Kenya Certificate Of Secondary Education (KCSE 2013) Chemistry Paper 3 with Marking Scheme

1. You are provided with:
   
   • solution A, aqueous copper (II) sulphate;
   • solid B, iron powder;
   • 0.02 M acidified potassium manganate (VII), solution C.
     You are required to determine the molar heat of displacement of copper by iron.
     
     Procedure 1
     Using a burette, place 50.0 cm³ of solution A in a 100 ml beaker. Measure the temperature of the solution and record it in table 1 below. Add all solid B provided at once and start a stop watch. Stir the mixture thoroughly with the thermometer and record the temperature of the mixture after every one minute in the table. Retain the mixture for use in procedure II below.
     
     
     1.      
        1. Plot a graph of temperature (vertical axis) against time in the grid provided. (3 marks)
           
        2. From the graph, determine the;
           1. highest change in temperature. ΔT; (1 mark)
           2. time taken for the reaction to bee completed. (¹/₂ mark)
        3. Calculate the heat change for the reaction. (Specific heat capacity of solution is 4.2 Jg^-1K^-1; Density of the solution is 1 gcm^-3). (2 marks)
           
           
   • PROCEDURE II
     Carefully decant the mixture obtained in procedure I into a 250 ml volumetric flask. Add about 10 cm³ of distilled water to the residue in the 100 ml beaker. Shake well, allow the mixture to settle and carefully decant into the volumetric flask. Immediately, add about 50 cm³ of 2 M sulphuric (VI) acid to the mixture in the volumetric flask. Add more distilled water to make 250.0cm³ of solution. Label this as solution D.
     Fill a burette with solution C. Using a pipette and a pipette filler, place 25.0 cm³ of solution D in a 250 ml conical flask. Titrate solution D against solution C until the first permanent pink colour is obtained. Record your results in table 2 below. Repeat the titration two more times and complete the table. Retain the remaining solution C for use in question 3.
     
     Determine the average volume of solution C used. (¹/₂ mark)
   • Calculate the number of moles of:
     1. aqueous potassium manganate (VII) used; (1 mark)
     2. iron (II) ions in 25.0 cm³ of solution D. (1 mole of MnO^-4 reacts with 5 moles of Fe²⁺). (1 mark)
     3. aqueous iron (II) ions in 250 cm³ of solution D. (1 mark)
   • Calculate the molar heat of displacement of copper by iron. (2 marks)
2. You are provided with solid E. Carry out the following tests and write our observations and inferences in the spaces provided.
   1. Place all of solid E in a boiling tube. Add about 10 cm³ of distilled water and shake thoroughly. Filter the mixture into another boiling tube. Retain the filtrate for use in tests 2(b) below. Dry the residue using pieces of filter papers.
      1. Transfer about half of the dry test-tube. Heat the residue strongly and test any gas produced using a burning splint.
         
      2. Place the rest of the residue in a dry test-tube. Add 4 cm³ of 2M hydrochloric acid. Retain the mixture for test (iii) below.
         
      3. To 2 cm³ of the solution obtained in (ii) above, add 6 cm³ of aqueous ammonia dropwise.
         
   2.     
      1. To 2 cm³ of the filtrate obtained in (a) above, add about 3 cm³ of aqueous ammonia. (Excess)
         
      2. To 2 cm³ of the filtrate, add about 2M of hydrochloric acid.
         
      3. To 2 cm³ of the filtrate, add one or two drops of barium nitrate solution.
         
3. You are provided with solid G. Carry out the tests in (a) and (b) and write your observations and inferences in the spaces provided. Describe the method used in part (c)
   
   1. Place about one third of solution G on a metallic spatula and burn it in a bunsen burner flame.
      
   2. Dissolve all of the remaining solid G in about 10 cm³ of distilled water in a boiling tube. Use the solution for tests (b)(i), (ii) and (c).
      1. Place 2 cm³ of the solution in a test-tube and add 2 drops of acidified potassium manganate (VII); solution C.
         
      2.  To 2 cm³ of the solution, add all of sodium hydrogen carbonate provided.
   3. Determine the pH of the solution obtained in (b) above.

MARKING SCHEME

Procedure I

1. Table 1
   

   Time (Min.)	0	1	2	3	4	5	6	7
   Temperature ("C)	23.0	26.0	30.0	33.0	34.0	35.0	35.0	35.0

   ½ mark for each correct entry,
   Maximum (3 marks)
   1.    
      1.   
         
      2.    
         1. ΔT = 35 - 23 = 12°C. (1 mark)
         2. 3 minutes 36 seconds. (½ mark)
      3. ΔH = 50 x 4.2 x 12
         = 2520 joules.
         (2 marks)
         

Procedure II
Table 2

(4 marks)

1. Average volume = 24.5+24,0 + 24.0
                                          3
   = 24.17 cm³ (½ mark)
2.    
   1. Moles of MnO₄ = 0.02 x 24.17 
                                     1000
      = 4.83 x 10^-4 √½ (1 mark)
   2. Moles of Fe²⁺ = 5 x 4.83 x 10⁴ √½
      = 2.417 x 10^-3 √½ (1 mark)
   3. Moles of Fe²⁺ in 250 cm³ = 2.417 x 10³ x 10 √½
      = 2.417 x 10² √½ (1 mark)
3. Molar heat of displacement =     2520      (1 mark)
                                             2.417 x 10^-2 √½
   = 104261.48 Joules (1 mark)

2.

1. 
   

   (i) Observations - White solid turns yellow - Splint extinguished - On cooling solid is white - Colourless, odourless gas. (max. 1 mark)

   Inferences Probably Co, gas given off. :: C023 - or HCO3 , ZnO formed (max. 1 mark)

   
   

   (ii) Observations effervescence/bubbles - colourless, odourless gas (1 mark)

   Inference C023  present (1 mark)

   
   

   (iii)Observations White ppt soluble in excess (1 mark)

   Inference Zn2+ present (1 mark)

2.    
   

   (i) Observations White ppt soluble in excess (1 mark)

   Inference Pb2+ or Al3+ Mg2+ (1 mark)

   
   

   (ii) Observations No effervescence - No white ppt (1 mark)

   Inference CO2-3 SO2-3 absent Pb2+ absent (1 mark) or AI3+ and Mg2+ present (1 mark)

   
   

   (iii) Observations White ppt (1 mark)

   Inference SO2-4 present (1 mark)

3.

1.    
   

   (i) Observations Melts and then burns with a sooty/ smoky/Luminous flame/yellow flame. (1 mark)

   Inference Long chain organic compound or C = C or H-C=C-H (1 mark)

2. 
   

   (ii) Observations Not decolourised (1 mark)

   Inference ROH C = C or C = C absent (1 mark)

   
   

   (iii) Observations Effervescence/bubbling Colourless gas (1 mark)

   Inference Carboxylic acid present. H or H,O or RCOOH (1 mark)

   
   

   Method used - Add 2 drops of universal indicator to solution. - Match the colour of solution to the pH chart paper - Read off pH. (2 marks)

   Inferences - pH is 1 or 2 - Solution is strongly acidic  (1 marks)