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Chapter 20

Chapter 20. Redox Reactions. What is oxidation?. Oxidation occurs when an atom loses electrons. (OIL) Oxidation may or may not involve oxygen Oxidation may or may not involve burning material. What is reduction?. Reduction occurs when an atom gains electrons. (RIG)

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Chapter 20

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  1. Chapter 20 Redox Reactions

  2. What is oxidation? • Oxidation occurs when an atom loses electrons. (OIL) • Oxidation may or may not involve oxygen • Oxidation may or may not involve burning material

  3. What is reduction? • Reduction occurs when an atom gains electrons. (RIG) • Reduction and oxidation occur simultaneously. • These reactions are referred to as “REDOX” • OIL RIG or LEO the Lion says GER

  4. Reduction usually occurs with nonmetals. Why? -Because they have a high electron affinity (love of electrons), and high electronegativity.

  5. Redox reactions forming Ions… • Oxidation usually occurs with metals, due to their low ionization energies. • They to lose electrons readily

  6. Example reactions • Lio Li+ + e- since lithium went from 0 to +1 it lost an electron. Is this oxidation or reduction? A: Oxidation • Clo +e-  Cl- • Since chlorine went from 0 to -1, did oxidation or reduction occur? A: Reduction

  7. Oxidizing and reducing Agents • A reducing agent is a substance that has the potential of causing another substance to be reduced.(metals) • An oxidizing agent is a substance that has the potential of causing another substance to be oxidized.(nonmetals)

  8. Redox with covalent Compounds • Nonmetals can undergo changes in oxidation state when forming covalently bonded compounds • Oxidation number depends on electronegativity of the bonded atoms (bond polarity) • Elements involved in redox reactions often have several different oxidation states For example…

  9. Corrosion • Refers to reaction of oxygen with a metal • What can accelerate this process? -salts -acids -water Some metals are resistant to corrosion. Name some… Why would they be resistant?

  10. Some metals are resistant to corrosion because their electrons are not easily lost (Au & Pt) • Some metals naturally form an oxide (thin layer)which protects the metal from further corrosion (Al & Zn). Like our chains!

  11. Ways to prevent corrosion? • Paint, plastic, oil, or other metals may be applied to slow or prevent the corrosion process. • If coated with other metals, metal coating “sacrifices” itself to save the “important” metal -This depends on activity of metals -Less active metal keeps its electrons (therefore does not corrode as easily)

  12. Section 20.2Oxidation Numbers • OBJECTIVES • Determinethe oxidation number of an atom of any element in a pure substance.

  13. Section 20.2Oxidation Numbers OBJECTIVES Defineoxidation and reduction in terms of a change in oxidation number, and identify atoms being oxidized or reduced in redox reactions.

  14. Assigning Oxidation Numbers • An “oxidation number” is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction. • Generally, a bonded atom’s oxidation number is the charge it would have if the electrons in the bond were assigned to the atom of the more electronegative element

  15. Rules for Assigning Oxidation Numbers • The oxidation number of any uncombined element is zero. • The oxidation number of a monatomic ion equals its charge.

  16. Rules for Assigning Oxidation Numbers • The oxidation number of oxygen in compounds is -2, except in peroxides, such as H2O2 where it is -1. • The oxidation number of hydrogen in compounds is +1, except in metal hydrides, like NaH, where it is -1.

  17. Rules for Assigning Oxidation Numbers • The sum of the oxidation numbers of the atoms in the compound must equal 0. 2(+1) + (-2) = 0 H O (+2) + 2(-2) + 2(+1) = 0 Ca O H

  18. Rules for Assigning Oxidation Numbers • The sum of the oxidation numbers in the formula of a polyatomic ion is equalto its ionic charge. X + 4(-2) = -2 S O X + 3(-2) = -1 N O thus X = +6 thus X = +5

  19. Try these! • HNO3 • N2O3 • NH3 • NH4Cl

  20. Oxidation Numbers • Do the worksheets! • Half reaction example… Cl2 + 2Fe2+ → 2Cl− + 2Fe3+ Fe2+ + Energy → Fe3+ + e− Cl2 + 2e− → 2Cl− + Energy

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