KCSE 2015 Chemistry Paper 1 with Marking Scheme

1.        
   1. Give the name of the first member of the alkene homologous series. (1 mark)
   2. Describe one chemical test that can be used to distinguish butanol from butanoic acid. (2 marks)
      
      
2.     
   1. Name the raw material from which sodium is extracted. (1 mark)
   2. Give a reason why sodium is extracted using electolysis. (1 mark)
   3. Give two uses of sodium metal. (1 mark)
      
      
3.     
   1. What is meant by lattice energy? (1 mark)
   2. Study the energy level diagram below and answer the question that follow:
      
      What type of reaction is represented by the diagram? (1 mark)
4.     
   1. State the Boyles law. (1 mark)
   2. A gas occupies 500 cm³ at 27⁰C and 100,000 Pa. What will be its volume at 0⁰C and 101325 Pa? (2 marks)
5. Calculate the mass of Zinc oxide that will just neutralise dilute nitric (V) acid containing 12.6 g of nitric (V) acid in water. (Zn = 65.0; O = 16.0; H = 1.0; N = 14.0). (3 marks)
6. Describe how sodium carbonate is used to remove water hardness. (2 marks)
7. Hydrogen chloride gas can be prepared by reacting sodium chloride with an acid.
   
   1. Write an equation for the reaction between sodium chloride and the acid. (1 mark)
   2. Give two chemical properties of hydrogen chloride gas. (1 mark)
   3. Give two uses of hydrogen chloride. (1 mark)
8. When solid A was heated strongly, it gave off water and a solid residue. When water as added to the solid residue, the original solid A was formed.
   
   1. What name is given to the process described? (1 mark)
   2. Give one example of solid A. (1 mark)
9. The set up below was used to investigate the reaction between hydrogen gas and copper (II) oxide.
   
   
   1. Name substance A. (1 mark)
   2. State the observation made in the combustion tube. ( 1mark)
   3. Explain the observation made in (b) above. (1 mark)
10. The atomic number of an element T, is 15.
    
    1. Write the ionic configuration of the ion T^3-. (1 mark)
    2. Write the formula of an oxide of T. (1 mark)
11. Dilute sulphuric (VI) acid was electrolysed using platinum electrodes.
    Name the product formed at the anode and give a reason for your answer. (2 marks)
12. The curve below shows the variation of time against temparature for the reaction between sodium thiosulphate and hydrochloric acid.
    
    
    1. Write the equation for the reaction between sodium thiosulphate and dilute hydrochloric acid. (1 mark)
    2. Explain the shape of the curve. (2 marks)
13. Dry ammonia and dry oxygen were reacted as shown in the diagram below.
    
    
    1. What is the purpose of the glass wool. (1 mark)
    2. What products would be formed if red hot platinum was introduced into a mixture of ammonia and oxygen. (1 mark)
14. The table below shows behaviour of metals R, X, Y and Z. Study it and answer the questions that follow:
    
    
    1. Arrange the metals in the order of their reactivity starting with the most reactivity. (2 marks)
    2. Name the metal which is likely to be: (1 mark)
       1. X
       2. Y
15. Given the following substances: wood ash, lemon juice and sodium chloride.
    
    1. Name one commerical indicator that can be used to show whether wood ash, lemon juice and sodium chloride are acidic, basic or neutral. (1 mark)
    2. Classify the substances in 15(a) above as acids, bases or neutral. (2 marks)
       

       Acid	Base	Neutral

16. The flow chart below shows various reactions of aluminium metal. Study it and answer the questions that follow:
    
    
    1.       
       1. Other than water, name another reagent that could be R. (1 mark)
       2. Write the formula of reagent Q. (1 mark)
    2. Write the equation for the reaction in step 5. ( 1mark)
17.      
    1. One of the allotropes of sulpur is rhombic sulphur, name the other allotrope. (1 mark)
    2. Concerntrated sulphuric (VI) acid reacts with ethanol and copper.
       State the property of the acid shown in each case. (2 marks)
       
       1. Ethanol
       2. Copper
18. Study the standard electrode potentials below and answer the questions that follow.
    
    
    1. Which of the metals is the strongest reducing agent? (1 mark)
    2. What observations will be made if a silver coin was dropped into an aqueous solution of copper (II) sulphate/ Explain. (2 marks)
19. A radioactive substance weighing M kg took 1900 years for the original mass to reduce to 15 kg. Given that the half life of the radioactive substance is 380 years;
    
    1. Determine the original mass of the radioactive substance. (2 marks)
    2. State two uses of radioactivity in medicine. (1 mark)
20. A crystal of iodine, heated gently in a test tube gave off a purple vapour.
    
    1. Write the formula of the substance responsible for the purple vapour.
    2. What type of bond is broken when the iodine crystal is heated gently? (1 mark)
    3. State one use of iodine. (1 mark)
21. Describe how samples of lead (II) sulphate, ammonium chloride and sodium chloride can be obtained from the mixture of the three. (3 marks)
22. Study the flow chart below and answer the questions that follow.
    
    
    1. Name the process T. (1 mark)
    2. State the formula of W. (1 mark)
    3. State two uses of X. (1 mark)
23. The table below is part of the periodic table. The letters are not the actual symbols of the elements. Study it and answer the questions that follow.
    
    
    1. Select an element which is stored in parrafin in the laboratory. (1 mark)
    2. How do the ionic radii of E and I compare? Explain. (2 marks)
24. The curve below is a cooling curve for water. Study it and answer the questions that follow.
    
    
    1. Explain what happens to the molecules of water in the region BC in the terms of kinetic energy. (2 marks)
    2. In what state is the water in region DE? (1 mark)
25. Starting with barium nitrate solution, describe how a pure sample of barium carbonate can be prepared in the laboratory. (3 marks)
26. A hydrocarbon contains 14.5% of hydrogen. If the molar mass of hydrocarbon is 56, determine the molecular formula of the hydrocarbon. (C = 12.0; H = 1.0) (3 marks)
27.     
    1. Describe how carbon (VI) oxide can be distinguished from carbon (II) Oxide using calcium hydroxide solution. (2 marks)
    2. What is the role of carbon (IV) oxide in fire extinguishing? (1 mark)
28.       
    1. State one source of alkanes. (1 mark)
    2. Ethane gas was reacted with 1 mole of bromine gas. State one observation made during this reaction. (1 mark)
29. An electric current was passed through several substances and the results obtained recorded in the table below.
    
    Which of these substances is likely to be
    
    1. magnesium (1 mark)
    2. hexane (1 mark)
    3. lead (II) bromide (1 mark)

MARKING SCHEME

1.    
   1. Ethene(1 mark)
   2. Add water to each compound. Add Na2CO₃ or NaHCO₃ to a soluble salt of each sample(1 mark)
      C₄H₁₀O no effervescence
      C₄H₆O- effervescence
      or add acidified potassium dichromate (VI) and warm
      C₄H₁₀O-turns from orange to green
      C₄H₆O₂,- no change
      Or add acidified Potassium Manganate (VII)
      C₄H₁₀O- will be decolourised
      C₄H₆O₂- no change
2.      
   1. Brine (NaCl) (1 mark)
   2.      
      • Sodium is very reactive (use electrolysis) (1 mark)
      • More reactive than carbon.
   3. Uses (1 mark)
      Sodium lamps, coolant in nuclear reactors
      Sodium cyanide, sodium amalgam
      Na₂O_2, Extraction of titanium, etc.
3.      
   1. Enthalpy change, when one mole of crystal lattice is broken into its ions in gaseous state. (1 mark)
   2. Endothermic reaction (process) (1 mark)
4.    
   1. Boyle's law: The volume of a fixed mass of a gas is inversely proportional to its pressure or the product of pressure and volume is constant at a fixed temperature. (1 mark)
   2. P₁V₁ = P₂V₂
       T₁          T₂
      100,000 x 500 x 273
      101325 x(273+27)
      = 449 cm³  (2 marks)
5. Equation (1 mark)
   ZnO₍₈₎ + 2HNO₃ → Zn(NO₃)_2.aq + H₂O
   RFM  HNO₃ = 63
   Moles of HNO₃ = 12.6 
                              63
   Moles of Zinc Oxide = ½ x 0.2
   = 0.1 m
   Mass of ZnO = 81
   Mass of ZnO that reacted = 0.1 x 81
   = 8.1 g
6. Add Na₂CO₃ to water. Ca2+ and Mg²⁺ ions precipitate as carbonates (insoluble)
7.    
   1. Equation: NaCl_(s) + H₂SO_4(l) → NaHSO₄₍₈₎ + HCI_(g)  (1 mark)
   2. Chemical properties of HCI
      1. Forms white fumes with ammonia gas (NH4CI)
      2.    
         • Forms FeCl₂ with Fe metal (green solution)
         • Forms white precipitate with Ag+/Pb2+ ions
   3. Uses:
      
      • Pickling metals
      • Forms chlorides with metals
      • Analysis of lab chemicals
      • Manufacture of HCI_(aq)
      • Manufacture vinyl Chloride (PVC) used in chloroethene (1 mark)
8.    
   1. Type of reaction: Reversible reaction/temporary reaction. (1 mark)
   2.    
      • Copper (II) Sulphate salt (Crystals)
      • Copper (II) Chloride hydrated.
        Any other hydrated salts e.g. Cobalt (II) Chloride
9.      
   1. Substance A (1 mark)
      
      • Calcium Oxide
      • fused calcium chloride
      • Accept any other answer
   2. Black Copper (II) Oxide (Solid) changes to brown (1 mark)
      Colourless liquid formed on the cooler part of the combustion tube.
   3. Copper (II) Oxide is reduced to Copper metal.
10.      
    1. 2.8.8 (1 mark)
    2. T₂O₃ /T₂O₅, or P₂O₃/P₂O₅  (1 mark)
11. Product at the anode = Oxygen and water (1 mark)
    Reasons (1 mark)
    OH ions are preferentially discharged to form oxygen
12.    
    1. Equation Na₂S₂O_3aq + 2HCl_aq—> 2NaCl_aq + SO_2(g) +H,₂O_(aq) + S_(s) (1 mark)
    2. Explain:
       • As the temperature increases, the time taken for the reaction to take place decreases.
         Explanation
       • Increase in temperature, leads to increase in kinetic energy, thus increasing the frequency of fruitful /successful collision, hence decrease in time taken for the reaction to take place. (2 marks)
13.      
    1. The purpose of the glass wool. It spreads the oxygen evenly/increase surface area. or enriches the air with oxygen. (1 mark)
    2. Forms NO, Nitrogen ½ (II) Oxide and steam ½ (1 mark)
14.    
    1. Reactivity series starting with the most reactive
       XRZY (2 marks)
    2. X could be potassium Y could be copper
       Accept any other metal
15.    
    1. Universal indicator / litmus paper
    2. Acid, base, neutral. (3 marks)
16. Reagent R - Sodium hydroxide / KOH (1 mark)
    Reagent Q Cl₂ or HCI (1 mark)
    Step V 2 Al_(s) + 3H₂SO_4(aq) → Al₂(SO₄)₃ + 3H_2(g)(1 mark)
17.      
    1. Monoclinic sulphur /Beta sulphur/ Prismatic sulphur
    2.    
       1. Dehydrating property(1)
       2. Oxidising property (1)
18.    
    1. Calcium
    2. No observable change (1) silver is below copper in the reactivity series so it cannot displace it. (1)
19.      
    1. No. of half-lives = 1900 = 5(1)
                                   380
       480 ___ 240___120___60___30___15 (1) 
    2.      
       • Sterilising surgical instrumen
       • Detecting diseases like goitre
       • Detecting ulcers
       • Treating cancer
       • Detecting fracture/flaw
20. Formula of Iodine I₂ (1)
    Weak Van der Waals (1)
    Antiseptic (1)
21. Heat the mixture and collect AICI, as sublimate
    Add water to the remaining sodium chloride dissolves
    Filter to obtain Lead (II) Sulphate as residue .
    Evaporate filtrate to obtain sodium chloride. 
22. Process T - Fermentation (1)
    W-CH,COONa (1)
    Uses of X - Making polythene (1)
    Manufacture of ethanoic acid
23.    
    1. Element stored under paraffin G (1 mark)
    2. E is smaller than I (1). E has two energy levels while I has 3 energy levels. (1)
24. The molecules of water are
    1. Loosing heat (1). The kinetic energy decreases and the molecules move closer to each other (1)
    2. Solid state (1)
25. Add a soluble carbonate (1)
    Filter the mixture
    Wash residue with distilled (1) water and dry residue (1)
26. H = 14.5
    C = (100 – 14.5 = 85.5%)
    C : 4
    Moles 85.5 : 14.5
               12       1
    7.12 : 14.5 
    7:12  : 14.5
    7.12    7.12
    :2.03
    Ratio 1  :  2
    EF CH₂
    MF  = (CH₂)_n = 56
    n = 4
    MF = C₄H₈
27.      
    1. Bubble the gases in calcium hydroxide solution.(1) Carbon (II) Oxide does not react () while Carbon (IV) Oxide forms a white precipitate. (*)
    2. Carbon (IV) Oxide cuts the supply of oxygen.
28.    
    1. Sources of alkanes
       • crude oil/petroleum
       • natural gas/biogas
    2. The brown/red/orange/yellow colour of bromine is discharged/discoloured
29.    
    1. B(1)
    2. A(1) 
    3. C (1)