Chemistry Paper 2 Questions and Answers - KCSE 2021 Past Papers

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QUESTIONS

  1. Table 1 gives the properties of two compounds, A and B.
       
    1. Table 1
      B
      white, crystalline, efflorescent white, crystalline, deliquescent
      State and explain the observation made when each of the compounds is left exposed in air:
      1. Compound A (2 marks)
      2. Compound B (2 marks)
    2. In an experiment to determine the formula of hydrated magnesium sulphate, a sample was heated in a crucible until a constant mass was obtained. The results are shown in Table 2.
      Table 2
      Mass of crucible 25.62 g
      Mass of crucible + solid before heating 28.08 g
      Mass of crucible + solid after heating 26.82 g
      Using the information in Table 2, determine the formula of the hydrated salt
      (Mg - 24.0; S = 32.0; O=16.0; H = 1.0). (3 marks)
    3. Figure 1 shows analysis of an alloy containing two metals.
      1
      1. Give the name of another product formed in step 1. (1 mark)
      2. Write the formula of the complex ion present in the colourless solution obtained in step 2 (1 mark)
      3. Identify the metals in the alloy, (2 marks)
  2. The flow chart in Figure 2 shows the processes involved in the manufacture of sulphuric(VI) acid.
    2
    1. Explain how the sulphur used in this process is obtained. (2 marks)
    2. Give one advantage of using air in chamber 1 instead of using oxygen gas. (1 mark)
    3. Identify substances: (1)
      1. M (1 mark)
      2. N (1 mark)
    4.      
      1. In chamber 2, drying and purification take place. Give a reason why this is necessary (1 mark)
      2. The reaction in chamber 3 is highly exothermic.
        1. Explain why high temperature is required for the reaction in chamber 3. (1 mark)
        2. State how heat produced in chamber 3 can be utilised in this process
    5. Give a reason why this method of manufacture is known as 'contact process". (1 mark)
    6. Emission of gases in the sulphuric(VI) acid plant may lead to environmental pollution.
      1. State the evidence that could be used to show that the sulphuric(VI) acid plant causes pollution (1 mark)
      2. Explain how the pollution identified in 2(1)(i) can be controlled. (1 mark)
  3.    
    1. Chemical reactions occur as a result of collisions of particles. Give a reason why not all collisions are effective. (1 mark)
    2. State and explain how the following factors affect the rate of reaction:
      1. Surface area of reactants. (1 mark)
      2. Pressure. (1 mark)
    3.  In an experiment to determine the rate of a reaction, marble chips were added to excess 2M hydrochloric acid. The equation for the reaction is:
      CaCO3(s) + 2HCl(aq) → CaCI2(aq) + CO2(g) + H2O(l)
      The volume of carbon(IV) oxide produced was measured at 25°C and recorded after every 30 seconds. Table 3 shows the results obtained.
      Table 3
      Time (seconds) 0 30 60 90 120 150 180 210 240
      Volume of CO2 (cm3) 0 62 92 113 124 130 132 133 133
      1. On the grid provided, plot a graph of volume of carbon(IV) oxide (vertical axis) against time (horizontal axis). (3 marks)
        3
      2. Using the graph, determine the rate of reaction at the:
        1. 45th second. (1 mark)
        2. 105th second. (1 mark)
      3. Give a reason for the differences in the two rates. (1 mark)
      4. Using the graph, determine the mass of marble chips that reacted (2 marks)
        (Ca=40.0; C = 12.0; 0 - 16.0;)
        Molar gas volume at room temperature and pressure = 24000 cm").
  4.      
    1. Sea water contains approximately 3% sodium chloride. Describe how sodium chloride is obtained from sea water. (3 marks)
    2. The solubility of sodium chloride is 36.2 g in 100 g of water at room temperature. Determine the concentration in moles per litre of a saturated aqueous sodium chloride at room temperature (Na= 23.0; Cl= 35.5; density of water = 1.0 gem"). (2 marks)
    3. Ammonia is highly soluble in water.
      1. Explain how aqueous ammonia is prepared starting with ammonia gas. (2 marks)
      2. On the axes provided, sketch a curve showing how solubility of ammonia gas varies with temperature. (1 mark)
        4
      3. Give a reason for the shape of the curve. (1 mark)
    4. Water hardness is due to the presence of magnesium and calcium ions. Explain how these ions get into sources of water. (2 marks)
  5.    
    1. Figure 3 shows part of a Periodic Table.
      5
      1. Select from the table the most reactive:
        1. metal. (1 mark)
        2. non-metal. (½ mark)
      2. Select an element with the highest first ionisation energy. (1 mark)
      3.      
        1. Name the method used to obtain argon from its source. (1 mark)
        2. Give one industrial use of argon. (1 mark)
      4. Explain each of the following observations:
        1. The melting point of lithium is higher than that of potassium. (1 mark)
        2. The melting point of chlorine is lower than that of iodine. (1 mark)
      5. The following ions have the same number of electrons: N2-, Mg2+, 02, Na+
        Arrange them in order of increasing ionic size. Give a reason for the order. (2 marks)
    2. Use Table 4 to answer the questions that follow.
      Table 4
      Property Substance
      I J K
      Melting point (°C) 993 113 -38.9 -85
      Boiling point (°C) 1695 183 357 -60
      Electrical conductivity at room temperature Does not conduct Does not conduct Conducts Does not conduct
      Electrical conductivity in molten state Conducts Does not conduct Conducts Does not conduct
      1. Identify the substance which is a gas at room temperature.
        Give a reason. (1 mark)
      2. Name the particles responsible for electrical conductivity in substance: IH (1 mark)
        1. H
        2. J (1 mark)
      3. Identify the type of forces that hold the particles together in:
        1. H (1 mark)
        2. Κ (1 mark)
  6. Figure 4 shows a flow chart involving reactions of some organic compounds.
    6
    1. Write the formula and give the names of compounds:

      1. Name:
        Formula:
      2. Y
        Name:
        Formula:
    2. Give the reagents and conditions necessary for carrying out:
      1. Step 3. (1 mark)
      2. Step 5. (1 mark)
    3. Step 1 can be carried out using concentrated sulphuric(VI) acid and heat. Name another reagent and conditions that can be used to carry out Step 1. (1 mark)
    4. Give the name of the type of reaction that takes place in
      1. Step 1. (1 mark)
      2. Step 5. (1 mark)
    5.      
      1. Write an equation for the reaction in step 6. (1 mark)
      2. State the observations made in step 6. (1 mark)
  7.      
    1. Using the oxidation numbers of chlorine, explain why the following is a redox reaction.
      HCIO3(aq) + 5HCl(aq) → 3CI2(g) + 3H2O(I) (2 marks)
    2. Use the following standard reduction potentials to answer the questions that follow:
        Half cell reactions Eº/V 
      PSO4(s) + 2e → Pb(s) + SO42-(aq) -0.36
      II  PBO2(s) + SO42- (aq) + 4H+(aq) + 2e → PbSO4 (s) + 2H2O(1) +1.69
      III Fe3+(aq) + e → Fe2+(aq) +0.77
      IV Zn2+(aq) + 2e → Zn(s) -0.76
      MnO4-2-(aq) + 8H+(aq) + 5e → Mn2+(aq) + 4H2O(I) +1.51
      VI  O2(g)+ 2H+ (aq) + 2e → H2O2(aq)  +0.68
      VII Fe2+(aq) + 2e → Fe(s) -0.44
      VIII Ca2+(aq) + 2e → Cu(s) +0.34
      1. The half cells I and II are combined to form an electrochemical cell.
        1. Write an equation for the cell reaction. (1 mark)
        2. Calculate the e.m.f of the cell. (1 mark)
      2. Draw a labelled diagram for the electrochemical cell formed using half cells III and IV. (3 marks)
      3. State and explain the observations made when a few drops of acidified potassium manganate(VII) are added to hydrogen peroxide.
        (3 marks)
      4. Coating iron with zinc is a more effective way of corrosion prevention than coating it with copper. Explain. (2 marks)

MARKING SCHEME

  1.      
    1.      
      1. Powder
        loss water of crystalization
      2. Dissolve/color loss liquid
        Absorbs water vapour
    2.   
        MgSO4 H2O
      Mass(g) 1.20 1.26
      Moles 1.20
       120
      1.26
       18
        0.01
      0.01
      0.07
      0.01
        1 7
      MgSO4.7H2O
    3.         
      1. Hydrgen gas
      2. [Zn(OH)42-
      3. Zinc/Zn
        Copper/Cu
  2.        
    1. Three concentric pipes
      Super heated water through outer pipe
      Hot compressed air through inner pipe
      Molten sulphur through middle pipe
    2. Air is cheap/Economical/Readily available
    3.    
      1. Concentrated sulphur(VI) acid/ H2SO4
      2. Water/H2O
    4.      
      1. Impurities poisons catalyst/make it inefficient
        1. Increase rate of reaction
          Effective collisions/Fruitful
        2. Preheat SO2 & O2/ reactants
          Recycling of heat
    5. Reactants come in contact with catalyst
    6.      
      1. Rusting of mettalic structures
        Stone structure wearing/ chlorosil
        Death of aquatic life
        Wilting of plants
        Respiratory diseases
        Acid rain loweing pH of soil
      2. Passing through Ca(OH)2/CaO
        Scrubbing
  3.      
    1. Particles not possess necessary kinetic energy /activation of energy
      Particles collide in wrong orientation
    2.        
      1. Increases rate of reaction
        Move particles are in contact
        More collision per unit time
        More particles exposed
      2. Increases rate of reaction
        Increases manner of collision
        Molecules of gaseous reactants close
        Decrease volume/frequency of collision
    3.      
      1.     
        7
      2.      
        1. Tangent 45
          Calculations from graph
          dy2 - dy1 
          dx2 - dx
          Ans = cm3/sec
        2. Tangent 105
          Calculations from graph
          dy2 - dy1 
          dx2 - dx
          Ans = cm3/sec
      3. Rate at 45th is greater than 105th Hence faster rate of reaction/
        Rate at 105 sec is low to reduce reactants
      4. Moles =  133  
                       
         24000
        Moles ratio 1:1
        CaCO
        5.54 x 10-3 x 100 RFM
        100 x 133
           24000
        Ans = 0.554g
  4.      
    1. Heat/Boil/Evaporate to saturation
      Allow to cool; crystals formed
      or
      Sea water trapped in pan/shallow pond 
      solid crystallises out
    2. RFM NaCl = 58.5
      36.2 x 1000 = 362
           100
       362  
      58.5
      6.188M or 6.19M
    3.      
      1. Pass inserted funnel to prevent suck back
      2.      
        8
      3. Solubility decreases with increase in temperature becasue the gaseous particles gain energy and escape from solution
    4. Formation of carbon acid reacts with rocks with Ca & Mg salts leading to Ca2+ and Mg2+ ions
  5.      
    1.     
      1.      
        1. Cs
        2. F
      2. Hl
      3.        
        1. Fractional distillation
        2. Used in flourescent bulbs/lamps
      4.      
        1. Lithium has stronger metallic bonds than potassium
        2. Iodine has more forces than chlorine
          Iodine is stronger/more intermolecular forces
      5. Mg2+; Na+; O2-; N3-
        Protons decreases from Mg to nitrogen hence nuclear attraction decreases from Mg to N
    2.      
      1. K; boiling point below room temperature
      2.      
        1. ions/ mobile ions
        2. electrons/ delocalized electrons
      3.      
        1. electrostatic forces/ ionic bonds
        2. weak van der waals forces/ intermolecular
  6.        
    1.      
      1. Magnesium propoxide (CH3CH2CH2O)2Mg
      2. Sodium propanoate CH3CH2COONa
    2.      
      1. H+/ KMnO4/H+K2Cr2O7
        Warm/ Heat/ High temperature
      2. Propane. High temperature/
        High pressure/
        340K - 360K
        Heat catalyst
        30 - 40
    3. Al2O3 300ºC/ Aluminium Oxide/High temperature/ Heat
    4.        
      1. Dehydration
      2. Addition polymerization
    5.        
      1. CH3CHCH2 + Br2 → CH3CHBr2
      2. Brown/Yellow/Orange/decolorized
  7.        
    1. Oxidatopn of Cl in HClO from
      +5 to 0  Reduction
      -1 to 0 - Oxidation
    2.      
      1.        
        1. PbO2(s) + 2SO42-(aq) + 4H+ + Pb → 2PbSO4(s) +2H2O(I) 
        2. +1.69 = 0.36
          + 2.05V
      2.    
        10
      3. H+/KMnO4 Decolorized/ Purple to colourless
        Effervescence/ Bubbles of colourless gas
        H2O2 oxidised to O2 gas/ Production of O2S4
        MnO4- 
        Mn2+ 
      4. Zinc is more reactive than iron
        Iron is more reactive than copper/copper is less reactive than iron

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