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)
Exp 19A: Oxidation-Reduction Reactions
etransfer
X or shift of Y
electrons
Oxidation: loss of electrons
Mg  Mg2+ + 2eReduction: gain in electrons
½ O2 + 2e-  O2-
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
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
Redox Activity of Halogens
F2 > Cl2 > Br2 > 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
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)
Exp 19A: Oxidation-Reduction Reactions
Cl2 Solution
• Reaction
2OCl- + 4H3O+ + 2e-  Cl2 + 6H2O
2Cl-  Cl2 + 2e2OCl- + 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
Exp 19A: Oxidation-Reduction Reactions
Br2 Solution
• Reaction
2BrO3- + 12 H3O+ + 10e-  Br2 + 18H2O
10 Br-  5 Br2 + 10e2BrO3- + 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
Exp 19A: Oxidation-Reduction Reactions
I2 Solution
• Reaction
2Cu2+ + 2I- + 2e-  2CuI
2I-  I2 + 2e2Cu2+ + 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
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
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
Exp 19A: Oxidation-Reduction Reactions
Exp 2: Reactions of Halogens with Halides (II)
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•
•
•
•
•
•
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
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
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
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
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
Exp 19A: Oxidation-Reduction Reactions
Exp 3: Reactions of Halogens and Halides
(1) Cl- + Br2: NR: Br2 is still present (yellow-orange color)
(2) Cl- + I2: NR: I2 is still present (violet color)
(3) Cl2 + 2Br-  2Cl- + Br2 (yellow-orange color)
(4) I2 + 2Br-: NR: I2 is still present (violet color)
(5) Cl2 + 2I-  2Cl- + I2 (violet color)
(6) Br2 + 2I-  2Br- + I2 (violet color)
Exp 19A: Oxidation-Reduction Reactions
Exp 3: Reactions of Permanganate and Iron
(7)
2MnO4- + 16H3O+ + 10e-  2Mn2+ + 8H2O
10Br-  5Br2 + 10e2MnO4- + 10Br- +16H3O+  2Mn2+ + 5Br2 + 8H2O (yellow-orange
color)
(9)
2MnO4- + 16H3O+ + 10e-  2Mn2+ + 8H2O
10I-  5I2 + 10e2MnO4- + 10I- +16H3O+  2Mn2+ + 5I2 + 8H2O (violet color)
(8)
Fe3+ + Br-  No color, No Reaction
(10)
2Fe3+ + 2 e-  2Fe2+
2I-  I2 + 2e2Fe3+ + 2I-  2Fe2+ + I2 (violet color)
Exp 19A: Oxidation-Reduction Reactions
Exp 3: Reactions of Permanganate and Iron
(11)
6MnO4- + 6e-  6MnO42I- + 3H2O  IO3- + 6H3O+ + 6e6H3O+ + 6OH-  6H2O
6MnO4- + I- +6OH-  6MnO42- + IO3- + 3H2O
green color in aqueous layer
no color in cyclohexane
no formation of a nonpolar halogen
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