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Ch. 20: Oxidation-Reduction Reactions

Ch. 20: Oxidation-Reduction Reactions. LEO the lion goes “GER”—losing electrons is oxidation and gaining electrons is reduction!. 20.1: The Meaning of Oxidation and Reduction (REDOX for short)

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Ch. 20: Oxidation-Reduction Reactions

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  1. Ch. 20: Oxidation-Reduction Reactions LEO the lion goes “GER”—losing electrons is oxidation and gaining electrons is reduction! • 20.1: The Meaning of Oxidation and Reduction (REDOX for short) • --oxidation is the loss of electrons or gain of oxygen; reduction is the gain of electrons or loss of oxygen--the substance that loses electrons is the reducing agent; the substance that accepts electrons is the oxidizing agent--no oxidation occurs without reduction and vice versa; oxidation and reduction occur simultaneously • 20.2: Oxidation Numbers • --An increase in the oxidation number of an atom or ion indicates oxidation; a decrease in the oxidation number of an atom or ion indicates reduction • --assigning oxidation numbers (generally the same as charges on monatomic ions; 0 for free elements; -2 for oxygen; +1 for hydrogen) • 20.3: Balancing Redox Equations • Two methods—oxidation number charges and half-reactions

  2. 20.1 20.1: The Meaning of Oxidation and Reduction Salt on icy roads can make driving safer, but the salt that clings to the metallic parts of cars can cause them to corrode or rust relatively quickly. This corrosion is one example of a chemical reaction called oxidation-reduction. You will learn about oxidation-reduction reactions.

  3. 20.1 What Are Oxidation and Reduction? • The substance gaining oxygen is oxidized, while the substance losing oxygen is reduced. • The substance oxidized is the reducing agent, while the substance reduced is the oxidizing agent • These reactions are called oxidation-reduction reactions and are also known as redoxreactions

  4. 20.1 What Are Oxidation and Reduction? A bunsen burner oxidizes the methane in natural gas to carbon dioxide and water.

  5. 20.1 What Are Oxidation and Reduction? Carbon is oxidized when charcoal burns.

  6. 20.1 What Are Oxidation and Reduction? Iron is oxidized when it rusts. The common reactant in the last three equations is oxygen. It is the oxidizing agent (therefore, it is reduced. In this example, iron is oxidized.

  7. 20.1 What Are Oxidation and Reduction? Electron Shift in Redox Reactions Losing electrons is oxidation. Gaining electrons is reduction (LEO goes “GER”) • Oxidation is now defined to mean complete or partial loss of electrons or gain of oxygen. • Reduction is now defined to mean complete or partial gain of electrons or loss of oxygen.

  8. 20.1 What Are Oxidation and Reduction?

  9. 20.1 What Are Oxidation and Reduction?

  10. 20.1 What Are Oxidation and Reduction? • The substance that loses electrons is called the reducing agent. The substance that accepts electrons is called the oxidizing agent.

  11. 2Na+S2Na+ + S2- Therefore, Na 2Na1+ + 2e- and S+2e-S2- and Na loses e-, oxidation, reducing agentS gains e-, reduction, oxidizing agent Al loses e-, oxidation, reducing agentO gains e-, reduction, oxidizing agent

  12. 20.1 What Are Oxidation and Reduction? Redox With Covalent Compounds Generally the more electronegative element is reduced

  13. 20.1 What Are Oxidation and Reduction? GER LEO reduction oxidizing agent gain of electrons loss of oxygen decrease in oxidation number oxidation reducing agent loss of electrons gain of oxygen increase in oxidation number

  14. 20.1 Corrosion Iron, a common construction metal often used in the form of the alloy steel, corrodes by being oxidized to ions of iron by oxygen.

  15. 20.1 Corrosion Resistance to Corrosion Aluminum resists corrosion because it forms a protective coating of aluminum oxide.

  16. 20.1 Corrosion Controlling Corrosion Painting a surface protects it from corrosion. Chromium metal also serves as a protective coating.

  17. 20.1 Corrosion Zinc blocks attached to the steel hull of this ship oxidize instead of the iron, preventing corrosion.

  18. 20.1 Section Quiz. 1. Choose the correct words for the spaces. During oxidation, an element _______ electrons. During reduction, an element ________ electrons. a. gains, loses b. loses, gains c. gains, donates d. gains, accepts

  19. 20.1 Section Quiz. 2. Choose the correct words for the spaces. During reduction, a substance may _______ oxygen or ________ hydrogen. a. gain, lose b. lose, gain c. gain, gain d. lose, lose

  20. 20.1 Section Quiz.. 3. The element that donates electrons in a redox reaction is called the reducing agent. This means the reducing agent is always a. the more active element. b. the less active element. c. oxidized. d. reduced.

  21. 20.1 Section Quiz. 4. Which substance is the reducing agent in the following chemical reaction? Fe2O3 + 3CO  2Fe + 3CO2 a. Fe2O3 • CO • Fe d. CO2

  22. 20.1 Section Quiz. 5. The following equations describe the corrosion of iron to iron hydroxides in moist conditions. 2Fe(s) + O2(g) + 2H2O(l)  2Fe(OH)2(s) 4Fe(OH)2(s) + O2(g) + 2H2O(l) 4Fe(OH)3(s) What substances in the equations are being oxidized? a. Fe(s) and Fe(OH)2(s) b. the O2(g) in both reactions c. the O2(g) and H2O(l) in both reactions d. Fe(OH)2(s) and Fe(OH)3(s)

  23. 20.2 20.2: Oxidation Numbers When aluminum or magnesium powder in fireworks are heated to high temperatures in the explosion, they burn with an intense white light. As elements burn, their oxidation numbers change. You will learn about how oxidation and reduction are defined in terms of a change in oxidation number.

  24. 20.2 Assigning Oxidation Numbers • An oxidation number is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction. • As a general rule, a bonded atom’s oxidation number is the charge that it would have if the electrons in the bond were assigned to the atom of the more electronegative element.

  25. 20.2 Assigning Oxidation Numbers The oxidation number of any element in the free or uncombined state is zero.

  26. 20.2 Assigning Oxidation Numbers

  27. 20.2 Assigning Oxidation Numbers Oxidation numbers are often written above the chemical symbols in a formula. Potassium chromate K2CrO4 Cr oxidation number = +6 Chromium oxide Cr2O3 Cr oxidation number = +3

  28. Determine the oxidation number of each element in the following. • S2O3 • N2O2 • NO3- • CaCO3 a. S is +3; O is -2 b. N is +2; O is -2 c. N is +5; O is -2 d. Ca is +2; C is +4; O is -2

  29. 20.2 Oxidation-Number Changes in Chemical Reactions An increase in the oxidation number of an atom or ion indicates oxidation. A decrease in the oxidation number of an atom or ion indicates reduction.

  30. a. H2 is oxidized; O2 is reduced b. O is oxidized; N is reduced

  31. 20.2 Section Quiz. 1.Which of the following oxidation numbers is INCORRECT? a. ClO3– ; Cl +5, O –2 b. SO32– ; S +4, O –2 c. H2O2; H +1, O –1 d. Ca(ClO)2; Ca +2, Cl +2, O –2

  32. 20.2 Section Quiz. 2. What is the oxidation number of Co in CoCl63–? a. –3 b. –6 c. +9 d. +3

  33. 20.2 Section Quiz. 3. In the reaction PbO2(aq) + 4HBr(aq)  PbBr2(aq) + Br2(l) + 2H2O(l) which element has the greatest change in oxidation number? a. hydrogen b. oxygen c. bromine d. lead

  34. 20.3 20.3: Balancing Redox Equations When concentrated hydrochloric acid (HCl) and nitric acid (HNO3) are mixed with gold, oxidation and reduction reactions produce gaseous nitrogen monoxide (NO) and soluble, stable AuCl4– ions. You will learn how to write and balance chemical equations for redox reactions such as this one.

  35. 20.3 Identifying Redox Reactions If the oxidation number of an element in a reacting species changes, then that element has undergone either oxidation or reduction. Therefore, the reaction as a whole must be a redox reaction. K + H2O KOH + H2 2Zn + HCl ZnCl2 + H2

  36. 20.3 Identifying Redox Reactions A redox reaction takes place between nitrogen and oxygen when lightning bolts heat the air. • A color change can signal a redox reaction.

  37. for Conceptual Problem 20.4 a. Not a redox reaction b. Redox reaction; H is oxidized, Cu is reduced

  38. 20.3 Two Ways to Balance Redox Equations • oxidation-number-change method • Using half-reactions • Using Oxidation-Number Changes • In the oxidation-number-change method, you balance a redox equation by comparing the increases and decreases in oxidation numbers. • Using Half-Reactions • A half-reaction is an equation showing just the oxidation or just the reduction that takes place in a redox reaction. • In the half-reaction method, you write and balance the oxidation and reduction half-reactions separately before combining them into a balanced redox equation.

  39. 20.3 Choosing a Balancing Method • Balancing by oxidation number change usually works well if the oxidized and reduced species appear only once on each side of the equation. • Balancing by the half-reaction method works best in redox reactions where the same element is both oxidized and reduced and in reactions that take place in acidic or alkaline solution.

  40. 20.3 Oxidation-number-change method Balance using the oxidation-number-change method: • Step 1Assign oxidation numbers to all the atoms in the equation. • Step 2Identify which atoms are oxidized and which are reduced.

  41. 20.3 Oxidation-number-change method • Step 3Use one bracketing line to connect the atoms that undergo oxidation and another such line to connect those that undergo reduction. • In a balanced redox equation, the total increase in oxidation number of the species oxidized must be balanced by the total decrease in the oxidation number of the species reduced.

  42. 20.3 Oxidation-number-change method • Step 4Make the total increase in oxidation number equal to the total decrease in oxidation number by using appropriate coefficients. • Step 5 Finally, make sure that the equation is balanced for both atoms and charge.

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