Chemistry Paper 1 Questions and Answers - KCSE 2021 Past Papers

QUESTIONS

1.      
   1. Draw a labelled diagram showing the atomic structure of ²⁴₁₂ mg. (2 marks)
   2. The atomic number of phosphorus is 15. Draw a dot (•) and cross (x) diagram for the compound formed when phosphorus reacts with chlorine, atomic number 17. (1 mark)
2.      
   1. State the condition under which a Bunsen burner produces a luminous flame. (1 mark)
   2. Write an equation for the reaction that takes place in a luminous flame assuming the laboratory gas is butane. (1 mark)
   3. One of the regions in the non-luminous flame is the unburnt gas region. Describe how the presence of this region can be shown using a wooden splint. (1 mark)
3.      
   1. The elements sodium, magnesium and aluminium belong to group I, II and III respectively. Select the element with the highest electrical conductivity and give a reason. (1 mark)
   2. Complete Table 1 to show the products of electrolysis for concentrated sodium chloride and molten sodium chloride.
      Table 1
      

      Compound	Anode	Cathode
      Concentrated sodium chloride
      Molten sodium chloride

      (2 marks)
4. A small piece of sodium metal was placed in a beaker containing pure water revision.
   1. State two observations made during the reaction. (1 mark)
   2. State and explain another observation made when a drop of phenolphthalein is added to the mixture in the beaker. (1 mark)
   3. Explain why it is not advisable to carry out this experiment using potassium metal. (1 mark)
5. Describe how a pure sample of copper(II) nitrate crystals can be prepared using recycled copper wire. (3 marks)
6. The following apparatus and chemicals are used to investigate the percentage of air used when iron rusts: iron filings, 100 ml measuring cylinder, trough and water.
   1. Draw a setup of the experiment. (2 marks)
   2. Write an expression to show how the percentage of air used is calculated at the end of the experiment (1 mark)
7. Figure I shows a graph of atomic radius of some group I and group II elements.
   
   1. Explain why the atomic radius of sodium is higher than that of:
      1. lithium (1 mark)
      2. magnesium (1 mark)
   2. Predict the atomic radius of calcium. (1 mark)
8. Compound D with formula, C₃H₄ was reacted with excess hydrogen chloride gas.
   1. Give the name of compound D. (1 mark)
   2. Draw two possible structures of the products formed. (2 marks)
9. Study the setup in Figure 2 and answer the questions that follow.
   
   
   1. State the precaution that should be taken in carrying out the experiment. Give a reason. (1 mark)
   2. State the observations made in the boiling tube. (2 marks)
10. Consider the following reaction:
    N_2(g) + 3H_2(g) ⇌ 2NH_3(g)
    The enthalpy change is 92.4 kJ per mole of nitrogen.
    1. Give the enthalpy change per mole of ammonia. (1 mark)
    2. State and explain how each of the following affects the yield of ammonia: 
       1. Increase in temperature. (1 mark)
       2. Finely divided iron. (1 mark)
11. Study the flow chart in Figure 3 and answer the questions that follow.
    
    
    1. Identify solid E. (1 mark)
    2. Name the type of reaction that takes place in chamber 1. (1 mark)
    3. Write an equation for the reaction that takes place in chamber 2. (1 mark)
12. Compounds H and J have the following structures.
    
    
    1. Give the names of:
       1. Compound H. (1 mark)
       2. Compound J. (1 mark)
    2. State the conditions necessary for H and J to react. (1 mark)
13. Rhombic sulphur is one of the allotropes of sulphur
    1. Draw the structure of rhombic sulphur. (1 mark)
    2. Describe the observations made when rhombic sulphur is heated from room temperature until it boils. (1 mark)
14. The molar enthalpy of solution for potassium sulphate (K,SO) is +23.8 kJ.
    1. On the axes provided, draw a labelled energy level diagram for the dissolution process of potassium sulphate in water. (2 marks)
       
    2. Calculate the enthalpy change when 5:12 gof potassium sulphate is completely dissolved in water (K = 39:0;5=32,0; 0 - 16.0)(1 mark)
15.        
    1. State Gay-Lussac's law. (1 mark)
    2. 180 cm³ of nitrogen(II) oxide gas was reacted with 400 cm³ of oxygen gas.
       1. Write an equation for the reaction. (1 mark)
       2. Calculate the total volume of the gases at the end of the reaction. (3 marks)
16. Describe how the setup in Figure 4 can be used to distinguish between 50.0 cm of 0.2M hydrochloric acid and 50.0 cm of 0.2 M ethanoic acid using pieces of 6 m length of magnesium ribbon and a stop watch. (3 marks)
    
17. Describe how dilute nitrie(V) acid and blue litmus papers can be used to distinguish between solid samples of sodium carbonate and sodium sulphite. (3 marks)
18.      
    1. Describe how propanone can be used to extract a pure sample of sunflower oil. (2 marks)
    2. State why sodium hydroxide solution is not suitable for the extraction of sunflower oil. (1 mark)
19. 31.5 cm³ of concentrated nitric(V) acid was diluted to 500 cm³. 10.0 cm³ of the dilute acid required 25.0 cm³ of 0.4M sodium hydroxide for neutralisation.
    1. Calculate concentration of the:
       1. dilute acid. (1 mark)
       2. concentrated acid. (1 mark)
20. Figure 5 shows part of a radioactive decay series.
    
    1. Write a nuclear equation for the formation of nuclide K from nuclide X. (1 mark)
    2. The half-life of nuclide X is 47 minutes. Determine the percentage of nuclide X that remains after 188 minutes. (2 marks)
21. Aluminium is extracted from aluminium oxide by electrolysis.
    1. Other than the cost of electricity, give another reason why this method is expensive. (1 mark)
    2. Calculate the mass of aluminium obtained when a current of 20A is used for 5 hours.
       (1 Faraday - 96500 C; Al - 27.0)(2 marks) 
22. Explain each of the following observations:
    1. Articles made of copper turn green when left exposed in air over a long period of time. (1 mark)
    2. Addition of aqueous ammonia to a solution containing copper(II)ions produces a deep blue solution (1 mark)
23.      
    1. State what is meant by relative atomic mass of an element. (1 mark)
    2. A compound of carbon and element X with formula, CX, contains 3.6% carbon by mass. Calculate the relative atomic mass of X. (2 marks)
24. Carbon(II) oxide can be prepared by dehydration of ethanedioic acid
    1. Complete the following equation to show the reaction that takes place. (1 mark)
       H₂0₂0₄
    2. Name another reagent that can be used to prepare carbon(II) oxide by dehydration (1 mark)
25. Figure 6 shows an incomplete diagram of a setup for laboratory preparation of nitrogen gas.
    
    
    1. Complete the setup in Figure 6 to show how nitrogen gas can be collected. (2 marks)
    2. The nitrogen prepared using this setup is purer than that obtained from air. Give a reason. (1 mark)
26. Hydrazine,    is used as a fuel in rockets. Using the bond energies in Table 2, calculate the enthalpy change for combustion of hydrazine.
    N₂H₄ (1) +0₂(g) → N₂(g) + 2H₂O(g)
    Table 2
    

    Bond	Bond Energy kJ/mol
    N-H	388
    N-N	163
    O=O	496
    N≡N	944
    O-H	463

    (3 marks)
27.      
    1. Table 3 gives the standard reduction potentials of some group VII elements.
       Table 3
       

       Reduction equations	Eº/V
       CI2 + 2e→2CI-	+1.36
       Br2 +2e→2Br-	+1.07
       I2 + 2e→21-	+0.54

       State and explain the reactions that take place when aqueous bromine is added to a sample of sea water containing both chloride and iodide ions. (2 marks)
    2. Give a reason why potassium iodide is added to table salt

MARKING SCHEME

1.          
   1.    
      
      Nuclear composition
      12p
      12n
      Electrons
      OR
      3energy levels
      Nuclear with occupied protons and neutrons
   2. 
      
      1 or 0
      Use of only dots or crosses (penalise fully)
2.      
   1. When airhole/collar is closed or fully closed 
   2. CH_4(g) + 4O_2(g) → C_(s)  + 3CO_(g) + CO_(g) +5H₂O_(I) 
      OR
      CH_4(g) + 4O_2(g) → C_(s)  + CO_(g) + CO_(g) +5H₂O_(I) 
   3. Slip a wooden splint across the middle part of the flame. The central part remains unburnt / uncharred while the outer part burns
      
3.        
   1. Aluminium
      It has 3 delocalised electrons while sodium and magnesium has one and two respectively
   2.     
      

      Compound	Anode	Cathode
      Concentrated sodium chloride	Chlorine or CL2	Hydrogen H2
      Molten sodium chloride	Chlorine or CL2	Sodium Na

4.        
   1.       
      • The piece of metal darts/floats
      • Melts into silvery ball
      • Production of effervescence/hissing sound
      • The beaker becomes warm
   2. Solution turns pink because sodium hydroxide/alkaline solution is formed
   3. Potassium reacts explosively with water/ more vigorously/ more violent
5.      
   • Heat the copper wire in air to form copper
   • Add excess copper_(II) oxide to dilute nitric acid
   • Filter to remove unreacted copper_(II) oxide
   • Heat the resulting solution to saturation
   • Allow it to cool to form crystals 
   • Dry/filter the crystals
6.          
   1.    
      
   2. Initial height of air column - Final height of air column
                       Initial height of air column
      OR
      Initial height of water - Final height of water
                       Initial height of water
7.      
     
   1.        
      1. Na = 2.8.1
         Li = 2.1
         Sodium has 3 energy levels while lithium has two
         or
         Li = 2
         Na = 2.8.1
      2. Mg = 2.8.2
         Na = 2.8.1
         The effective nuclear charge is higher in magnesium than sodium. Mg has a higher number of protons
   2. 208± 2
      Ithout showing on the graph
      Extrapolate to 20 on x-axis and mark to value
8.           
   1. Propyne 
      prop 1 -yne
      prop -1,2-iodene
   2. ,,
9.        
   1. The experiment should be carried out in a fume chamber out in open since carbon_(II) oxide is poisonous
   2. A white percipitate is formed which dissolves to form a colourless solution
10.        
    1. - 92.4 ⇒ =46.2 KJmol^-1
          2
    2.        
       1. It lowers the yield of ammonia since the forward reaction is exothermic or backward reaction is endothermic
       2. No effect
          A catalyst has no effect on the position of the equilibrium
11.      
    1. Potassium magnate VII or maganese(iv) oxide lead(vi) oxide
    2. Red ox/oxidation
    3. CI_2(g) + 2NaOH → NaCl_(aq) + NaOCl_(aq) + H₂O
12.      
    1.        
       1. 3-methylpentanol/ 3-methylpentanol
       2. Butanoic acid
    2.    
       • Concentrated sulphuric VI acid/ sulphuric acid
       • Warm/heat/ temperature between 3-6ºC
13.        
    1.    
        
    2. Yellow solid forms amber liquid
       As the temperature increases the liquid becomes darker and vicious
       Then it turns dark red/brown and less vicious
14.      
    1.    
       
       OR
       
    2. RFM of K₂SO₄ = 174
       moles of K₂SO₄ = ^5.22/₁₇₄ = 0.03
       ΔH = 0.03 x 23.8 = 0.714KJ
15.        
    1. When gases react, they do so in volumes that bear simple ratios to one another and to the products if gaseous at constant temperature and pressure
    2.        
       1. 2NO_(g) + O_2(g) → 2NO_2(g) 
       2. 2NO + O₂ → 2NO₂ 
          using ratio
          Volume of oxygen =180 x 1
                                              2
          = 90cm³
          Volume of oxygen unreacted = 400 - 90
          = 310
          Volume of NO₂ = 18cm³ 
          Total volume = 310 + 180
          = 490cm³ 
16.           
    • Any contact between 
    • Mg and hydrochloric acid
    • Mg and ethanoic acid
    • Using a stopwatch to show the difference
    • Conclusion - HCl takes a shorter time
      
      
    • Put a 6cm Mg ribbon in conical flask and add 50cm³ HCl. Using a stopwatch, record the volume of gases collected at a time inferral e.g. 15cm³
    • Repeat the experiment using 50cm³ of ethanoic acid
    • More/higher volume of gas will be collected when HCl is used than ethanoic acid at same inferral of time
      OR
      The reaction will take a shorter time to completion when HCl is used than when ethanoic acid used
17. To two different test tubes containing the samples add nitric acid
    • Place blue items at the mouth of test tubes
    • Both turn red
    • Thereafter one of them is bleached
    • The sample that produces bleaching on the litmus is sodium sulphite
18.        
    1.      
       • Crush the sunflower seeds using motar and pestle
       • Add propane and stir
       • Decant
       • Leave the extract on sunlight for propane to evaporate leaving oil behind
    2. It will react with oil to form soap
19.      
    1.        
       1. Moles of NaOH = 0.4 x 25 = 0.01
                                         1000
          Moles of HNO₃ = 0.01
          Molarity of HNO₃ = 0.01 x 1000
                                               10
       2. C₁V₁ = C₂V₂ 
          1 x 500 = 15.9M
            31.5
    2. Add acid to water
20.      
    1.    
       
    2. 188 = 4halflives
       100 - 50 - 25 - 6.25
       OR
       x - ½x - ¼x - ¹/₈x - ¹/₁₆x
       % = ¹/₁₆ x 100 = 6.25%
21.          
    1. The graphite anode has to be replaced periodically
    2. Q = It
       = 20 x 5 x 60 = 360000
       moles = 360000 = 1.244moles
       3 x 96500
       mass = 1.244 x 27
       = 33.588g
22.        
    1. Due to formation of copper_(II) carbonate. Since copper reacts with carbon_(IV)  oxide/CO₂
    2. Due to formation of complex ion of tetramine copper_(II) ions
       OR
       due to formation of tetramine copper _(II) ions
23.      
    1. Is the mass of one atom of an element compound to the mass of carbon - 12
    2.       
       

       Let RAM 7 x be n	C	X
       RAM	12	n	96.4 = 4   n    0.3
       % mass	3.6	96.4	96.4 = 4 x 0.3   n
       No. of moles	3.6   12	96.4   n	n = 96.4        1.2
       	0.3	96.4   n
       Ratio	1	4	= 80.3

24.        
    1. H₂C₂O₄ → CO_(g) + CO_2(g) + H₂O_(I) 
    2.      
       • Methanoic
       • Sodium methanate
25.      
    1.  
            
    2. It has impurities such as noble gases
26. N₂H₄ + O₂ → N₂ + H₂O
    Bonds broken
    4 x 388 = 1552
    1 x 163 = 163
    1 x 496 = 496
    = 2211
    Bonds formed
    1 x 944 = 944
    2 x 463 = 1852
    = -2796
    Enthalpy of combination = -2796 + 2211
    = 585KJmol^- 
27.          
    1. Br_2(aq) + 2I^-_(aq) → 2Br^-_(aq) + I_2(aq)
       Br_2(aq) + 2CI^- → No reaction 
       Bromine will oxidize iodide ions to iodine since it has more positive E^θ  
       Bromine will not displace chlorine since E^θ for Cl^- is more positive
    2. Potassium iodide is a source of iodine is needed to regulate functioning of thyroid gland