INSTRUCTIONS TO CANDIDATES

• Answer ALL the questions in the spaces provided in the question paper.
• KNEC Mathematical tables and silent non-programmable electronic calculators may be used.
• All working MUST be clearly shown where necessary
• Candidates should check the question paper to ensure that all pages are printed as indicated and no questions are missing.
• Candidates should answer all the questions in English.

1. The electrical conductivity of barium hydroxide solution was measured after each addition of 1.0 cm³ of dilute sulphuric (VI) acid from a burette. The graph below was obtained.
   
   
   1. Write the chemical equation of the reaction that took place.                            (1 mark)
   2. Explain the graph between,
      1. Q and R         (1 mark)
      2. R and S        (1 mark)

2. A mass of 14.2 g sodium nitrate saturated 32.1 cm³ of water at 32 ^oC. Determine the solubility of sodium nitrate at 32 ^oC. (Density of water = 1g/cm³).  (2 marks)

3. Explain why sulphur is a solid while oxygen is a gas at room temperature. (2 marks)
4. Study the electrode potential in the table below and answer the questions that follow.
                                                     E volts
   Cu²⁺ _(aq) + 2e-   → Cu_(s)            +0.34
   Mg²⁺ _(aq) + 2e-  → Mg_(s)           -2.38
   Ag⁺ _(aq)     + e-  → Ag _(s)           +0.80
   Ca²⁺ _(aq) + 2e-   → Ca _(s)           -2.87
   1. Identify the strongest reducing agent.     (1 mark)
   2. What would be observed when magnesium ribbon is dipped in solution of copper (II) sulphate. Explain.  (2 marks)

5. In a reaction, 0.65 g of impure zinc oxide reacted with 100 cm³ of 0.15 M nitric (V) acid.
   1. Write equation of the reaction.    (1 mark)
   2. Calculate percentage purity of the zinc oxide sample.    (2 marks)
      (Zn = 65, O = 16)

6. The set up below can be used for the laboratory preparation of product W.
   
   
   1. Write chemical equation for the reaction that takes place in the retort flask.   (1 mark)
   2. Explain why product W appears yellow in colour. How is the colour removed? (2 marks)

7. The table below shows information of four elements A, B, C and D. Study it and answer the questions that follow. The letters do not represent the actual symbols of the elements.
   

   Element	Electronic arrangement	Atomic radius	Ionic radius
   A	2.8.2	0.136	0.065
   B	2.8.7	0.99	0.181
   C	2.8.8.1	0.203	0.133
   D	2.8.8.2	0.174	0.099

   
   
   1. Which two elements have similar properties?                                                    (1 mark)
   2. Explain why B ionic radius is larger than its atomic radius.                             (2 marks)

8. The production of ammonia gas involves a reversible reaction as shown.
   
   
   1. What condition is necessary for the chemical equilibrium to be established?   (1 mark)
   2. Suggest two conditions that are likely to shift the equilibrium from right to left. (2 marks)

9. Describe how chloride ions are tested in a solution. (2 marks)
10. The empirical formula of X is CH₂Br. Given that 0.235 g of X occupies a volume of 56 cm³ at 546 K and 1 atmosphere pressure, determine its molecular formula.
    (H = 1.0, C = 12.0, Br = 80.0, molar gas volume at STP = 22.4 dm³)    (3marks)

11. When a piece of sodium metal is place in cold water in a beaker it melts producing a hissing sound, as it moves on the surface of the water. Explain these observations.      (3 marks)

12. The set-up below was used to prepare a hydrocarbon. Study it and answer the questions that follow.
    
    
    1. Identify solid X and gas Y.
       Solid X      (1 mark)
       Gas Y      (1 mark)
    2. Write a chemical equation for the complete reaction between gas Y and bromine vapour.    (1 mark)

13.  
    1. When excess chlorine gas is bubbled through cold, dilute sodium hydroxide solution, the resulting solution acts as a bleaching agent. Using an equation, explain how the resulting solution acts as a bleaching agent.   (1 mark)
    2. What is observed when chlorine gas is bubbled through a solution of potassium bromide? Explain.   (2 marks)

14.  
    1. Explain why the pH of 1.0 M hydrochloric acid is 1 while that of 1.0 M ethanoic acid is 5.0.  (1 mark)
    2. How can a precipitate of barium sulphate be distinguished from that of barium sulphite? (2 marks)

15. A, B, C and D are dyes present in a mixture. In a given solvent, C is more soluble than B and A is more soluble than C. D is the least soluble. Draw an ascending paper chromatogram showing how they would appear when separated using the solvent. (2 marks)
16. The reaction below refers to the preparation of lead (II) sulphate starting with lead metal. 
            
    1. Name solution A(1 mark)
    2. Write an ionic equation for the reaction in (a) above.                                      (1 mark)
    3. Explain why it is not possible to prepare residue Z using lead metal and dilute sulphuric acid.  (1 mark)

17. The diagram below represents a Bunsen burner flame. Study it and answer the questions that follow.
    
    
    1. Under what condition is the represented flame produced?                            (1 mark)
    2. Which of the regions shown represents the hottest part of the flame?           (1 mark)
    3. Name region Y                                                                                                (1 mark)

18. A typical electrolysis cell uses a current of 40,000 amperes. Calculate the mass in kilograms of aluminium produced in one hour. (Al = 27, 1 Faraday = 96,500 coulombs)   (3 marks)

19.  
    1. Distinguish between endothermic and exothermic reaction. (1 mark)
    2. Nitrogen reacts with oxygen to form nitrogen (II) oxide according to the following reaction 
       N_2(g) + O_2(g) → 2NO_(g) ∆H = +180 kJmol^-1
       Draw an energy level diagram for this reaction including the activation energy. (2 marks)
       

20.  
    1. When a compound T was heated, a brown gas and a residue which was yellow when hot and white when cold were formed. Identify the:
       1. Brown gas    (1 mark)
       2. Residue       (1 mark)
    2. Name a suitable drying agent for ammonia gas.        (1 mark)

21. Give the structural formula of the following organic compounds.
    1. 2-Methylbutane
    2. Pent-2-ene
    3. Ethylpropanoate

22. When iron and steam are heated in a closed container, a dynamic equilibrium is reached.   
      
    1. Define the dynamic equilibrium.                                                                     (1 mark)
    2. What is the effect on equilibrium if magnesium is added? Explain.               (2 marks)

23. State and explain the observations that would be made when burning magnesium is lowered into a gas jar of sulphur (IV) oxide.   (3 marks)

24. A mixture contains barium sulphate, calcium chloride and dry ice. Describe how the components can be separated. (3 marks)

25. In the redox reaction below: 
    2H⁺_(aq) + Cr₂O₇^2-_(aq) + 3SO_2(aq) → 2Cr³⁺_(aq) + 3SO₄^2-_(aq) + H₂O_(l) 
    Identify the reducing agent. Explain.   (2 marks)
26.  
    1. Explain why aluminium utensils do not corrode as easily as iron utensils although aluminium is higher than iron in the reactivity series.   (1 mark)
    2. State two uses of aluminium other than utensils making.                               (2 marks)

27. The diagram below represents a set up that was used to react lithium with steam. Study and answer the question that follows.
    
    
    1. Write an equation for the reaction that takes place. (1 mark)
    2. Why is it not advisable to use potassium metal in place of lithium in the above set-up? (1 mark)
    3. The gas produced above is used for welding. Which other gas is combined with it? (1 mark)
28. Using dots (.) and crosses (×) to represent valence electrons, show bonding in:
    1. Nitrogen trifluoride (N = 7, F = 9)    (1 mark)
    2. Sodium oxide (Na = 11, O = 8)     (1 mark)

29. The graphs below were drawn by measuring the volume of hydrogen produced with time when excess zinc metal in different physical states were reacted with 50 cm³ of 2 M hydrochloric acid.
    
    
    1. Which curve corresponds to the reactions involving powdered zinc?            (1 mark)
    2. Both curves eventually flatten out at the same level of hydrogen. Explain.   (1 mark)

MARKING SCHEME

1.  
   1. H₂SO_4(aq) + Ba(OH)_2(aq) → BaSO_4(s) + H₂O_(l)
   2.  
      1. Conductivity decreases because Ba²⁺ and OH^- ions are converted to insoluble BaSO_4(s) and H₂O_(l) which do not conduct electricity.
      2. Conductivity increases due to the increase of H₂SO_4(aq) beyond the neutralization point or equivalence point.

2. Mass of water = 32.1 cm³ × 1 g/cm³ = 32.1 g √
   32.1 g of water = 14.2 sodium nitrate
   100 g of water = 100 × 14.2 = 44.24 g √
                                 32.1

3. Oxygen is above sulphur in group six, √ the intermolecular force of attraction between molecules increases down the group with increases in size of the molecule. √

4.  
   1. Ca / calcium √
   2. The blue colour of copper (II) sulphate will fade; √½ brown solid will be deposited on magnesium ribbon.√½
      Magnesium reduces the copper (II) ions (blue) to copper metal (brown) and itself oxidised to colourless magnesium ions √ or
      Mg_(s) + Cu²⁺_(aq) → Mg²⁺_(aq) + Cu_(s)

5.  
   1. ZnO_(s) + 2HNO_3(aq) → Zn(NO₃)_2(aq) + H₂O_(l) √
   2. mol HNO₃ = 0.15 × 100/1000 = 0.015 √½
      mol ZnO 0.015/2 = 0.0075 √½
      g ZnO = 0.0075 × 76 = 0.57 √½
      % ZnO = 0.57/65 = 87.7 √½

6.  
   1. KNO_3(s) + H₂SO_4(l) → KHSO_4(s) + HNO_3(l) √
   2. Because of dissolved NO_{2 (g)} √ (formed from thermal decomposition of HNO₃); the colour can be removed by bubbling air through the acid. √

7.  
   1. A and D √
   2. B ion has an extra electron√ which reduces the overall nuclear attraction.√

8.  
   1. Reaction be carried out in a closed system or vessel √
   2. -Increase in temperature √
      -Reduction in pressure √

9. To the solution, add a few drops of lead nitrate solution. √ A white ppt that dissolves on heating confirms Cl^- ions √ or add a few drops of silver nitrate and a white ppt is formed.

10. (1 × 56)/546 = (1 × V₂)/273
    V₂ = 28 cm³ √
    28 cm³ = 0.235 g
    22400 cm³ = (22400 ×0.235)/28 = 188 g √
    n = ¹⁸⁸/₉₄ = 2
    MF = (CH₂Br)₂ or     C₂H₄Br₂ √

11. - Melting: reaction is exothermic. √1
    - Hissing sound: due to hydrogen gas evolved. √1
    - Moving on the surface: sodium is less dense than water and gets propelled by escaping H₂ gas. √1

12.  
    1. Solid X-Calcium carbide / CaC₂ √, Gas Y-Ethyne / C₂H₂ / CH≡CH √
    2. CH≡CH_(g) + 2Br_2(g) → CHBr₂CHBr_2(l) √

13.  
    1. NaOCl_(aq) + dye → NaCl_(aq) + dye-O (colourless compound) √
    2. Colourless solution turns brown √; Chlorine being more reactive than bromine oxidises bromide ions to bromine.√

14.  
    1. As a strong acid, hydrochloric acid is completely dissociated giving a high concentration of H⁺ ions while ethanoic acid is a weak acid and is partially dissociated. √1
    2. Sulphite dissolves in dilute hydrochloric acid or nitric acid with evolution of sulphur (IV) oxide √ while the sulphate does not dissolve.√
15.  
    
    Diagram = 1 mark, Labelling = 1 mark

16.  
    1. Nitric (V) acid √
    2. Pb²⁺_(aq) + SO₄^2-_(aq) → PbSO_4(s) √
    3. An insoluble coating of PbSO_4(s) would prevent contact of the metal with the acid and stop further reaction. √

17.  
    1. Produced when air-hole is open √
    2. X √
    3. Green-blue region√

18. Quantity of electricity = 40,000 × 60 × 60 √
    3 × 96,500 coulombs produce 27 g of Al √
    = 13.43 kg √

19.  
    1. Endothermic reaction- absorbs heat from the surrounding √½
       Exothermic reaction- releases heat to the surrounding √½
    2.  
       

20.  
    1.  
       1. Nitrogen (IV) oxide /NO₂ √
       2. Zinc oxide/ZnO √
    2. Calcium oxide √

21.  
    1. CH₃─CH₂CHCH₃ √
               │
             CH₃
    2. CH₃CH₂CH═CHCH₃ √                                                                                      
    3.        O
              ║
       CH₃CH₂C─O─CH₂CH₃ √

22.  
    1. The forward and backward reactions takes place at the same rate.√
    2. Backward reaction is favoured or equilibrium shift to the left √; Magnesium reacts with steam lowering the concentration which is replaced through backward reaction.√

23. Magnesium continues to burn for some time, √½ white fumes √½ and yellow specks or solid are formed√½. Heat produced by burning magnesium decomposes sulphur (IV) oxide to sulphur (yellow solid) and oxygen √½. Magnesium burns in the oxygen √ ½ to form magnesium oxide (white fumes). √½

24. Heat the mixture √½, dry ice sublimes √½, add water to the mixture √½, filter √½, and filtrate is calcium chloride, residue is barium sulphate √½, heat the filtrate to saturation and then cool to allow crystals of calcium chloride to form. √½

25. Reducing agent SO₂ √
    Oxidation number of S has increased from +4 in SO₂ to +6 in SO₄^2-.
    OS of S in SO₂
    S + 2(-2) = 0
    S = +4 √½
    OS of S in SO₄^2-
    S + 4(-2) = -2
    S= +6 √½

26.  
    1. Aluminium form a protective layer of aluminium oxide on the surface which prevents further oxidation whereas iron does not.√
    2.  
       • Food packaging
       • Head lamp reflectors
       • Overhead electrical cables
       • Construction of aircrafts and ships (Duralumin alloy)
       • Reducing agent in thermite process
       • Abrasive (corundum or emery)
         Any two = 2mks

27.  
    1. 2Li_(s) + H₂O_(g) → Li₂O_(s) +H_2(s) √
    2. K is highly reactive and the reaction would be more vigorous and explosive.√
    3. Oxygen gas√
28.  
    

29.  
    1. Curve A                      
    2. In both cases, hydrochloric acid has been used up in the reaction and the total amount of hydrogen produced is the same. √