KCSE 2014 Chemistry Paper 3 Questions with Marking Scheme

QUESTIONS

1. You are provided with:
   1. solution J containing copper (II) ions
   2. solution K, 0.1 M sodium thiosulphate
   3. aqueous potassium iodide, solution L
   4. starch indicator, solution M
      
      You are required to determine the:
      
      
   5. concentration of copper (II) ions in solution J
   6. enthalpy change of reaction between copper (II) ions and hydroxide ions.
      
      PROCEDURE I
      
      
   1. Using a pipette and a pipette filler, place 25.0 cm³ of solution J in a 250 ml volumetric flask. Add distilled water to make upto the mark. Label this as solution J₂. Retain solution J for use in procedure II.
      
      
   2.  Place soluton K in a burette. Using a clean pipette and pipette filler, place 25.0 cm³ of solution J₂ in a 250 ml conical flask. Add 10cm³ of potassium iodide, solution L. Shake well, then add 2 cm³ of starch indicator, solution M. Titrate until a blue-black colour appears and continue titrating until the blue-black colour just disappears.
      Record you readings in Table 1 below. 
      
      
   3. Repeat step (b) two more times and complete Table 1. (3 marks)
      
      
      
      Calculate the:
      1. average volume of solution K used. (1 mark)
      2. moles of sodium thiosulphate used; (2 mark)
      3. concentration in moles per litre of copper (II) ions in solution J given that the number of moles of copper (II) ions in 25.0 cm³ of solution J₂ are the same as the moles of sodium thiosulphate used. (2 1/2 marks)
         
         
         PROCEDURE II
         
   1. Using a clean burette, place 5.0 cm³ of solution N into each of the six (6) test-tubes.
      
      
   2. Using an 100 ml measuring cylinder, place 20 cm³ of solution J in a 100 ml plastic beaker. Measure the temperature of solution J and record in Table 2 below.
      
      
   3. To solution J in the beaker, add sodium hydroxide, solution N from one of the test-tubes. Stir the mixture with the thermometer and record in Table 2, the maximum temperature reached. Continue with step (d) IMMEDIATELY.
      
      
   4. Add the sodium hydroxide, solution N from another test-tube to the mixture obtained in (c) above, stir and record the maximum temperature reached in Table 2. Continue adding the sodium hydroxide, solution N from each of the other four test-tubes, stirring the mixture and recording the maximum temperature each time and complete Table 2.
      
      
      
      
      1. On the grid provided, plot a graph of temperature (vertical axis) against volume of sodium hydroxide solution N added. (3 marks)
         
         
         
         
      2. Using the graph, determine the:
         1. volume of sodium hydroxide, solution N that reacted completely with 20 cm³ of solution J; (2 marks)
         2. temperature change, ΔT, for the reaction: (1 mark)
            
            
      3. Enthalpy of the reaction per mole of copper (II) ions. (Heat capacity = 4.2 J g^-1k^-1, density of the mixture = 1.0 gcm^-3) (3 marks)
         
         
2. You are provided with substance P. Carry out the tests below and write your observations and inferences in the spaces provided.
   
   
   1. Describe the appearance of substance P. (1 mark)
      
      
   2. Place about one-third of substance P in a dry test-tube and heat it strongly.
      observations(1 mk) | Inferences(1mk)
      
      
   3. Place the remaining amount of substance P in a boiling tube. Add about 10 cm³ of distilled water and shake well. Retain the mixture for tests in (d) below.
      observations(1 mk) | Inferences(1mk)
      
      
   4. Use about 2 cm³ portions of the mixture obtained in (c) for tests (i) to (iii) below.
      
      
      1. Add two to three drops of aqueous barium nitrate to the mixture.
         observations(1 mk) | Inferences(1mk)
         
         
      2. Add five drops of dilute nitric (V) acid to the mixture.
         observations(1 mk) | Inferences(1mk)
         
         
      3. Add to the mixture aqueous sodium hydroxide dropwise until in excess.
         observations(1 mk) | Inferences(1mk)
         
         
         
   5. Give the formula of the cation and anion present in substance P.
      Cation ......................................... (¹/₂ mark)
      Anion .......................................... (¹/₂ mark)
      
      
      
3. You are provided with an organic substance Q. Carry out the following tests and write your observations and inferences in the spaces provided.
   1. Place about one-third of substance Q on a metallic spatula and ignite it with a Bunsen burner flame. 
      observations(1 mk) | Inferences(1mk)
      
      
   2. Place the remaining amount of substance Q in a boiling tube and add 10 cm³ of distilled water. Heat the mixture and allow it to boil for about 30 seconds. Divide the mixture while still hot into two portions.
      
      1. To the first portion, add solid sodium hydrogen carbonate provided. 
         observations(1 mk) | Inferences(1mk)
         
         
      2. To the portion, add two or three drops of acidified potassium manganate (VII).
         observations(1 mk) | Inferences(1mk) 

MARKING SCHEME

1. Table 1    (PROCEDURE I) 
   

   	I	II	III
   Final burette reading	41.20	19.20	38.00
   Initial burette reading	22.00	0.10	19.00
   Volume of solution K used (cm3)	19.20	19.10	19.00

   1. Average  = 19.2 + 19.1+ 19.0 = 19.10 cm³  (3 marks)
                                   3
   2. Moles of Sodium thiosulphate = 19.1 × 0.1
                                                         1000
      = 0.00191 (1)
      Therefore: Moles of Copper ions in 25 cm³ = 0.00191
      Moles in 250 cm³ = 0.00191 × 10
      = 0.0191 (1)
      Concentration of Copper ions =0.0191 × 1000  (1)
                                                              25
      = 0.764 M (1/2)
2. Table 2           (PROCEDURE II)
   

   Volume of NaOH added (cm3)	0	5	10	15	20	25	30
   Maximum Temperature (°C)	22.5	24.5	26.5	27.0	27.0	26.5	26.0

   
   
   1.  Graph
      (3 marks)
   2.                        
      1. 13.0 ± 0.2
         1 mark for working
         1 mark for value
      2. ▵T = 5.2°C ± 0.1
         1 mark 
   3. ▵H = 33 × 5.2 × 4.2
      = 720.72 J (1)
      Moles of Cu²⁺ = 20 × 0.764
                                  1000
      = 0.01528 (1/2)
      1 mole = 720.721  (1)
                    0 01528
      = -47.2 KJMol^-1 (1/2)
3.  QUESTION 2
   1. White crystalline substance. (1 mark) 
   2. Observations                               
      Colourless liquid condenses on the cool parts of T-Tube leaving behind a white solid(1 mark)
      Inferences
      Hydrated salt or salt contains water of crystallisation(1 mark)
   3. Observations    
      Solid dissolves to form colourless solution.(1 mark)
      Inferences
      P is soluble in water No coloured ions(1 mark)
   4.                        
      1. Observations  
         White PPt formed (1 mark)
         Inferences
         SO₄^2-, SO₃^2- or CO₃^2- present  (2 marks)
      2. Observations 
         No effervescence or no bubbles (1 mark)
         Inferences
         SO₄^2-, present or SO₃^2- or CO₃^2- absent (1 mark)
      3. Observations 
         White PPt (1 mark)
         Inferences
         Mg²⁺ present (1 mark)
   5. Cation =  Mg²⁺ or Magnesium ions (1/2 )
      anion  = SO₄^2- or Sulphate ions (1/2)
4. QUESTION 3                
   1. Observations 
      Burns with a yellow sooty flame or luminous flame. (1 mark)
      Inferences
      
      Organic compound with high C:H ration
      aromatic compound, long chain organic compound. (1 mark)
   2.              
      1. Observations 
         Efferescence observed (1 mark)
         Inferences
         Has a - COOH group or carboxylic/alkanoic acid. (1 mark)
      2. Observations 
         Decolourised(1 mark)
         Inferences
         could be  an alcohol or has  
         (1 mark)