Post Lab Discussion

G. De Vera

A. Na2C2O4 + KMnO4 (Neutral Media)

- Slight / complete decolorization of permanganate solution with BROWN precipitate

Red’n: 3e- + 2H2O + MnO4- MnO2(s) + 4OH-

Oxid’n: 4OH- + C2O42- 2CO3

2- + 2H2O + 2e-

4OH- + 2MnO4- + 3C2O4

2- 2MnO2(s) + 6CO3

2- + 2H2O

B. Na2C2O4 + KMnO4 (Acidic Media)

- Formation of colorless to faint pink due to Mn2+

(evolution of gas may be observed)

Red’n: 5e- +8H+ + MnO4- Mn2+ + 4H2O

Oxid’n: C2O42- 2CO2 + 2e-___________

16H+ + 2MnO4- + 5C2O4

2- 2Mn2+ + 10CO2 + 8H2O

C. Na2C2O4 + KMnO4 (Basic Media)

- Formation of emerald-green solution due to MnO42-

Red’n: e- + MnO4- MnO4

2-

Oxid’n: 4OH- + C2O42- 2CO3

2- + 2H2O +2 e-

4OH- + 2MnO4- + C2O4

2- 2MnO42- + 2CO3

2- + 2H2O

D. FeCl3 + KSCN

- Formation of blood red solution (Fe3+ + SCN- FeSCN2+)

E. FeSCN2+ + Zn(s)

- Decrease in the color intensity of FeSCN2+ (blood-red) solution. (Dissolution of some Zn metal)

Red’n: e- + FeSCN2+ Fe2+ + SCN-

Oxid’n: Zn(s) Zn2+ + 2e-

Zn(s) + 2 FeSCN2+ Zn2+ + 2Fe2+ + 2SCN-

F. FeCl3 + K4Fe(CN)6

- Formation of Prussian-blue ppt- 4Fe3+ + 3[Fe(CN)6]4- Fe4[Fe(CN)6]3(s)

G. Fe4[Fe(CN)6]3(s) + Zn

- Decrease in the amount of Prussian-blue ppt

Red’n: 4e- + Fe4[Fe(CN)6]3(s) 4Fe2+ + 3[Fe(CN)6]4-

Oxid’n: Zn(s) Zn2+ + 2e-

2Zn(s) + Fe4[Fe(CN)6]3(s) 2Zn2+ + 4Fe2+ + 3[Fe(CN)6]4-

G. Fe4[Fe(CN)6]3(s) + Zn

- A white ppt may be observed due to the reaction of Zn2+ with [Fe(CN)6]4-

Red’n: 4e- + Fe4[Fe(CN)6]3(s) 4Fe2+ + 3[Fe(CN)6]4-

Oxid’n: Zn(s) + 2[Fe(CN)6]4- Zn4[Fe(CN)6]2(s) + 8e-

4Zn(s) + 2Fe4[Fe(CN)6]3(s) Zn4[Fe(CN)6]2(s) + 8Fe2+ + 4[Fe(CN)6]4-

1. Oxidation of [Co(NH3)6]2+ to [Co(NH3)6]3+

with H2O2 as RA

Oxid’n: [Co(NH3)6]2+ [Co(NH3)6]3+ + e-

Red’n: 2e- + H2O2 2OH-

2[Co(NH3)6]2+ + H2O2 [Co(NH3)6]3+ +

2OH-

2. Balanced RedOx reactions:

a. 6K3Fe(CN)6 + Cr2O3 + 10KOH 2K2CrO4 + 6 K4Fe(CN)6 + 5H2O

b. 2Cu(NH3)4Cl2 + 7 KCN + H2O 2K2Cu(CN)3 + KCNO + 6NH3 + 2NH4Cl + 2 KCl

c. CoCl2 + 7KNO2 + 2HCl K3Co(NO2)6 + NO +H2O + 4 KCl

d. 2[Cr(Cl)6]3- + Zn 2Cr2+ + [ZnCl4]2- + 8Cl-

3. E0cell = E0

red + E0oxid

a. Water oxidation: 2 H2O(l) O2(g) + 4H+ (10-7M) + 4e- Eoxid = -

0.815V

E0cell = E0

red + E0oxid > 0 * E0

red > -E0oxid > 0.815 V

- All Co3+ species with E0red value greater than 0.815 V can

oxidize water.

b. Water reduction 2H2O(l) + 2e- H2(g) + 2OH- E0

red = -0.828 V

-Only Co(CN)63- can reduce water.

5. Suppose oxalic acid was used instead of oxalate salt:

a. Equation: 6H+ + 2MnO4- + 5H2C2O4 2Mn2+ + 10CO2(g)

+ 8H2O

b. Physical / Chemical changes:-Decolorization of purple solution (formation of colorless

to faint pink solution)

c. How one could make the reaction at a reasonably rapid speed?

- Addition of MnSO4 may catalyze the reaction (recall autocatalysis experiment)

6. Consider Part B and Part C of the experiment:

a. Role of Zn – reducing agentb. Reagents that can replace Zn:

Any metal above Zn in the activity series

- convenient sources of understanding complex redox chemistry of any element in its various oxidation states

Example of a Latimer Diagram

a. From L to R, the species of the element towards right are in LOWER oxidation state.

b. The std electrode potential value for the red’n half rxn involving any pair of the species joined by an arrow is shown above the arrow.

c. For generation of E0 value for a combination of processes:

d. Separate diagrams for different media.

Latimer Diagram for Fe

Exercise: Draw a Latimer Diagram for copper

Exercise: Draw a Frost Diagram for Cu.

a. Oxidation state increases from L Rb. Slope of the line joining 2 pts = Std Potential value

of the couple formed by the 2 speciesc. *steeper line corresponds to higher potential and

more spontaneity:

Identify the following:a. Strongest oxidizing agentb. Strongest reducing agentc. Unstable towards

disproportionationd. Tends to form via

comproportionatione. Most stable Cr species

Construct a Frost diagram for titanium in acidic, aqueous solution from the data on the Latimer diagram.

a. Which is the most stable form of titanium under these conditions?

b. Does any species on the diagram disproportionate under these conditions? If so, what are the products of its disproportionation?

c. Calculate the standard reduction potential for the half reaction: