ADVANCED LEVEL CHEMISTRY PRACTICAL ASSESSMENT

Candidate Number:                          Centre Number:71637

Name:___________________________             Date:_______________

 

THE PREPARATION AND PROPERTIES OF CHROME ALUM


INTRODUCTION
In this experiment you will prepare chrome alum KCr(SO4)2.12H2O by the reduction of potassium dichromate with ethanol. You are aiming for maximum yield of a very pure product.


Points to note.
1. Chrome alum is somewhat soluble in water and hence the volume of water used should be kept to a minimum.

2. The solution should be kept below 60oC since above this temperature complexes of chromium (III) sulphate form. These complexes contain sulphate in a non-ionisable form and are difficult to crystallise.


APPARATUS                                                                                       CHEMICALS
Weighing bottle                                                             Potassium dichromate (VI)
Spatula                                                                                   Concentrated sulphuric acid
400 cm3 beaker                                                                     Methylated spirits
Bunsen burner                                                                      2M sodium hydroxide
50 cm3 measuring cylinder                           2M barium chloride
Thermometer (-10oC to 110oC)
Watch glass


A             PREPARATION OF CHROME ALUM

PROCEDURE


1. Weigh out 20.0g of potassium dichromate (VI). Record the mass. Record the colour of the solid.


2. Place the crystals in a 400cm3 beaker and add 150cm3 of hot water. Stir until all the crystals are dissolved. Record the colour of the solution.


3. Cool the solution and carefully add 16cm3 of concentrated sulphuric acid. Cool the solution and record any observations.


4. Add 40cm3of methylated spirit using a dropping pipette. Stir constantly with a thermometer. Do not permit the temperature to rise above 60oC. Record any observations you make during the procedure.


5. Cover the beaker with a watch glass. Set the solution aside to crystallise.


6. When the crystallisation is complete, filter off the crystals and wash with a little water.


7. Transfer the product to a dry filter paper and allow to dry in air.


8. Weigh the crystals and record the mass.


9. Describe the appearance of the crystals.

 

PART B OBSERVATION AND DEDUCTION EXERCISE

PROCEDURE


Dissolve a few of the crystals in cold water. Record the colour of the solution. Divide the solution into four portions (a, b, c, d).


Test (a) - (d) as follows


(a) Add dilute sodium hydroxide solution dropwise. Observe and record all changes.
Now add dilute sulphuric acid dropwise to this solution. Observe and record any changes you notice.


(b) Add barium chloride solution followed by dilute hydrochloric acid. Observe and record all changes noticed.


(c) Heat portion (c) to boiling, concentrate the solution and allow it to stand for a few minutes.
Observe and record all changes noticed.


(d) Heat portion (d) to boiling, add excess 2M barium chloride solution, filter the mixture and allow the filtrate to stand for a few minutes.
Observe and record all changes noticed.


QUESTIONS


1. Why was concentrated sulphuric acid used and not the dilute acid?


2. Suggest a possible reductant.


3. Calculate your percentage yield of crystals.


4. Explain the reason for the yield being less than 100%.


5. Would you expect the solution of chrome alum prepared by you to show any differences from one made by mixing solutions of potassium sulphate and chromium (III) sulphate in the ratio 1:1.?

 

6.  Explain, using equations, the reaction of the solution of chrome alum with sodium hydroxide.


7. Explain, using equations, the effect sulphuric acid had on the chrome alum/sodium hydroxide  reaction mixture.


8. Explain, using equations, how barium chloride solution reacted with chrome alum solution. Why was hydrochloric acid used in this reaction?


9. Explain the effect of heat on the chrome alum solution. Repeat the explanation for tests (c) and (d).

 

ADVANCED LEVEL CHEMISTRY PRACTICAL ASSESSMENT CHECKLIST

 

Candidate Number:                                                                             Centre Number: 71637

 

Name:                                                                                                                    Date:                                     

 

THE PREPARATION AND PROPERTIES OF CHROME ALUM

 

 

SKILL AREA 1   PLANNING                                                                           Student Mark

 

 

SKILL AREA 2   MANIPULATION                                                 Student Mark

Goggles                                                                                                                    1                                          

Using balance (including weighing bottle                              2                                      

Dissolving dichromate in water                                                      2                                

Adding methylated spirits dropwise                                                             2                                

Checking temperature                                                                                      1                                

Filtering, washing, drying crystals                                       3                                          

Analysis test                                                                                                          4                                          

                                                                                                                                                [15]                                        

 

 

SKILL AREA 3   MEASURING & RECORDING                          Student Mark

Records mass of potassium dichromate                                              1                                         

Records colour of potassium dichromate                                            1                                         

Records colour of potassium dichromate soln.                                   1                                         

Records colour change with methylated spirits                                  1                                         

Records reaction with methylated spirits                                             1                                         

Records mass of chrome alum crystals                                                1                                         

Records appearance of chrome alum crystals                    2                                         

Records colour of chrome alum solution                                             1                                         

Records results of test (a)                                                                      2                                         

Records results of test (b)                                                                      1                                         

Records results of test (c)                                                                      1                                         

Records results of test (d)                                                                      2                                         

 

 

SKILL AREA 4   OBSERVATION & RECORDING                    Student Mark

Colour of potassium dichromate solid                                                      1                                     

Colour of potasium dichromate in solution                            1                      

Colour change with methylated spirits                                           1                                               

Observing reaction with acid and alcohol                                           1                                         

Appearance of crystals                                                                                          2                                         

Colour of chrome alum solution                                                                            1                                         

Observes reaction with sodium hydroxide                                          2                                         

Observes reaction with sulphuric acid                                                 1                                         

Observes reaction with barium chloride                                               2                                         

Observes effect of heat                                                                                           1                                         

Observes effect of test (d)                                                                     2                                         

                                                                                                                                                 [15]                                       

 

 

SKILL AREA 5   CONCLUDING & COMMUNICATING           Student Mark

Question 1                                                                                                                                 2                                         

Question 2                                                                                                                                 2                                         

Question 3                                                                                                                                 2                                         

Question 4                                                                                                                                 2                                         

Question 5                                                                                                                                 2                                         

Question 6                                                                                                                                 2                                         

Question 7                                                                                                                                 2                                         

Question 8                                                                                                                                2                                         

Question 9                                                                                                                                4                                         

                                                                                                                                                 [20]