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Section 4.4

Section 4.4. Oxidation-Reduction Reactions. Review of Reactions. Precipitation: cations and anions form insoluble ionic compound Neutralization : H + and OH - ions form H 2 O. A third kind of reaction involves electron transfers between reactants 

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Section 4.4

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  1. Section 4.4 Oxidation-Reduction Reactions

  2. Review of Reactions • Precipitation: cations and anions form insoluble ionic compound • Neutralization: H+ and OH- ions form H2O A third kind of reaction involves electron transfers between reactants Oxidation-reduction or REDOX reactions

  3. Oxidation-Reduction Reactions • An oxidation occurs when an atom or ion loses electrons. • A reduction occurs when an atom or ion gains electrons.

  4. Oxidation-Reduction Reactions One cannot occur without the other.

  5. Oxidation Numbers • Oxidation number: assigned to each element in a neutral compound or charged entity • Oxidation # = oxidation • Oxidation # = reduction

  6. 4 Rules for Oxidation Numbers 1. Elements in their elemental form have an oxidation number of 0. -Ex: H in H2 2. The oxidation number of a monatomic ion is the same as its charge. -Ex: K+ = +1; S2- = -2

  7. 3. Nonmetals tend to have negative oxidation numbers, except: • Oxygen is −2, except in the peroxide ion (O22-) in which it is −1. • Hydrogen is −1 when bonded to a metal, +1 when bonded to a nonmetal. • Fluorine always is −1. The other halogens are −1, BUT positive when combined with oxygen (oxyanions)

  8. 4. The sum of the oxidation numbers in a neutral compound is 0. The sum of the oxidation numbers in a polyatomic ion is the charge on the ion. -Ex: H3O+ = 3(+1) + (-2) = +1

  9. Oxidation Numbers

  10. Oxidation of Metals by Acids/Salts • Many kinds of redox reactions • Focus: redox reaction of metals with acid or salt A + BX AX + B Ex: Zn(s) + 2HBr(aq)  ZnBr2 (aq)+ H2 (g) What type of reaction does this look like?

  11. Displacement Reactions • The ion in a solution is displaced through oxidation of an element.

  12. Displacement with Metals and Acids • Metal + acid  salt + H2 Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g) 0------------------------+2 +1------------------------------0

  13. Displacement with Metals and Salts • Metals can be oxidized by aqueous solutions of salts Molecular Equation Fe(s) + Ni(NO3)2(aq)  Fe(NO3)2 (aq) + Ni(s) Net Ionic Equation Fe(s) + Ni 2+ (aq)  Fe 2+ (aq) + Ni(s)

  14. Remember… • Whenever one substance is oxidized (loses electrons/becomes more positive), some other substance must be reduced (gains electrons/becomes less positive).

  15. Activities Series • List of metals arranged in order of decreasing ease of oxidation • Alkali and alkaline earth metals: Active metals (most easily oxidized ) • Transition metals at the bottom of list:Noble metals (low reactivity) Any metal on the list can be oxidized by the ions of elements below it

  16. Activity Series

  17. Homework • 4.49-4.52 and 4.58

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