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UNIT 7 Redox Titrations

 

8.0  Introduction

9.0  Objective

10.0  Main Content

3.1Theory

 3.2  Experiment

4.0  Conclusion

5.0  Summary

6.0  Tutor Marked Assignments

7.0  References/Further Reading

 

1.0 Introduction

Reactions in which substances undergo changes in oxidation number are referred to as oxidationreduction reactions or redox reactions. Oxidation is defined as an algebraic increase in oxidation number, or a process in which electrons are lost. Reduction is defined as an algebraic decrease in oxidation number or a process in which electrons are gained. Oxidation-reduction processes must occur simultaneously. The species that gains electrons is called the oxidizing agent, therefore it is reduced. The species that loses electrons is called the reducing agent, therefore, it is oxidized.

2.0 Objective

At the end of this unit should be able to perform an experiment in two parts

ecolebooks.com
  • To standardize a potassium permanganate solution will be against a sample of potassium oxalate.
  • To use the standard permanganate solution to find the concentration of iron (II) in a ferrous solution Image From EcoleBooks.com

 

 

  1. Main Content
  2. Theory

Potassium permanganate,Image From EcoleBooks.com, is a strong oxidizing agent. Permanganate,Image From EcoleBooks.com, is an intense dark purple color. Reduction of purple permanganate ion to the colorless Image From EcoleBooks.com ion, the solution will turn from dark purple to a faint pink color at the equivalence point. No additional indicator is needed for this titration. The reduction of permanganate requires strong acidic conditions. In this experiment, permanganate will be reduced by oxalate, Image From EcoleBooks.comin acidic conditions. Oxalate reacts very slowly at room temperature so the solutions are titrated hot to make the procedure practical. The unbalance redox reaction is shown below.

 

Image From EcoleBooks.comImage From EcoleBooks.com
Image From EcoleBooks.com

 

In part I of this experiment, a potassium permanganate solution will be standardized against a sample of potassium oxalate. Once the exact normality Image From EcoleBooks.com of the permanganate solution is determined, it can be used as a standard oxidizing solution. In part II of this experiment, the standard permanganate solution will be used to find the concentration of iron (II) in a ferrous solution Image From EcoleBooks.com

The unbalanced redox reaction is shown below.

Image From EcoleBooks.com(acidic solution)

 

Phosphoric acid will be used to ensure that the ferric product, Image From EcoleBooks.comremains in its colorless form.

 

3.2 Experiment

Equipment and Reagents (Day 1)

  • Image From EcoleBooks.com solid  
  • weighing paper  
  • burette
  • 500 mL Florence Flask  
  • Image From EcoleBooks.com
    Image From EcoleBooks.com  
  • Ring Stand
  • Rubber Stopper    
  • Analytical Balance
  • Burette Clamp
  • Hot plate or Bunsen burner  
  • 250 mL Erlenmeyer flask  
  • Image From EcoleBooks.com

 

 

 

 

Procedure (Day 1)

Part (I) – Preparation of a 0.1 N Image From EcoleBooks.com Solution.

  1. On a centigram balance, weigh about 1.0 gImage From EcoleBooks.com crystals on a piece of weighing paper.

Add the crystals to a Image From EcoleBooks.com Florence Flask.

  1. Add about Image From EcoleBooks.com of distilled water to the flask.
  2. Heat the solution with occasional swirling to dissolve the Image From EcoleBooks.com crystals. Do not boil the solution. This may take about 30 minutes.
  3. Allow the solution to cool and stopper. You will need this solution for both day 1 and day 2.

 

Part (II) – Standardization of a Image From EcoleBooks.com solution.

  1. On weighing paper, weigh about Image From EcoleBooks.com of Image From EcoleBooks.com on the analytical balance. Record the exact mass. Transfer the sample to a Image From EcoleBooks.com Erlenmeyer flask.
  2. Rinse and fill the burette with the Image From EcoleBooks.comsolution.
  3. Add Image From EcoleBooks.com of distilled water and Image From EcoleBooks.com of 6 N Image From EcoleBooks.comto the oxalate sample in the

Erlenmeyer flask. Swirl to dissolve the solids.

  1. Heat the acidified oxalate solution to aboutImage From EcoleBooks.com. Do not boil the solution.
  2. Record the initial burette reading. Because the Image From EcoleBooks.comsolution is strongly colored, the top of the meniscus may be read instead of the bottom.
  3. Titrate the hot oxalate solution with the Image From EcoleBooks.comsolution until the appearance of a faint pink color.
  4. Record the final burette reading and calculate the volume of Image From EcoleBooks.comused in the titration.
  5. Discard the titration mixture down the drain and repeat the titration with a new sample of oxalate for a total of 2 trials.
  6. An oxalic acid solution may be used to wash the burette and the titration flask if a brown stain remains in the glassware. Calculations
  • Using the half-reaction method, write a balanced redox equation for the reaction of permanganate with oxalate in an acidic solution.
  • Calculate the equivalent weight of the oxalate reducing agent from the molar mass of the oxalate sample and the equivalence of electrons lost by the reducing agent in the oxidation halfreaction.Image From EcoleBooks.com

    Image From EcoleBooks.com

  • Use the sample mass and the equivalent weight to calculate the number of equivalents of oxalate in each sample.

    Image From EcoleBooks.com

At the equivalence point, the equivalence of the reducing agent is equal to the equivalence of the oxidizing agent.

Image From EcoleBooks.com

  • Calculate the normality of the Image From EcoleBooks.comsolution from the equivalence of the oxidizing agent and the volume used in the titration.
  • Calculate the average normality of the permanganate solution.

 

 

 

Equipment and Reagents (Day 2)

  • Unknown Image From EcoleBooks.comsolution  
  • Image From EcoleBooks.com solution  
  • Burette Clamp
  • 250 mL Erlenmeyer Flask  
  • 25 mL pipet  
  • Ring Stand
  • Image From EcoleBooks.com      
  • Pipet bulb

 

Procedure (Day 2)

Part (III) – Determination of the Mass of Iron in a Ferrous Solution.

  1. Pipet a Image From EcoleBooks.com sample of the unknown Image From EcoleBooks.com solution into a Image From EcoleBooks.com Erlenmeyer flask.
  2. Add Image From EcoleBooks.comof distilled water and 1Image From EcoleBooks.com of Image From EcoleBooks.com into the flask.
  3. Fill a burette with the standard Image From EcoleBooks.comsolution and record the initial burette reading.
  4. Titrate the sample with the standard Image From EcoleBooks.com to a faint pink end-point and record the final burette reading. Calculate the volume of Image From EcoleBooks.com used.
  5. Discard the ferric solution down the drain and repeat the titration with a new sample of the ferric solution for a total of 2 trials.
  6. After all trials, discard the purple permanganate solution in the appropriate waste container in the fume hood.
  7. Oxalic acid may be used to remove any brown stains left on the glassware.

 

4.0 Conclusion/Calculations

  1. Using the half-reaction method, balance the redox reaction of permanganate with iron (II) in acidic media.
  2. Calculate the equivalence of Image From EcoleBooks.comtitrated.

Image From EcoleBooks.com

mass concentrations for the ferrous unknown solution.

At the equivalence point, the equivalence of the oxidizing agent is equal to the equivalence of the reducing agent.

Image From EcoleBooks.com

Determine the normality of the ferric reducing agent.

Image From EcoleBooks.com

  1. Calculate the molarity (mol/L) of the ferrous solution.

    Image From EcoleBooks.com

    Image From EcoleBooks.com

    (n = moles of electrons lost in the oxidation half-reaction.)

  2. Calculate the mass concentration (Image From EcoleBooks.com) of iron in the unknown solution by multiplying the molar mass of iron by the molarity of the ferrous solution.

Image From EcoleBooks.com

 

  1. Calculate the average mass concentrations for the ferrous unknown solution.

5.0 Summary

In this unit, you have been able to perform an experiment with two parts. In the first part, you standardized a potassium permanganate solution against a sample of potassium oxalate to determine the exact normality Image From EcoleBooks.com of the permanganate solution .In the second part of the experiment, you used the standard permanganate solution to find the concentration of iron (II) in a ferrous solution Image From EcoleBooks.com

6.0 Tutor Marked Assignments (TMAs)

  1. Explain the following terms
    1. oxidizing agent
    2. reducing agent
  2. A solution contains both iron (II) and iron(III) ions. A 50.0 mL sample of the solution is titrated with 35.0 mL of Image From EcoleBooks.com
    Image From EcoleBooks.com, which oxidizes Image From EcoleBooks.com to Image From EcoleBooks.com The permanganate ion is reduced to manganese (II) ion. Another Image From EcoleBooks.com sample of solution is treated with zinc metal, which reduces all the Image From EcoleBooks.comto Image From EcoleBooks.com
    . The resulting solution is again titrated with of

Image From EcoleBooks.com
Image From EcoleBooks.com, this time Image From EcoleBooks.com is required. What are the concentrations of Image From EcoleBooks.com to Image From EcoleBooks.com in the solution?

Solution: 1) The stoichiometric relationship of permanganate to Fe(II):

Image From EcoleBooks.com

is five to one.

  1. Calculate moles of Fe(II) reacted:

(Image From EcoleBooks.com (Image From EcoleBooks.com) = Image From EcoleBooks.com
Image From EcoleBooks.com,

(Image From EcoleBooks.com) (Image From EcoleBooks.com) = Image From EcoleBooks.com

  1. Determine the TOTAL iron content:

(Image From EcoleBooks.com
Image From EcoleBooks.com,

(Image From EcoleBooks.com (Image From EcoleBooks.com

  1. Determine Fe(III) in solution and its molarity:

    Image From EcoleBooks.com

    Image From EcoleBooks.com

  2. Determine molarity of Fe(II):

Image From EcoleBooks.com

 

 


 




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EcoleBooks | UNIT  7    Redox Titrations - INORGANIC CHEMISTRY PRACTICALS

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