CHEMISTRY PAPER 3 - 2019 KCSE STAREHE MOCK EXAMS (QUESTIONS AND ANSWERS)

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INSTRUCTIONS TO CANDIDATES.

  • Answer ALL the questions in the spaces provided in the question paper.
  • You are NOT allowed to start working with the apparatus for the first 15 minutes of the 2 ¼ hours allowed for this paper. This time is to enable you to read the question paper and make sure you have all the chemicals and apparatus that you may need.
  • All working MUST be clearly shown where necessary.
  • KNEC Mathematical tables and silent non-programmable electronic calculators may be used.
  • Candidates should answer all the questions in English.
  1. You are provided with:
    • Solid G, magnesium powder.
    • Solution N, 0.5 M Ferrous ammonium sulphate hexahydrate.
    • Solution M, containing 3.68 g/l, acidified potassium manganate (VII).

      You are required to determine the:
    • Molar enthalpy of displacement of iron (II) ions with Solid G.
    • Mass of magnesium that reacts with Solution N.

      Procedure 1
      1. Using a measuring cylinder, place exactly 100 ml of solution N in 250 ml plastic beaker. Allow to stand for about a minute. Measure the initial temperature of the solution and record it in table 1.
      2. Add all of the solid G at once to the solution. Stir the mixture carefully with the thermometer and record the highest temperature reached in table 1. Retain the mixture for use in procedure II.
        Table 1(2 marks)

        Final temperature (oC)

         

        Initial temperature (oC)

         

        Change in temperature, ∆T

         
        Procedure 1I
      3. Filter the mixture obtained in procedure I into a 250 ml volumetric flask. Wash the residue with distilled water and add into the flask. Add more water to make up to the mark. Label this as solution F.
      4. Using a pipette and a pipette filler, place 25.0 ml of solution F into a 250 ml conical flask. Titrate solution F with Solution M until a permanent pink colour just appears. Record the readings in table 2. Repeat the titration two more times and complete table 2.

        Table 2(4 marks)

        Titration number

        I

        II

        III

        Final burette reading

             

        Initial burette reading

             

        Volume of Solution M used, cm3

             
      5. Calculate the average volume of Solution M used.                                          (1 mark)
      6. Given the equation for the reaction between manganate (VII) and iron (II) as:
        MnO-4(aq) + 5Fe2+(aq) + 8H+(aq) → Mn2+(aq) + 5Fe3+(aq) + 4H2O(l)
        Calculate the number of moles of:
        1. Potassium manganate (VII) used. (K = 39, Mn = 55, O = 16)                     (1 mark)
        2. Iron (II) ions in 25.0 ml solution F.    (1 mark)
        3. Iron (II) ions in 250 ml solution F.       (1 mark)
        4. Iron (II) ions in 100 ml solution N after the reaction with magnesium.     (1 mark)
        5. Iron (II) ions in 100 ml solution N before the reaction with magnesium.   (1 mark)
        6. Iron (II) ions that reacted with magnesium.   (1 mark)
      7. Calculate the mass of magnesium that reacted. (Mg = 24)   (1 mark)
        Calculate the:
        1. Heat change for the displacement reaction of iron (II) with magnesium.
          (Assume for the mixture, specific heat capacity = 4.2 Jg-1K-1 and density = 1.0 gcm-3)(1 mark)
        2. Molar enthalpy of displacement of iron (II) with magnesium.                    (1 mark)
  1. You are provided with solid L. Carry out the tests below. Record your observations and inferences in the spaces provided.
    1. Place about half of solid L in a clean test-tube and add about 8 ml of distilled water and shake. Retain the other half of solid L for use in part (c) while the solution is for use in part (b) (i) to (iv).

      Observations

      Inferences

         
    2. Divide the solution obtained in part (a) above into four portions.
      1. To the first portion, add 2 M ammonium hydroxide solution drop-wise until in excess.

        Observations

        Inferences

           
      2. To the second portion add 2 – 3 drops of barium nitrate solution.

        Observations

        Inferences

           
      3. To the third portion add about equal volume of chlorine water.

        Observations

        Inferences

           
      4. To the fourth add 2 – 3 drops of lead (II) nitrate solution.

        Observations

        Inferences

           
    3. Clean a metallic spatula and rinse it with distilled water. Dry the spatula on a Bunsen burner using non-luminous flame for about one minute and then allow it to cool. Place a little of solid L on the spatula and heat it strongly with a non-luminous flame.

      Observations

      Inferences

         
  1. You are provided with:
    • Solid W
    • Bromine water
    • Acidified potassium manganate (VII)
    • Sodium carbonate
    • Solid W is suspected to be an organic compound.
      1. Using the reagents provided describe the tests that could be performed consecutively to confirm which type of organic compound is Solid W. Record the tests and expected observations in the spaces provided.
        1.  

          Test 1

          Expected Observations

             
           
        2.  

          Test 1

          Expected Observations

             
           
        3.  

          Test 1

          Expected Observations

             
      2. Carry out the tests described in (a) above using solid W and record the observations and inferences in the spaces provided.
        1. Test 1

          Test 1

          Expected Observations

             
        2. Test 2

          Test 1

          Expected Observations

             
        3. Test 3

          Test 1

          Expected Observations

             


MARKING SCHEME

  1.  
    1. Table 1 = 3 marks
      Final temperature P½ (± 2 S.V.)
      Initial temperature P½ (± 2 S.V.)
      ∆T = Final - Initial temperature P
      Note: Exothermic reaction otherwise award for initial temperature only.
      Temperature value should be a whole number or with a 0.5 decimal and use of decimals should be consistent otherwise penalize ½ mark.
    2. Table 2 and averaging (5 marks)        
      • Complete table . . . . . . (1 mark)

        Conditions
      • 3 titrations done (1 mark)
      • 2 titrations done (½ mark)
      • 1 titration done (0 mark)

        Penalties
        Penalise ½ mark only once for any of the following
      • Wrong arithmetic / subtraction
      • Inverted table
      • Unrealistic burette readings and titre values i.e. burette readings beyond 50cm³, unless explained or titre values less than 1cm³ or in hundreds.

        Use of decimal (1 mark) (Tied to 1st and 2nd row only)
        Conditions
      • Accept 1 or 2 decimal places used consistently, otherwise penalise fully.
      • If 2d.p used, the second decimal place must be '0' or '5', otherwise penalise fully.
      • Accept inconsistency in the use of zeros as the initial burette reading e.g. 0, 0.0 or 0.00.

        Accuracy (1 mark)
        Compare the candidate's correct titre values with the school value and tick (ü) the chosen value if it earns a mark and award as follows:
      • If at least one titre value is within ± 0.1 of SV…… (1 mark)
      • If at least one titre value is within ± 0.2 of S. V….. (½ mark)
      • If none is within ± 0.2 of S.V.….(0 mark)

        NOTE
      • The school value must be written above the table.
      • If there was wrong arithmetic or no subtraction, then work out the correct value and compare the correctly worked out value with the S.V and award accordingly.
      • If no school value is given or cannot be worked out from the teachers' values as per the principles of averaging then; sample from all the candidates; correct average titre values.

        Principles of averaging (1 mark)
        Values averaged must be consistent within ± 0.2 of each other.
      • 3 consistent titrations done and averaged . . . . . 1 mark
      • 3 titration done but only 2 are consistent and averaged . . . . 1 mark
      • Only 2 titrations done, are consistent and averaged . . . . 1 mark
      • 3 consistent titrations done but only 2 are averaged. ....  0 mark
      • 3 inconsistent titrations done and averaged . . . . . 0 mark

        NOTE.
      • The mark for principles of averaging is awarded after marking the working and answer for calculating the average volume of solution C.
      • Penalise ½ mark if no working is shown but correct answer is given.
      • Penalise fully if no working shown but answer given in wrong or wrong working done and correct answer given
      • Accept rounding off or truncation of answer to at least 2 decimal places, otherwise penalise ½ mark for rounding off to less than 2 decimal places, unless the answer works out exactly to a whole number or to 1 decimal place.
      • Final accuracy . . . . . . . (1 mark)

        Compare the candidates' correct averaged titre with the school value used in III and award as follows:
      • If within ± 0.1 of school value . . . . . . 1 mark
      • If within ± 0.2 of school value . . . . . . 1 mark
      • If not within ± 0.2 of S.V. . . . . . . . . . 0 mark

        NB
      • If wrong principles of averaging is used by the candidate, then pick the correct values (if any), average and award accordingly.
    3.  
      1. Moles KMnO4 = 3.68 / 158 × average titre / 1000 P
        Conditions and penalties for all the working
        - Accept correct answer for ½ mark if working is not shown.
        - Penalise fully for any strange figure used in the working
        - Answer to be to at least 4 decimal places, otherwise penalise ½ mark for rounding off to less than 4dp, unless it works out exactly to lesser number of decimal places.
        - Answer should be as expected, otherwise penalise ½ mark for wrong arithmetic.
        - Units may not be shown, but if shown must be correct, otherwise penalise ½ mark for wrong units.
      2. Iron (II) ions in 25.0 ml solution F = answer in (d) (i) × 5 P½ = Correct answer P½
      3. Iron (II) ions in 250 ml solution F = answer in (d) (ii) × 10 P ½ = Correct answer P½
        Iron (II) ions in 100 ml solution N after the reaction with Mg = Answer in (iii) P½ = Correct answer P½
      4. Iron (II) ions in 100 ml solution N before the reaction with Mg = 0.5 × 100 / 1000 √½ = 0.05 P½
      5. Iron (II) ions that reacted with magnesium = answer in (d) (v) - answer in (d) (iv) P½ = Correct answer P½
    4. Calculate the mass of Mg that reacted = Answer in (d) (vi) × 24 P½ = Correct answer P½
    5.  
      1. ∆H for reaction in J = 100 × 4.2 × ∆T in (a) P½ = Correct answer P½ or ∆H for reaction in kJ = 100 / 1000 × 4.2 × ∆T in (a)
      2. Molar enthalpy = answer in (f) (i) / answer in (d) (vi) P = Correct answer P½

        Conditions
        - Ignore formula for working ∆H, otherwise penalise ½ mark for wrong formula.
        - Penalise ½ mark for wrong or no units or for wrong or no sign on correct answer.
  2.  

    Observations

    Inferences

    (a)Dissolves to form a colourless solution P

                                                               

    Soluble salt // absence of coloured ions// Cu2+, Fe2+, Fe3+  absent P                   

    (b) (i) No white ppt P               

    Zn2+(aq), Al3+(aq) and Pb(aq) absent

    3 – mentioned – P1mk
    2 – mentioned– P½mk
    1 – mentioned – 0mk

    (ii) No white ppt P

    SO42-(aq), SO32-(aq) and CO32-(aq) absent

    3 – mentioned – P1mk
    2 – mentioned – P½mk
    1 – mentioned – 0mk

    (iii) Brown solution /yellow solution P  

    Br-(aq) or I-(aq) P1mk (P½mk each)

    (iv) Bright yellow ppt P      

    I-(aq) P

    (c) Blue-purple or lilac P

    KP

  1.  
    1.  
      1.  

        Test 1

        Expected Observations

        To about 2 cm3 of W solution in a test tube, add 2-3 drops of acidified potassium dichromate (VI) solution and warm. P

        Orange colour of acidified potassium   

        dichromate (VI) changes to green P

         
      2.  

        Test 2

        Expected Observations

        To about 2 cm3 of W solution in a test tube, add 2-3 drops of bromine water. P

        Brown / yellow colour of bromine water 
        decolourised P

         
      3.  

        Test 3

        Expected Observations

        To about 2 cm3 of W solution in a test tube, add all of sodium carbonate provided. P

        Effervescence / bubbles of  colourless gas/ fizzing. P

         
    2.  

      Observations

      Inferences

      (i) Orange colour of acidified potassium dichromate (VI) changes to green P

      ─C═C─ or ─C≡C─ or – OH P

      (ii) Brown / yellow colour of bromine water decolourised P

      ─C═C─ or ─C≡C─P

      (iii) Effervescence / bubbles of colourless gas / fizzing. P

      – COOH P

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