Exp 19A: Oxidation-Reduction Reactions

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Exp 19A Oxidation-Reduction Reactions

• Oxidation-Reduction (Redox) reactions
• Net movement of electrons from one reactant to
  the other
• Movement from reactant with less attraction to
  reactant with more attraction for electrons
  (larger electron affinity)
• Occurs in both ionic and covalent bond formation
• Examples
• 2 Mg(s) O2(g) ? 2 MgO (s) ionic bond formation
• transfer of electrons (formation of
  cations and anions)
• H2(g) Cl2(g) ? 2 HCl(g) covalent bond
  formation
• shift in electron charge (dipole,
  polarity)

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Exp 19A Oxidation-Reduction Reactions
Oxidation loss of electrons Mg ? Mg2
2e- Reduction gain in electrons ½ O2 2e-
? O2-
transfer or shift of electrons
X looses electron(s)
Y gains electron(s)
X is oxidized
Y is reduced
X is the reducing agent
Y is the oxidizing agent
X increases its oxidation number
Y decreases its oxidation number

• reducing agents loose electrons while oxidizing
  agents gain electrons simultaneously
• a chemical change cannot be an oxidation
  reaction or just a reduction reaction
• it is always an oxidation-reduction reaction

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Exp 19A Oxidation-Reduction Reactions

• Goal of the Experiment (I)
• Observe redox reactions between halogens
  (oxidizers) and halogen ions (halides, reducers)
  in cyclohexane
• Halide ions (polar!) are insoluble in non-polar
  cyclohexane
• Halogens (non-polar!) are soluble in cyclohexane
• If a reaction takes place, the color in the
  cyclohexane may be different from the color in
  water, but ..
• You have to make very careful observations and
  THINK about the results

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Redox Activity of Halogens

• F2 gt Cl2 gt Br2 gt I2
• Redox potential and reactivity decreases down
  Group 7A
• A halogen higher in the periodic system oxidizes
  one that is lower in the periodic system
• Chlorine can oxidize bromide
• -1 0 0 -1
• 2 Br- (aq) Cl2 (aq) ? Br2 (aq) 2 Cl- (aq)
• Bromine cannot oxidize chloride
• 2 Cl- (aq) Br2 (aq) ? no reaction

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Exp 19A Oxidation-Reduction Reactions

• Goal of the Experiment (II)
• Observe redox reactions with permanganate and
  iron(III) ions
• Reduction of
• MnO4- (permanganate, purple) to MnO42-
  (manganate, green)
• MnO4- (permanganate, purple) to MnO2 (manganese
  oxide, black solid)
• MnO4- (permanganate, purple) to Mn2
  (manganese(II) ion, pink)
• Fe3 (iron(III), reddish brown) to Fe2(iron(II),
  pale green)

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Exp 19A Oxidation-Reduction Reactions

• Cl2 Solution
• Reaction
• 2OCl- 4H3O 2e- ? Cl2 6H2O
• 2Cl- ? Cl2 2e-
• 2OCl- 2Cl- 4 H3O ? 2Cl2 6H2O
• or
• OCl- Cl- 2 H3O ? Cl2 3H2O
• Preparation of 12 mL 0.050 M chlorine water (add
  in this order and in the fume hood and leave it
  there!)
• 1.70 mL bleach (NaOCl)
• 1.2 ml 0.50 M H2SO4
• 6.0 mL 0.20 M NaCl
• 3.1 mL H2O
• Put in large test tube

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Exp 19A Oxidation-Reduction Reactions

• Br2 Solution
• Reaction
• 2BrO3- 12 H3O 10e- ? Br2 18H2O
• 10 Br- ? 5 Br2 10e-
• 2BrO3- 10 Br- 12 H3O ? 6 Br2 18H2O
• or
• BrO3- 5 Br- 6 H3O ? 3 Br2 9H2O
• Preparation of 12 mL 0.050 M bromine water
• 1.0 mL 0.2 M KBrO3
• 5.0 mL 0.20 M NaBr
• 1.2 ml 0.50 M H2SO4
• Wait 15 minutes before adding
• 4.8 mL H2O
• Put in large test tube

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Exp 19A Oxidation-Reduction Reactions

• I2 Solution
• Reaction
• 2Cu2 2I- 2e- ? 2CuI
• 2I- ? I2 2e-
• 2Cu2 4I- ? 2CuI I2
• Preparation of 12 mL 0.050 M iodine water
• 6.0 mL 0.20 M CuSO4
• 6.0 mL 0.40 M NaI
• Filter over double filter paper to remove
  insoluble CuI
• Put solution in large test tube

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Exp 19A Oxidation-Reduction Reactions

• Exp 1
• Put 15 drops of Cl2 solution in small test tube
• Put 15 drops of Br2 solution in 2nd small test
  tube
• Put 15 drops of I2 solution in 3rd small test
  tube
• Add 1 mL cyclohexane to each tube
• Shake or swirl solution gently
• Record colors (if any) in cyclohexane layer
• Keep these tubes as reference colors

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Exp 19A Oxidation-Reduction Reactions

• Exp 2 Reactions of Halogens with Halides (I)
• Put 10 drops of 0.2 M NaCl in 2 clean and dry
  test tubes
• Add 10 drops of Br2 solution to tube 1
• Add 10 drops of I2 solution to tube 2
• Add 1 mL cyclohexane to each tube
• Shake or swirl
• Record color change for both the aqueous and the
  organic layer
• NO reaction cyclohexane will have color of
  aqueous layer
• Reaction color change in cyclohexane

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Exp 19A Oxidation-Reduction Reactions

• Exp 2 Reactions of Halogens with Halides (II)
• Clean test tubes
• Put 10 drops of 0.2 M NaBr in 2 clean and dry
  test tubes
• Add 10 drops of Cl2 solution to tube 1
• Add 10 drops of I2 solution to tube 2
• Add 1 mL cyclohexane to each tube
• Shake or swirl
• Record color change for both the aqueous and the
  organic layer

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Exp 19A Oxidation-Reduction Reactions

• Exp 2 Reactions of Halogens with Halides (III)
• Clean test tubes
• Put 5 drops of 0.4 M NaI in 2 clean and dry test
  tubes
• Add 10 drops of Cl2 solution to tube 1
• Add 10 drops of Br2 solution to tube 2
• Add 1 mL cyclohexane to each tube
• Shake or swirl
• Record color change for both the aqueous and the
  organic layer

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Exp 19A Oxidation-Reduction Reactions

• Exp 3 Reactions of Permanganate and Iron (I)
• Clean test tubes
• Put 15 drops of 0.2 M NaBr in two test tubes
• Add 1 drop of 0.1 M KMnO4 solution to tube 1
• Add 5 drops of 0.1 M FeCl3 solution to tube 2
• Add 5 drops of 3 M H2SO4 to each tube
• Add 1 mL cyclohexane to each tube
• Shake or swirl
• Record color change/observations for both the
  aqueous and the organic layer

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Exp 19A Oxidation-Reduction Reactions

• Exp 3 Reactions of Permanganate and Iron (II)
• Clean test tubes
• Put 8 drops of 0.4 M NaI in two test tubes
• Add 1 drop of 0.1 M KMnO4 solution to tube 1
• Add 5 drops of 0.1 M FeCl3 solution to tube 2
• Add 5 drops of 3 M H2SO4 to each tube
• Add 1 mL cyclohexane to each tube
• Shake or swirl
• Record color change/observations for both the
  aqueous and the organic layer

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Exp 19A Oxidation-Reduction Reactions

• Exp 3 Reactions of Permanganate and Iron (III)
• Clean test tubes
• Put 8 drops of 0.4 M NaI in one test tube
• Add 5 drop of 6.0 M NaOH solution to tube
• Add 1 drop of 0.1 M KMnO4 solution to tube
• Add 1 mL cyclohexane to tube
• Shake or swirl
• Record color change/observations for both the
  aqueous and the organic layer

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Exp 19A Oxidation-Reduction Reactions

• Exp 3 Reactions of Halogens and Halides
• Cl- Br2 NR Br2 is still present
  (yellow-orange color)
• Cl- I2 NR I2 is still present (violet color)
• Cl2 2Br- ? 2Cl- Br2 (yellow-orange color)
• I2 2Br- NR I2 is still present (violet
  color)
• Cl2 2I- ? 2Cl- I2 (violet color)
• Br2 2I- ? 2Br- I2 (violet color)

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Exp 19A Oxidation-Reduction Reactions

• Exp 3 Reactions of Permanganate and Iron
• (7) 2MnO4- 16H3O 10e- ? 2Mn2 8H2O
• 10Br- ? 5Br2 10e-
• 2MnO4- 10Br- 16H3O ? 2Mn2 5Br2 8H2O
  (yellow-orange color)
• (9) 2MnO4- 16H3O 10e- ? 2Mn2 8H2O
• 10I- ? 5I2 10e-
• 2MnO4- 10I- 16H3O ? 2Mn2 5I2 8H2O
  (violet color)
• (8) Fe3 Br- ? No color, No Reaction
• (10) 2Fe3 2 e- ? 2Fe2
• 2I- ? I2 2e-
• 2Fe3 2I- ? 2Fe2 I2 (violet color)

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Exp 19A Oxidation-Reduction Reactions

• Exp 3 Reactions of Permanganate and Iron
• (11) 6MnO4- 6e- ? 6MnO42-
• I- 3H2O ? IO3- 6H3O 6e-
• 6H3O 6OH- ? 6H2O
• 6MnO4- I- 6OH- ? 6MnO42- IO3- 3H2O
• green color in aqueous layer
• no color in cyclohexane
• no formation of a nonpolar halogen

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Exp 19A Oxidation-Reduction Reactions

• Post Lab
• Results sheets (p. 351-352)
• Post lab questions 1, 2a-d, 3a-c
• Give balanced equations for every reaction that
  happened
• If there was no reaction, write NR and indicate
  how you reached that conclusion
• Answer the questions