Nonmetals Concept Check Which of the following are oxidation reduction reactions? Identify the oxidizing agent and the reducing agent. a)zn(s) + 2HCl(aq) ZnCl 2 (aq) + H 2 (g) b)cr 2 O 7 (aq) + 2OH - (aq) 2CrO 4 (aq) + H 2 O(l) c)2cucl(aq) CuCl 2 (aq) + Cu(s) Copyright Cengage Learning. All rights reserved 13

14 Section 18.4 Balancing Oxidation Reduction Reactions by the Half-Reaction Method Half Reactions The overall reaction is split into two half reactions, one involving oxidation and one reduction. Has electrons as reactants or products 8H + + MnO 4 + 5Fe 2+ Mn Fe H 2 O Reduction: 8H + + MnO 4 + 5e Mn H 2 O Oxidation: 5Fe 2+ 5Fe e Copyright Cengage Learning. All rights reserved 14

15 Section 18.4 Balancing Oxidation Reduction Reactions by the Half-Reaction Method The Half Reaction Method for Balancing Equations for Oxidation Reduction Reactions Occurring in Acidic Solution 1. Identify and write the equations for the oxidation and reduction half reactions. 2. For each half reaction: A. Balance all the elements except H and O. B. Balance O using H 2 O. C. Balance H using H +. D. Balance the charge using electrons. Copyright Cengage Learning. All rights reserved 15

16 Section 18.4 Balancing Oxidation Reduction Reactions by the Half-Reaction Method The Half Reaction Method for Balancing Equations for Oxidation Reduction Reactions Occurring in Acidic Solution 3. If necessary, multiply one or both balanced half reactions by an integer to equalize the number of electrons transferred in the two half reactions. 4. Add the half reactions, and cancel identical species. 5. Check that the elements and charges are balanced. Copyright Cengage Learning. All rights reserved 16

17 Section 18.4 Balancing Oxidation Reduction Reactions by the Half-Reaction Method Cr 2 O 7 (aq) + SO 3 (aq) Cr 3+ (aq) + SO 4 (aq) How can we balance this equation? First Steps: Separate into half-reactions. Balance elements except H and O. Copyright Cengage Learning. All rights reserved 17

18 Section 18.4 Balancing Oxidation Reduction Reactions by the Half-Reaction Method Method of Half Reactions Cr 2 O 7 (aq) 2Cr 3+ (aq) SO 3 (aq) SO 4 (aq) Balance O s with H 2 O and H s with H + 14H + (aq) + Cr 2 O 7 (aq) 2Cr 3+ (aq) + 7H 2 O(aq) H 2 O(l) + SO 3 (aq) SO 4 (aq) + 2H + (aq) How many electrons are involved in each half reaction? Balance the charges. Copyright Cengage Learning. All rights reserved 18

19 Section 18.4 Balancing Oxidation Reduction Reactions by the Half-Reaction Method Method of Half Reactions (continued) 6 e- + 14H + (aq) + Cr 2 O 7 (aq) 2Cr 3+ (aq) + 7H 2 O(aq) H 2 O(l) + SO 3 (aq) SO 4 (aq) + 2H + (aq) + 2e- Multiply whole reactions by a whole number to make the number of electrons gained equal the number of electrons lost. 6 e- + 14H + (aq) + Cr 2 O 7 (aq) 2Cr 3+ (aq) + 7H 2 O(aq) 3(H 2 O(l) + SO 3 (aq) SO 4 (aq) + 2H + (aq) + 2e-) Combine half reactions cancelling out those reactants and products that are the same on both sides, especially the electrons. Copyright Cengage Learning. All rights reserved 19

20 Section 18.4 Balancing Oxidation Reduction Reactions by the Half-Reaction Method Method of Half Reactions (continued) 8 6e- + 14H + (aq) + Cr 2 O 7 (aq) 2Cr 3+ (aq) + 7H 2 O(aq) 3H 2 O(l) + 3SO 3 (aq) 3SO 4 (aq) + 6H + (aq) + 6e- 4 Final Balanced Equation: Cr 2 O 7 + 3SO 3 + 8H + 2Cr SO 4 + 4H 2 O Copyright Cengage Learning. All rights reserved 20

21 Section 18.4 Balancing Oxidation Reduction Reactions by the Half-Reaction Method Exercise When the reaction Ce 2+ + Co 2+ Ce 3+ + Co is balanced, the coefficient in front of Ce 2+ is a) 0 b) 1 c) 2 d) 3 Ce 2+ Ce 3+ +1e- 2e- + Co 2+ Co 2Ce 2+ + Co 2+ 2Ce 3+ + Co Copyright Cengage Learning. All rights reserved 21

22 Section 18.4 Balancing Oxidation Reduction Reactions by the Half-Reaction Method Exercise Balance the following oxidation reduction reaction that occurs in acidic solution. Br (aq) + MnO 4 (aq) Br 2 (l)+ Mn 2+ (aq) 10Br (aq) + 16H + (aq) + 2MnO 4 (aq) 5Br 2 (l)+ 2Mn 2+ (aq) + 8H 2 O(l) Copyright Cengage Learning. All rights reserved 22

23 Section 18.5 Electrochemistry: An Introduction Electrochemistry The study of the interchange of chemical and electrical energy. Two types of processes: Production of an electric current from a chemical reaction. The use of electric current to produce a chemical change. Copyright Cengage Learning. All rights reserved 23

24 Section 18.5 Electrochemistry: An Introduction Making an Electrochemical Cell 8H + + MnO 4 + 5e Mn H 2 O Fe 2+ Fe 3+ + e Copyright Cengage Learning. All rights reserved 24

25 Section 18.5 Electrochemistry: An Introduction Making an Electrochemical Cell If electrons flow through the wire charge builds up. Solutions must be connected to permit ions to flow to balance the charge. Copyright Cengage Learning. All rights reserved 25

26 Section 18.5 Electrochemistry: An Introduction Making an Electrochemical Cell A salt bridge or porous disk connects the half cells and allows ions to flow, completing the circuit. Copyright Cengage Learning. All rights reserved 26

27 Section 18.5 Electrochemistry: An Introduction Electrochemical Battery (Galvanic Cell) Device powered by an oxidation reduction reaction where chemical energy is converted to electrical energy. Anode electrode where oxidation occurs Cathode electrode where reduction occurs Copyright Cengage Learning. All rights reserved 27

28 Section 18.5 Electrochemistry: An Introduction Electrolysis Process where electrical energy is used to produce a chemical change. Nonspontaneous Copyright Cengage Learning. All rights reserved 28

29 Section 18.6 Batteries Lead Storage Battery Anode reaction oxidation Pb + H 2 SO 4 PbSO 4 + 2H + + 2e Cathode reaction reduction PbO 2 + H 2 SO 4 + 2e + 2H + PbSO 4 + 2H 2 O Copyright Cengage Learning. All rights reserved 29

30 Section 18.6 Batteries Lead Storage Battery Overall Reaction Pb(s) + PbO 2 (s) + 2H 2 SO 4 (aq) 2PbSO 4 (s) + 2H 2 O(l) Hydrometer to measure H 2 SO 4 concentration. As the battery discharges the sulfate of the acid precipitates with the lead taking it out of solution and reducing the acid concentration. As the battery is recharged the current goes into dissolving the lead sulfate restoring the acid concentration. Copyright Cengage Learning. All rights reserved 30

31 Section 18.6 Batteries Electric Potential The pressure on electrons to flow from anode to cathode in a battery, like water flow. Copyright Cengage Learning. All rights reserved 31

32 Section 18.6 Batteries Dry Cell Batteries Do not contain a liquid electrolyte. Acid version Anode reaction oxidation Zn Zn e Cathode reaction reduction 2NH MnO 2 + 2e Mn 2 O 3 + 2NH 3 + 2H 2 O Copyright Cengage Learning. All rights reserved 32

33 Section 18.6 Batteries Dry Cell Batteries Do not contain a liquid electrolyte. Alkaline version Anode reaction oxidation Zn + 2OH ZnO + H 2 O + 2e Cathode reaction reduction 2MnO 2 + H 2 O + 2e Mn 2 O 3 + 2OH Copyright Cengage Learning. All rights reserved 33

34 Section 18.6 Batteries Dry Cell Batteries Do not contain a liquid electrolyte. Other Types Silver cell Zn anode, Ag 2 O cathode Mercury cell Zn anode, HgO cathode Nickel-cadmium rechargeable Copyright Cengage Learning. All rights reserved 34

35 Section 18.7 Corrosion The oxidation of metals to form mainly oxides and sulfides. Some metals, such as aluminum, protect themselves with their oxide coating. Corrosion of iron can be prevented by coatings, by alloying and cathodic protection. Cathodic protection of an underground pipe. Copyright Cengage Learning. All rights reserved 35

36 Section 18.8 Electrolysis Forcing a current through a cell to produce a chemical change that would not otherwise occur. Copyright Cengage Learning. All rights reserved 36

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