KCSE 2016 Chemistry Paper 3 Questions with Marking Scheme

1. You are provided with the following:
   • Aqueous potassium iodide, solution A₁
   • Aqueous sodium thiosulphate, solution A₂
   • Acidic solution, solution A₃
   • Starch solution, solution A₄
   • Dilute hydrogen peroxide, solution B
   • Distilled water
     
     You are required to determine the rate of reaction between acidified potassium iodide and hydrogen peroxide
     
     Procedure
   • Step 1
     Using a 10 ml measuring cylinder, place 2 cm³ of solution A₁ into a 100 ml beaker. Add 2 cm³ of solution A₂ followed by 2 cm³ of solution A₃. Add 5 drops of solution A₄ Finally, add 12 cm³ of distilled water and shake the mixture.
     
     
   • Step 2
     Using a burette, place 6cm³ of solution B into a test tube. Pour the measured 6cm³ of solution B into the beaker in Step 1 above and immediately start a stopwatch/clock. Swirl the mixture and place on a white paper. Observe the mixture and immediately stop the stopwatch/clock when a blue colour appears.
     
     Record the time taken in table 2. Measure the temperature of the mixture and record in table 2. l'hese are the results of experiment 1.
     
     
   • Step 3
     Repeat steps 1 and 2 using the volumes of solutions given in table 1 for experiments 2,3,4,5 and 6 and record in table 2.
     
     
   • Table 1
     

     Experiment number	Volume of solutions (cm3)				Distilled Water (cm3)
     	A1	A2	A3	A4
     1	2	2	2	5 drops	12
     2	4	2	2	5 drops	10
     3	6	2	2	5 drops	8
     4	8	2	2	5 drops	6
     5	10	2	2	5 drops	4
     6	12	2	2	5 drops	2

     
     
     1. Calculate the rate of reaction 1^S-¹/_time for each experiment and fill in table 2.
   • Table 2 
     

     Experiment number	Volume of solution A Acidified Potassium iodide (cm)	Temperature (ºC)	Time (sec) t	Rate 1S-¹/t
     1	2
     2	4
     3	6
     4	8
     5	10
     6	12

     
     
     2. Plot a graph of rate of reaction vertical axis against volume of potassium iodide.(3 marks)
     3. Explain why it is necessary to record the temperature for each experiment (1 mark)
     4. If the experiment was repeated using 7 cm' of acidified potassium iodide, solution A determine:
        1. the volume of distilled water that would be used(1 mark)
        2. the time taken for the blue colour to appear.(2 marks
     5. Explain how the rate of the reaction is affected by the volume of acidified potassium iodide.(1 mark)
        
        
2. You are provided with Solution G. Carry out the following tests and record your observations and inferences in the spaces provided.
   1. Measure the pH of the solution using 2 cm³ of Solution G and universal indicator paper.
      

      Observation	inference

   2. To about 2 cm³ of G in a test tube, add dropwise dilute nitric (V) acid until no further change. Retain the mixture for use in tests (c)(i) and ( ii)
      

      Observation	inference

   3. Divide the mixture obtained in (b) above into two portions.
      1. To the first portion, add 3 drops of aqueous lead (II) nitrate.
         

         Observation	inference

      2. To the second portion, add 3 drops of aqueous barium nitrate.
         

         Observation	inference

   4. Describe a test that you would carry out to find out if Zn²⁺ ions are present in Solution G
      

      Observation	inference

   5. Carry out the test described in (d) above.
      

      Observation	inference

3. You are provided with Solid H. Carry out the following tests and record your observations and inferences in the spaces provided.
   1. Place all of Solid H in a boiling tube. Add about 10cm³ mixture thoroughly.
      

      Observation	inference

   2. To about 2cm³ of the mixture in a test tube, add about half of the solid sodium hydrogen carbonate.
      

      Observation	inference

   3. To the remaining amount of the mixture of H in the boiling tube add about 10cm of dilute hydrochloric acid. Shake thoroughly then filter the mixture. Wash the residue with distilled water. Dry the residue using filter papers. Use the residue for tests (i), (ii) and (iii) below.
      1. Place about one third of the residue in a test tube. Add about 10cm³ of distilled water and warm the mixture. To the warm mixture, add the remaining amount of solid sodium hydrogen carbonate.
         

         Observation	inference

      2. To about one third of the residue in a test tube, add about 5 cm³ of distilled water, shake the mixture and then add 3 drops of bromine water.
         

         Observation	inference

      3. To the remaining amount of the residue in the boiling tube, add about 10cm of distilled water, about 5 cm³ of dilute sulphuric (VI) acid and then about 5 cm³ of cthanol. Warm the mixture.
         

         Observation	inference

         
         
         

MARKING SCHEME

1. You are provided with the following:

   • Aqueous potassium iodide, solution A₁
   • Aqueous sodium thiosulphate, solution A₂
   • Acidic solution, solution A₃
   • Starch solution, solution A₄
   • Dilute hydrogen peroxide, solution B
   • Distilled water

     You are required to determine the rate of reaction between acidified potassium iodide and hydrogen peroxide

     Procedure

   • Step 1
     Using a 10 ml measuring cylinder, place 2 cm³ of solution A₁ into a 100 ml beaker. Add 2 cm³ of solution A₂ followed by 2 cm³ of solution A₃. Add 5 drops of solution A₄ Finally, add 12 cm³ of distilled water and shake the mixture.
   • Step 2
     Using a burette, place 6cm³ of solution B into a test tube. Pour the measured 6cm³ of solution B into the beaker in Step 1 above and immediately start a stopwatch/clock. Swirl the mixture and place on a white paper. Observe the mixture and immediately stop the stopwatch/clock when a blue colour appears.

     Record the time taken in table 2. Measure the temperature of the mixture and record in table 2. l'hese are the results of experiment 1.

   • Step 3
     Repeat steps 1 and 2 using the volumes of solutions given in table 1 for experiments 2,3,4,5 and 6 and record in table 2.
   • Table 1
     

     Experiment number	Volume of solutions (cm3)				Distilled Water (cm3)
     	A1	A2	A3	A4
     1	2	2	2	5 drops	12
     2	4	2	2	5 drops	10
     3	6	2	2	5 drops	8
     4	8	2	2	5 drops	6
     5	10	2	2	5 drops	4
     6	12	2	2	5 drops	2

     
     
     1. Calculate the rate of reaction 1^S-¹/_time for each experiment and fill in table 2.
        Expected results
        
        
   • Table 2 
     

     Experiment number	Volume of solution A Acidified Potassium iodide (cm)	Temperature (ºC)	Time (sec) t	Rate 1S-¹/t
     1	2	21	294	3.4 x 10-3
     2	4	21	147	6.8 x 10-3
     3	6	21	100	10 x 10-3
     4	8	21	73	13.9 x 10-3
     5	10	21	69	14.5 x 10-3
     6	12	21	50	20 x 10-3

     
     
     1. COMPLETE TABLE………………………………………………………………………………………5MKS
        
        Marks distribution for the table ,the marks were distributed using the conditions shown in Table below.
        1. Complete table with 18 entries (6 temperature,6 time and 6 correct rates ………..…5mks
        2. Incomplete table with 6 temperature, 6 time and 4 or 5 correct rates……………..4 mks
        3. Incomplete table with 6 temperature, 6 time and 3 correct rates………………4mks
        4. Incomplete table with 6 temperature, 6 time and 4 or 5 correct rates…………………3 mks
        5. incomplete table with 4 or 5 temperature, 4 or 5 time and 4 or 5 correct rates………..3mks 
        6. incomplete table with 4 or 5 temperature, 4 or 5 time and less than 3 correct rates...2 mks 
        7. incomplete table with 4 or 5 temperature, 4 or 5 time and 3 correct rates……...2 mks
        8. incomplete table with 3 temperature, 3 time and 3 correct rates………………..2 mks
        9. incomplete table with 3 temperature, 3 time and less than 3 correct rates………..2
        10. incomplete table with 2 temperature, 2 with or without correct rates…………….1mk
        11. incomplete table with 1 temperature, 1 time and with or without correct rates.….. mks
        12. incomplete table with only one entry award ………………………………………0 mk
        13. incomplete table with only time column mark out of 2 mks and penalize mk for each space not .filledon the time column
            • Penalties
            • Penalize mk once for unrealistic temperature values less than 10oC an more than 400C
            • Penalize mk once for unrealistic time values less than 5 seconds an more than 420 seconds
            • Penalize mk once for time in minutes
            • If no temperature is given penalize 1mk once
            • Accept given to at least 3 decimal places , Penalize mk once if a candidates gives rate in lesdivides s than 3dp places unless it works out exactly to a whole number.
            • Penalize mk once for wrong arithmetic error in working out rate if the error is outside + or – 2 units in third decimal place
            • Where only temperature readings are given ,penalize fully(award 0 mark) for complete table an award accordingly for decimal place and trend.
              
              NOTE
            • Reject rate in fractions and strange values and penalize mk for each entry upto a maximum of 1 mks accept /rate given in standard form or powers of 10
            • If fractions appear followed by a decimal points ignore the fractions and mark accordingly.
            • Where the values are constant in time column award a maximum of mk for complete table.
            • When two sets of values for temperature , time and rates are given award only mk for complete table, for trend and accuracy each of the two values must meet the criteria.
     2. USE OF DECIMALS   1mk
        • Tied to time mk and temperature mk column only subject to at least two readings in each case
          
          Conditions
        • Accept whole numbers ,one or 2 decimal place used for time column used consistently ,otherwise penalize mk
        • Accept for decimals given as whole number or one decimal place of 0.0 or 0.5 or two decimal place of 0.00, 0.25, 0.50 or 0.75 used consistently otherwise penalize mk
     3. ACCURACY   1MK
        • Tied to time column only for experiment number one, compare the candidate value with the school value if within the range of + or -2 seconds award 1mk
     4. TREND   1MK
        • Tied to time and temperature column only ,award mk if the time reading decrease continuously otherwise penalize mk , and mk if temperature is constant otherwise penalize mk
          
          
   • Graph 3mks
     • Labeled axis with units shown or not shown 1/2mks
     • Scale .must cover half of the grid 1/2mk
     • Plots…all points less one correctly plotted 1mk, if only half of the points are plotted penalize half a mark
     • Shape of the curve –straight line passing through at least two correctly plotted points plus the origin on extrapolation ward 1mk
     • If a student values of is strange or missing award mk for correct labeling of axis and penalize fully for the other marking points
       
       
       1. Plot a graph of rate of reaction 1/time vertical axis against volume of potassium iodide. (3 marks)
       2. Explain why it is necessary to record the temperature for each experiment. (1 mark)
          • Because rate of reaction also depends of temperature which remains constant
       3. If the experiment was repeated using 7 cm3 of acidified potassium iodide, solution A1, determine:
          1. The volume of distilled water that would be used. (1mark)
             • 14-7 = 7cm³
          2. The time taken for the blue colour to appear. (2marks)
             • Correct showing ………….. ¹/₂mk Correct reading ……………..¹/₂ mk
             •   Expression  ¹/_time ¹/₂mk ,correct answer ¹/₂mk
       4. Explain how the rate of reaction is affected by the volume of acidified potassium iodide. (1mark)
          • Rate of reaction increases with increase in volume of potassium iodide mk , this is because increase in volume leads to corresponding increase in concentration.
            
            
2. You are provided with Solution G. Carry out the following tests and record your observations and inferences in the spaces provided.
   1. Measure the pH of the solution using 2 cm³ of Solution G and universal indicator paper.
      

      Observation	inference
      pH 10	Weakly alkaline/ basic  reject weak base/ alkali
      pH 11 or 12	Strongly alkaline/basic reject strong base /alkali

   2. To about 2 cm³ of G in a test tube, add dropwise dilute nitric (V) acid until no further change. Retain the mixture for use in tests (c)(i) and ( ii)
      

      Observation	inference
      Effervescence /bubbles of a gas /fizzing reject ; fizzling, hissing , colourless gas on its own (1mark)	HCO3-, CO32-, SO32- 3 ions 1mk, 2 ions 1mk , 1 ion 0mk

   3. Divide the mixture obtained in (b) above into two portions.
      1. To the first portion, add 3 drops of aqueous lead (II) nitrate.
         

         Observation	inference
         White precipitate	SO42-, Cl- penalize 1 mark for each contradictory ion to a maximum of 2mks

      2. To the second portion, add 3 drops of aqueous barium nitrate.
         

         Observation	inference
         White precipitate Reject white precipitate dissolves,white precipitate soluble, white solution	SO42-,penalize 1 mark for each contradictory  ion to a maximum of 1mk

   4. Describe a test that you would carry out to find out if Zn²⁺ ions are present in Solution G
      

      Observation	inference
      Add aqueous ammonia 1mk, dropwiswe till in excess	presence of Zn2+
      White precitate mk solube in excess	Ignore soluble in excess if test was not complete

   5. Carry out the test described in (d) above.
      

      Observation	inference
      No white precipitate	Zn2+ absent

3. You are provided with Solid H. Carry out the following tests and record your observations and inferences in the spaces provided.
   1. Place all of Solid H in a boiling tube. Add about 10cm³ mixture thoroughly.
      

      Observation	inference
      dissolves mk to form a colourless solution mk to form a colourless solution mk accept colourless solution formed	Polar compound/salt present Accept soluble salt present(1mark)

   2. To about 2cm³ of the mixture in a test tube, add about half of the solid sodium hydrogen carbonate.
      

      Observation	inference
      No effervesce /no bubbles of a gas	H3O+ ,H+ , absent

   3. To the remaining amount of the mixture of H in the boiling tube add about 10cm of dilute hydrochloric acid. Shake thoroughly then filter the mixture. Wash the residue with distilled water. Dry the residue using filter papers. Use the residue for tests (i), (ii) and (iii) below.
      1. Place about one third of the residue in a test tube. Add about 10cm³ of distilled water and warm the mixture. To the warm mixture, add the remaining amount of solid sodium hydrogen carbonate.
         

         Observation	inference
         effervesce /bubbles of a gas fizzling,sizzling hissing or colourless gas (1mark)	H3O+ ,H+ , absent  Any of them 1mk, penalize fully for contradictory  ion

      2. To about one third of the residue in a test tube, add about 5 cm³ of distilled water, shake the mixture and then add 3 drops of bromine water.
         

         Observation	inference
         orange /yellow/red bromine water not decolurised (1mk) accept yellow colour of bromine water persists/retained reject orange/red colour of br reject orange/red colour of bromine retained reject brown or red brown as colour of bromine	-C ≡C-

      3. To the remaining amount of the residue in the boiling tube, add about 10cm of distilled water, about 5 cm³ of dilute sulphuric (VI) acid and then about 5 cm³ of cthanol. Warm the mixture.
         

         Observation	inference
         Pleasant smell /fruity smell Reject sweet smell/fruity sweet smell(1mark)	RCOOH/-COOH present reject COOH present(1mark)