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Chapter 14

Chapter 14. Oxidation-Reduction Reactions. Setting the Stage – Electron Flow. The flow of electrons is common. It occurs naturally (lightning) and as a result of human activity (electricity). Early electrical experiments involved chemically generated electricity (batteries).

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Chapter 14

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  1. Chapter 14 Oxidation-Reduction Reactions

  2. Setting the Stage – Electron Flow • The flow of electrons is common. • It occurs naturally (lightning) and as a result of human activity (electricity). • Early electrical experiments involved chemically generated electricity (batteries). • This electron flow results from one reactant having a greater affinity for electrons than the other. Malone and Dolter - Basic Concepts of Chemistry 9e

  3. Setting a Goal – Part ARedox Reactions – The Exchange of Electrons • You will learn of an important classification of chemical reactions that involves an exchange of electrons. Malone and Dolter - Basic Concepts of Chemistry 9e

  4. Objective for Section 14-1 • Calculate oxidation states and determine the species oxidized, the species reduced, the oxidizing agent, and the reducing agent in an electron exchange reaction. Malone and Dolter - Basic Concepts of Chemistry 9e

  5. Professor’s Little Jokes An electron is setting in jail with her room mate, who asks, “Why did they put you in prison?” The electron replies, “For making a forbidden transition”. What do two parting dipoles say to each other? Answer: “Debye”! A male polar bear and a female brown bear are sitting at a bar. The polar bear says, “Sorry babe, I just don't think The chemistry is right”. Question: "If H2O is water, what is H2O4?" Answer: “Drinking and washing, of course! Are you awake yet?” Malone and Dolter - Basic Concepts of Chemistry 9e

  6. 14-1 The Nature of Oxidation and Reduction and Oxidation States • Na reacts with Cl2 vigorously, such that the piece of Na glows white hot with the heat of the reaction. • The process forms ordinary table salt:2 Na(s) + Cl2(g)  2 NaCl(s) Malone and Dolter - Basic Concepts of Chemistry 9e

  7. Half Reactions • Electron exchange reactions can be viewed as the sum of two half reactions. • Half reactions represent either the loss of electrons or the gain of electrons as a separate balanced equation. • Sodium half reaction:Na  Na+ + e- • Na loses an electron to form a sodium ion. • A substance that loses electrons is said to be oxidized. Malone and Dolter - Basic Concepts of Chemistry 9e

  8. Half Reactions • Chlorine half reaction2 e- + Cl2 2 Cl- • The neutral chlorine molecule has gained two electrons to form chloride ions. • A substance that gains electrons is said to be reduced. Malone and Dolter - Basic Concepts of Chemistry 9e

  9. Terminology • Redox or oxidation-reduction reactions are reactions involving the exchange of electrons. • Oxidizing agent – the species that accepts electrons. • Reducing agent – the species that donates electrons. Malone and Dolter - Basic Concepts of Chemistry 9e

  10. Redox Reactions • Electrons are conserved in redox reactions. • For example, electrons gained in the reduction process must equal the electrons lost in the oxidation process. • A common, and unfortunate, redox reaction is the rusting of iron:4 Fe(s) + 3 O2 (g)  2 Fe2O3 (s) Malone and Dolter - Basic Concepts of Chemistry 9e

  11. Oxidation States or Numbers • The charge that an atom in a molecule or ion would have if all atoms were present as monatomic ions. • Similar to formal charge, but in this case, both electrons in the bond are assigned to the more electronegative atom sharing the electrons. Malone and Dolter - Basic Concepts of Chemistry 9e

  12. Rules for Assigning Oxidation State 1. OS of an element in its free, natural state is zero [Cu(s), Cl2 (g), Hg(l)] 2.The OS of a monoatomic ion is the same as the charge of the ionalkali metals +1 same as group #alkaline earths +2 same as group #oxygen O2-aluminum Al3+ Malone and Dolter - Basic Concepts of Chemistry 9e

  13. Rules for Assigning Oxidation State…..Cont’d 3.Halogens are in a -1 oxidation state in binary compounds whether ionic or covalent when bound to a less electronegative element (e.g. as in HCl). 4.Oxygen is O2- except in peroxides and superoxides. Oxygen is positive when bound to F. Malone and Dolter - Basic Concepts of Chemistry 9e

  14. Rules for Assigning Oxidation State…..Cont’d 5. H is usually +1. When bound to a less electronegative atom (usually a metal) it is -1 (as in LiH). 6. The sum of the OS of all of the atoms in a compound must equal the charge of the compound or ion.e. g. FeO has Fe2+ and O2- Malone and Dolter - Basic Concepts of Chemistry 9e

  15. Rules for Assigning Oxidation State…..Cont’d. Examples Malone and Dolter - Basic Concepts of Chemistry 9e

  16. Oxidation State Changes • Oxidation is the loss of electrons, and therefore results in an increase in the oxidation state. Reduction results in the decrease in the oxidation state. • Generally, only one element changes oxidation state in a compound, but this is not always true. Malone and Dolter - Basic Concepts of Chemistry 9e

  17. Objective for Section 14-2 • Balance redox reactions by the bridge method. Malone and Dolter - Basic Concepts of Chemistry 9e

  18. 14-2 Balancing Redox Equations: Oxidation State Method • The oxidation state method focuses on the atoms of the elements undergoing a change in oxidation state. • ConsiderHNO3 (aq) + H2S(aq)  NO(g) + S(s) + H2O(l) Malone and Dolter - Basic Concepts of Chemistry 9e

  19. Oxidation State Method….cont’d 1. Identify the atoms whose OS have changed. 2. Draw a bridge between the same atoms whose OS has changed, indicating the electrons gained or lost. Malone and Dolter - Basic Concepts of Chemistry 9e

  20. Oxidation State Method….cont’d 3. Multiply the two numbers of electrons (in this case +3 and -2) by whole numbers that produce a common number:3 × 2 = 6 2 × 3 = 6Use these multipliers as coefficients of the respective compounds or elements. Malone and Dolter - Basic Concepts of Chemistry 9e

  21. Oxidation State Method….cont’d 4. Balance the rest of the elements by inspection2 HNO3 + 3 H2S  2 NO + 3 S + 4 H2O Malone and Dolter - Basic Concepts of Chemistry 9e

  22. Objective for Section 14-3 • Balance redox reactions by the ion-electron method. Malone and Dolter - Basic Concepts of Chemistry 9e

  23. 14-3 Balancing Redox Equations: Ion-Electron Method • Ion-electron or half reaction method: 1. Separate the total reaction into half reactions. 2. Balance the half reactions separately. 3. Methodology depends on whether the reactions are done in acidic or basic conditions. Malone and Dolter - Basic Concepts of Chemistry 9e

  24. Half Reaction Method • ConsiderH+(aq) + Cl-(aq) + Cr2O72-(aq) Cr3+(aq) + Cl2(aq) + H2O(l) 1.Separate the molecule or ion that contains atoms of an element that have changed oxidation state and product containing the atoms of that element. (You don’t need to know the actual oxidation state). Malone and Dolter - Basic Concepts of Chemistry 9e

  25. Half Reaction Method…..Cont’d 1. Cr2O72- Cr3+ (reduction) 2.Balance the atoms other than hydrogen or oxygen.Cr2O72- 2Cr3+ 3.Balance the oxygen by adding H2O on the side missing the oxygen. Malone and Dolter - Basic Concepts of Chemistry 9e

  26. Half Reaction Method…..Cont’d 4. Cr2O72- 2Cr3++ 7 H2O 5. Balance the hydrogens by adding H+ on the other side of the equation from the H2O14 H+ + Cr2O72- 2Cr3++ 7 H2O 6. The atoms in the half reaction should now be balanced. Malone and Dolter - Basic Concepts of Chemistry 9e

  27. Half Reaction Method…..Cont’d 7. Add e- to balance the charge on both sides of the equation:6 e- + 14 H+ + Cr2O72- 2Cr3++ 7 H2O 8. Do the same for other half reaction:2 Cl-  Cl2 + 2 e- (oxidation) 9. Multiply each equation by a coefficient so that the number of electrons is the same in both half reactions. Malone and Dolter - Basic Concepts of Chemistry 9e

  28. Half Reaction Method…..Cont’d 10. 3 (2 Cl-  Cl2 + 2 e-) 6 Cl-  3 Cl2 + 6 e- 11. 6 e- + 14 H+ + Cr2O72- 2Cr3++ 7H2O6 Cl-  3 Cl2 + 6 e- _Add________________________________14 H+ + Cr2O72- + 6 Cl-  3 Cl2 + 2Cr3++ 7 H2O Malone and Dolter - Basic Concepts of Chemistry 9e

  29. Half Reaction Method –Alkaline Solution • In alkaline solution, OH- is the predominant species (besides water). • The simplest way to adjust for basic solution is to balance the reaction as if it occurred in acid, then neutralize the H+ by adding OH- to both sides. • The H+ will combine with the OH-. Malone and Dolter - Basic Concepts of Chemistry 9e

  30. Example Half equation for the reduction of hypochlorite to chloride in alkaline solution • 2 e- + 2 H+ + ClO- Cl- + H2O • 2 e- + (2 H+ + 2OH-) + ClO- Cl- + H2O + 2 OH- • 2 e- + 2 H2O + ClO- Cl- + H2O + 2 OH- • 2 e- + H2O + ClO- Cl- + 2 OH- Malone and Dolter - Basic Concepts of Chemistry 9e

  31. Spontaneous Redox Reactions • A spontaneous reaction occurs between the stronger oxidizing agent and the stronger reducing agent to form weaker oxidizing and reducing agents. • By testing reactions between pairs of atoms and molecules, we can determine a relative order of strength of oxidizing or reducing ability. Malone and Dolter - Basic Concepts of Chemistry 9e

  32. Setting a Goal – Part BSpontaneous and Nonspontaneous Redox Reactions • You will understand the extensive practical applications of redox reactions. Malone and Dolter - Basic Concepts of Chemistry 9e

  33. Objective for Section 14-4 • Using a table of relative strengths of oxidizing agents, determine whether a specific redox reaction is spontaneous. Malone and Dolter - Basic Concepts of Chemistry 9e

  34. 14-4 Predicting Spontaneous Redox Reactions • Conventionally, we write redox reactions as reductions in order of increasing reducing ability (Table 14-1; most reducing at the bottom) • We can get quantitative values for the comparative strength of a given reducing agent. These are known as reduction potentials. The species are compared in solution under standard conditions (1.00 M concentration and 1.00 atm). Malone and Dolter - Basic Concepts of Chemistry 9e

  35. Malone and Dolter - Basic Concepts of Chemistry 9e

  36. Activity Series Table • Table 14-1 is the activity series table. • A favorable reaction occurs between an element, ion or compound on the left (an oxidizing agent) with a species on the right (a reducing agent) that lies below it in the table. Malone and Dolter - Basic Concepts of Chemistry 9e

  37. Example: Reaction between Cl2 and H2O • Cl2 lies above H2O in Table 14-1 so it is a stronger oxidizing agent.Cl2 + 2 e-2 Cl-O2 + 4 H+ + 4 e- 2 H2O2 Cl2 + 2 H2O O2 + 4 H+ + 4 Cl- (slow but favorable) Malone and Dolter - Basic Concepts of Chemistry 9e

  38. Example: Reaction between Na and H2O • Na lies below H2O in Table 14-1, so water will oxidize Na.2 H2O + 2 e-H2 + 2OH-Na+ + e- Na2 H2O + 2 Na H2 + 2OH-+ 2 Na+ • Active metals are those that react with water and therefore lie below water in the activity series. Malone and Dolter - Basic Concepts of Chemistry 9e

  39. Example: Reaction between Fe and Acid solution • 2 H+ + 2 e- H2Fe2+ + 2 e- Fe2 H+ + Fe H2 + Fe2+ • Higher concentrations of acid make this reaction more favorable. • This is one of the problems with acid rain in that it accelerates the rusting of many metal structures. Malone and Dolter - Basic Concepts of Chemistry 9e

  40. Objectives for Section 14-5 • Describe the parts of a voltaic cell and how it generates electricity. • List the types of batteries discussed and a particular use or advantage of each type. Malone and Dolter - Basic Concepts of Chemistry 9e

  41. 14-5 Voltaic Cells • Use a favorable redox reaction to generate electrical energy through an external circuit. • First practical example was the Danielle cell.Zn(s) + Cu2+(aq)  Zn2+ (aq) + Cu(s)It was used to power telegraphs and other early electrical devices. • To generate electricity, we must separate the half cells physically. Malone and Dolter - Basic Concepts of Chemistry 9e

  42. The Daniell Cell • A Zn strip is immersed in a Zn2+ solution and in a separate compartment, a Cu strip is immersed in a Cu2+ solution. • Electrodes are the surfaces in a cell at which the reactions take place. • The Zn and Cu strips serve as electrodes. • Refer to Figure 14-4. Malone and Dolter - Basic Concepts of Chemistry 9e

  43. Figure 4.4 The Daniell Cell Malone and Dolter - Basic Concepts of Chemistry 9e

  44. Electrodes • Anode – the electrode at which the oxidation takes place. • Cathode – the electrode at which the reduction takes place. • Active electrodes, such as the Zn and Cu strips, participate in the redox reaction. • Inert electrodes merely provide a surface at which the reaction occurs. Malone and Dolter - Basic Concepts of Chemistry 9e

  45. Salt Bridge • Since negative charge is flowing through the external circuit, a means must be provided to keep charge balanced. • The salt bridge allows inert ions to pass between the two compartments while preventing the species involved in the redox reaction from mixing. Malone and Dolter - Basic Concepts of Chemistry 9e

  46. Common Voltaic Cells • Dry cells (flashlight batteries) • Lead-acid car batteries (keep in mind that a battery is a collection of one or more separate cells joined together in one unit) Malone and Dolter - Basic Concepts of Chemistry 9e

  47. Dry Cells • Not rechargeable, but cheap and portable. • Zn casing serves as the anode and a graphite rod serves as an inert cathode. • In between the two is an aqueous paste containing NH4Cl, MnO2 and carbon.anode: Zn(s)  Zn2+(aq) + 2 e-cathode: 2NH4+(aq) + 2MnO2(s) + 2 e-  Mn2O3(s) + 2 NH3(aq) + H2O(l) Malone and Dolter - Basic Concepts of Chemistry 9e

  48. The Dry Cell • The disadvantage of the dry cell is that it creates an acidic solution. • This acidic solution slowly dissolves the zinc, so the shelf life of the battery is only a few months. • Alkaline batteries use NaOH or KOH in place of NH4Cl, which is more expensive but the shelf life is much greater. The anode reaction becomes:Zn(s) + 2 OH-(aq)  ZnO(s) + H2O(l) + 2 e- Malone and Dolter - Basic Concepts of Chemistry 9e

  49. The Lead Acid Car Battery • Anode:Pb(s) + H2SO4(aq)  PbSO4(s) + 2H+(aq) + 2 e- • Cathode:2 e- + 2 H+(aq) + PbO2(s) + H2SO4(aq)  PbSO4(s) + 2 H2O(l) • Total reaction: Pb(s) + PbO2(s) + 2 H2SO4(aq)  2 PbSO4(s) + 2 H2O(l) Malone and Dolter - Basic Concepts of Chemistry 9e

  50. Other Batteries • Silver:Zn(s) + Ag2O(s)  ZnO(s) + 2Ag(s) • Mercury:Zn(s) + HgO(s)  ZnO(s) + Hg(l) • Ni/Cd:Cd(s) + NiO2(s) + 2 H2O(l)  Ni(OH)2(s) + Cd(OH)2(s) (rechargeable) Malone and Dolter - Basic Concepts of Chemistry 9e

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