CHM 420 Laboratory Report Experiment 7 PDF

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Summary

Experiment 7 : Redox TitrationObjectiveTo standardize the potassium permanganate solution and to determine the composition of metal by titration method.IntroductionElectron transfer reactions came to be called oxidation-reduction reactions, or simply redox reactions. The term oxidation was used to d...

Description

Experiment 7 : Redox Titration

Objective

To standardize the potassium permanganate solution and to determine the composition of metal by titration method.

Introduction

Electron transfer reactions came to be called oxidation-reduction reactions, or simply redox reactions. The term oxidation was used to describe the loss of electrons by one reactant while reduction to describe the gain of electrons by another.

An oxidizing agent is defined as a substance that removes electrons from another reactant in a redox reaction. Some redox titrations do not need an indicator because of the colour changing. For example, in the titration with Potassium Permanganate, a slight faint pale pink colour formed signals the endpoint of the titration because of the colour of the excess oxidizing agent potassium permanganate.

Potassium permanganate iron(II) in acidic solution while being reduced to manganese(II). The reactions are :

Reduction half equation :

MnO4- + 8H+ + 5e- → Mn2+ + 4H2O

Oxidation half equation :

[ Fe2+ → Fe3+ + e- ] 5 MnO4- + 5Fe2+ + 8H+ → Mn2+ + 5Fe3+ + 4H2O

Chemicals and apparatus

0.1 M potassium permanganate solution

Burette

Solid sodium oxalate (Na2C2O4)

Conical flask

1M sulfuric acid

Hot plate

Iron ore

Volumetric flask

Procedure

1.

Standardization of the Potassium Permanganate Solution Dried sodium oxalate, Na2C2O4 was weighed accurately between 0.2 to 0.25 g into a conical flask. 40 mL of 1M sulfuric acid solution was added. The solution was heated gently to 60°C and without delay, the solution was titrated with permanganate solution until the first persistent pink colour. Temperature was maintained at 55-57°C. The titration was repeated to obtain 2 acceptable results and the data was recorded in Table 7.1.

2.

Analysis of Ore

Iron ore was weighed accurately between 0.20 to 0.40 g into a conical flask. 30 mL of distilled water was added and then 30 mL of 1M acid sulfuric solution was added. A glass funnel was placed on the conical flask, the solution was heated gently on hot plate for 20-30 minutes until ore dissolves. The mixture was filtered into a 100 mL volumetric flask and filled up to the mark with 1.0M sulfuric acid solution. 25 mL of the solution is pipetted into a conical flask, the solution was heated gently to 60°C and then the solution was titrated with permanganate solution until the first persistent pink colour. The titration was repeqated to obtain 3 acceptable results and the data was recorded in Table 7.2.

Data

1. Standardization of potassium permanganate solution

Table 7.1 Trial 1

Trial 2

Weight of Na2C2O4 (g)

0.2352

0.2140

Final buret reading (mL)

15.30

13.20

Initial buret reading (mL)

22.00

21.00

Volume of KMnO4 (mL)

6.70

7.80

2. Analysis of Ore

Table 7.2 Trial 1

Trial 2

Trial 3

Weight of iron ore (g)

0.3685

0.2058

0.2649

Final buret reading (mL)

19.70

21.30

20.60

Initial buret reading (mL)

20.00

21.50

21.00

Volume of KMnO4 (mL)

0.30

0.20

0.40

Calculations

1. Standardization of the Potassium Permanganate Solution

Ionic equations Reduction half equation :

[ MnO4- + 8H+ + 5e- → Mn2+ + 4H2O ] 2

Oxidation half equation :

[ C2O4- → 2CO2 + 2e- ] 5 2MnO4- + 5C2O4- + 16H+ → Mn2+ + 10CO2 + 8H2O

Average weight of Na 2 C2 O4 =

(0.2352 + 0.2140 )g 2

= 0.2246g

Average volume of KMnO 4 =

(6.70 + 7.80 )mL 2

= 7.25mL

Mol of Na 2C 2O 4 =

0.2246g 134 gmol −1

= 1.68  10-3 mol From equation,

5 mol C2O4- ≈ 2 mol Mn2+ 1.68×10-3 mol C2O4- ≈ 6.72×10-4 mol Mn2

6.72 ×10 -4 mol 7.25  10 −3 L = 0.09M

Molarity of KMnO 4 =

2. Analysis of Ore

Ionic equations Reduction half equation :

MnO4- + 8H+ + 5e- → Mn2+ + 4H2O

Oxidation half equation :

[ Fe2+ → Fe3+ + e- ] 5 MnO4- + 5Fe2+ + 8H+ → Mn2+ + 5Fe3+ + 4H2O

Average weight of iron ore =

(0.3685 + 0.2058 + 0.2649 )g 3

= 0.2797g

Average volume of KMnO 4 =

(0.30 +0.20 +0.40 )mL 3

= 0.30mL

−

(

Mol of MnO 4 = (0.09M ) 3 10 −4 L

)

= 2.7 10 −5 mol

From equation,

1 mol Mn2+ ≈ 5 mol Fe2+ 2.7×10-5 mol Mn2+ ≈ 1.35×10-4 mol Fe2+

Mass of iron(II) titrated = 1.35 ×10-4 mol  55.85gmol -1 = 7.54  10 −3 g

Scale of titrated =

25mL 1 = 100mL 4

(

)

Total mass of iron(II) in original sample = 4 7.54 10−3 g = 0.0377g

0.0377g  100% 0.2797g = 13.48%

% of iron =

Questions

1. Calculate the following :

a) Actual molarity of potassium permanganate solution. Titration reaction :

2MnO4- + 5C2O42- + 16H+ → 2Mn2+ + 10CO2 + 4H2O

Average weight of Na 2 C2 O4 =

(0.2352 + 0.2140 )g 2

= 0.2246g

Average volume of KMnO 4 =

(6.70 + 7.80 )mL

= 7.25mL

2

Mol of Na 2C 2O 4 =

0.2246g 134 gmol −1

= 1.68  10-3 mol From equation,

5 mol C2O4- ≈ 2 mol Mn2+ 1.68×10-3 mol C2O4- ≈ 6.72×10-4 mol Mn2

6.72 ×10 -4 mol 7.25  10 −3 L = 0.09M

Molarity of KMnO 4 =

b) Concentration of iron(II) in iron ore solution (initial volume of 100mL).

0.0377 g 0.025 L = 1.508 gL−1

Concentrat ion of iron(II) =

c) Percent purify of iron(II) in unknown iron sample.

(

)

Total mass of iron(II) in original sample = 4 7.54 10−3 g = 0.0377g

0.0377g  100% 0.2797g = 13.48%

% of iron =

2. Explain why no indicator is needed in this experiment. The end point that the titration reach indicates that the solution was completely oxidized. The solution will turns from colourless to permanent colour change which is faint pale pink. The colour changing is due to the presence of KMnO4. There is no more solution left to oxidize it. Hence, KMnO4 acts as self-indicator.

3. Typically, a solid iron ore is dried in an oven before analysis. How would the percentage of Fe determined be affected if the ore had not been dried ? It would have a higher mass due to the extra moisture and so the percentage of iron would be smaller.

Discussion

1. Standardization of the Potassium Permanganate Solution

From equations, MnO4– is reduced to Mn2+ and C2O4– is oxidised to CO2 . The oxidation number of carbon in C2O4– changes from +3 to +4. In this case, colour of standard solution is changed because its reduction by sodium oxalate. After the standard solution was completely oxidized, the appearance of a faint pale pink colour produced by the addition of a little excess of unreacted potassium permanganate show that the end point has been reached. Moreover, temperature of sodium oxalate along with H2SO4 need to be maintained at 55-57°C . This is because the reaction takes place at higher temperature. During the titration, first manganous sulphate is produced and acts as a catalyst for the reduction of KMnO 4 by sodium oxalate. Thus, reaction rate in the beginning is slow and the rate of the reaction increases as the reaction proceeds.

2. Analysis of Ore

To prepare standard solution, iron ore was put into the conical flask and then 30 mL distilled water and 30 mL of 1M sulfuric acid were added into the conical flask. The mixture was heated on hot plate to dissolves the ore. The mixture was filtered into a 100 mL volumetric flask and the mark was filled up with sulfuric acid. Only 25 mL was pippeted from volumetric flask into conical flask to be titrated with KMnO 4. These volumes is used to calculate scale of titration to find mass of iron(II) in original sample.

From equation, The oxidation number of iron ore is +2. During reaction, Iron is oxidized and its oxidation number changes from +2 to +3. After the standard solution was completely oxidized, the appearance of a faint pale pink colour produced by the addition of a little excess of unreacted potassium permanganate show that the end point has been reached which is one excess drop of KMnO4 beyond that needed with the Fe2+ was added. From overall experiment, the presence of sulfuric acid is to supply the proton H+ and then preventing the formation of a manganese(IV) oxide precipitate. Formation of MnO2 which would not give a good end point and causing errors and quantitative complications. Diluted sulphuric acid also does not undergo any redox reactions under the conditions of this titration. Potassium permanganate acts as a self-indicator in this titration.

There are some precautions that must been followed in order to get accurate result. The air gaps must be removed from the burette and never forget to remove the funnel from the burette before noting the initial reading of the burette to get accurate reading of burette.

Conclusion

In the end of this experiment, a potassium permanganate solution was able to prepared and standardized. The molarity of Potassium permanganate calculated is 0.09 M. The composition of metal has been determined by titration method which is 13.48%.

References Neil D. Jespersen & Alison Hyslop. (2014). Chemistry: The Molecular Nature of Matter, 7th Edition. John Wiley and Sons, Incorporated.

Christopher Spohrer, Luvleen Brar. (2019). Oxidation-Reduction Reactions. Retrieved

on

October

October

26,

2019

from

https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Mo ules_(Analytical_Chemistry)/Electrochemistry/Redox_Chemistry/Oxidation-Reductio n_Reactions. Jim Clark. (2002). Definitions Of Oxidation And Reduction (Redox). Retrieved on October

2016,

2019

https://www.chemguide.co.uk/inorganic/redox/definitions.html

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