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CHEMISTRY Paper 2 Questions and Answers - KCSE 2022 Past Papers

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QUESTIONS:

  1.  
    1. Aluminium and phosphorus form oxides with general formula M,O,. Complete Table 1 by writing the properties of the oxides. (3 marks) 
       Table 1
       Property  Al2O3  P2O3
       Structure    
       Bonding    
       Acid/base character    
    2. The grid in Figure 1 shows part of the Periodic Table. Use it to answer the questions that follow.
      CHEMP21
      1. Give the total number of elements that can be placed in:
        1. period I   (1⁄2 mark)
        2. period 5  (1⁄2 mark)
      2. Place each of the following elements in the grid:
        1. Element X, whose atomic number is 14 (1 mark)
        2. Element Y, with the highest first ionisation energy (1 mark)
        3. Element Z, with the lowest first ionisation energy (1 mark)
        4. Element L, whose ion L2- has electron arrangement 2.8 (1 mark)
        5. Element D, whose ion D2+ has electron arrangement 2.8.8 (1 mark)
        6. Element Q, a halogen with the highest atomic radius (1 mark)
        7. Element R, a period 3 element that exists as a monoatomic gas (1 mark)
  2. The general formula of alkanols is CH2+OH.
    1. Draw the structure and give the name of the alkanol with n = 5. (2 marks)
      Structure
      Name
    2. Table 2 gives the boiling points of some alkanols.
       n  Boiling point/°C
       2  78.5
       3  97.2
       4  117.0
      1. On the grid provided, draw the graph of boiling point against number of carbon atoms, n. (3 marks)
      2. From the graph, determine the boiling point of the alkanol with n = 5. (1 mark)
      3. The boiling point of the alkanol with n = 2 is much higher than that of butane. Explain (C 12.0; H= 1.0; O 16.0). (2 marks)
    3. Alkanols are used as fuel.
      1. Give another use of alkanols. (1 mark)
      2. Write an equation for the combustion of the alkanol with n = 2. (1 mark)
      3. Use the bond energies in Table 3 to calculate the enthalpy change of combustion of the alkanol with n = 2. (3 marks)
         Bond  Energy kJ/mol
         C-C  348
         C-H  412
         C-O  360
         O-H  463
         0=0  496
         C = 0  743
  3.  
    1. Use the following cell notation to answer the questions that follow:
      Al(s)/Al3+(aq) // Pb2+(aq)/Pb(s); Ecell = +1.53 V
      1. State what the symbol / represents. (1 mark)
      2. Write the equation for the cell reaction. (1 mark)
      3. Given that E value for Pb2+(aq) / Pb(s) is -0.13 V calculate the E value for Al3+(aq)/Al(s). (2 marks)
      4. State one use of electrochemical cells. (1 mark)
    2. Figure 2 shows a cell used to electrolyse water.
      CHEMP23B
      1. State why it is necessary to add dilute sulphuric(VI) acid to the water.(1 mark)
      2. State the electrode at which oxygen is produced and give a reason. (1 mark)
      3. Write an equation for the formation of oxygen. (1 mark)
      4. After electrolysing the water for 88 seconds, the volume of oxygen gas collected was 23.0 cm3. Determine the:
        1. volume of hydrogen gas collecte (1 mark)
        2. amount of current used (3 marks)
          (1 Faraday = 96500C)
  4.  
    1. State and explain how a catalyst affects:
      1. rate of a reaction   (2 marks)
      2. yield of the products (1 mark)
    2. Rates of reactions are measured using various methods. In the decomposition of hydrogen peroxide, the rate is measured by recording the volume of oxygen gas produced with time.
      Other than measuring volume of gas produced, describe a method that can be used to measure the rates of each of the following reactions.
      1. Sodium thiosulphate with hydrochloric acid.
        S,O,2-(aq) + 2H+(aq)  → S(s)+ SO2(g) + H2O(1)  (2 marks)
      2. Acidified potassium manganate(VII) with iron(II) sulphate.
        MnO, (aq) + 5Fe2+(aq) + 8H+(aq) → Mn2+(aq) + 5Fe3+(aq) + 4H,O(1) (1 mark)
    3. In an experiment, the rate of decomposition of 50 cm3 of hydrogen peroxide in the presence of manganese(IV) oxide was measured. Figure 3 shows a graph of the results obtained.
      CHEMP24C
      1. Write an equation for the reaction. (1 mark)
      2. Using the graph, determine the maximum number of moles of oxygen produced. (Molar gas volume at room temperature and pressure=24dm3). (2 marks)
      3. Calculate the concentration in moles per litre of hydrogen peroxide. (2 marks)
      4. Determine the rate of decomposition at the 18th second. (1 mark)
      5. State and explain one factor that would increase the rate of decomposition of 50 cm3 of the hydrogen peroxide. (1 mark)
  5.  
    1. Explain how concentrated sulphuric(VI) acid can be prepared from sulphur(VI) oxide gas and distilled water. (2 marks)
    2. Concentrated sulphuric(VI) acid acts as a dehydrating and as a drying agent.
      1. Give an example of a gas that can be dried using concentrated sulphuric (VI) acid.(1 mark)
      2. Complete the following equations to show how concentrated sulphuric(VI) acid acts as a dehydrating agent.
        1. C12H22O11    →      (1 mark)
        2. CoCl2  - 6H2O  → (1 mark)
      3. State the observations made when concentrated sulphuric(VI) acid dehydrates:
        1. C12H22O11           (1 mark)
        2. CoCl2 6H2 O  (1 mark)
    3. State the properties of concentrated sulphuric(VI) acid which are illustrated by the following reactions:
      1. S(s) + 2H2SO4(1) →  3SO2(g) + 2H2O(1)   (1 mark)
      2. NaNO3(s) + H2SO4(1)    →  HNO3(aq) + NaHSO4(s)  (1 mark)
    4. When a mixture of 5 cm3 ethanol, 1 cm3 concentrated sulphuric (VI) acid and 5 cm3 ethanoic acid was heated in a beaker, a pleasant smelling compound was formed.
      1. state the role of the concentrated sulphuric(VI) acid. (1 mark)
      2. write the formula of the pleasant smelling compound.(1 mark)
  6. Various types of cells are used to electrolyse concentrated sodium chloride. One of them is the mercury cell.
    1. Name another type of cell used. (1 mark)
    2. The mercury cell uses titanium or graphite as anode and mercury as cathode. State why steel is not used for the anode. (1 mark)
    3. At the anode, chloride ions and not hydroxide ions are oxidised. Give a reason. (1 mark)
    4. Describe using equations, how sodium hydroxide and hydrogen are produced in the cell. (3 marks)
    5. Give two reasons why it is necessary to recycle the mercury used in the cell. (2 marks)
    6. The products of electrolysis of concentrated sodium chloride find extensive use in industries. State the role of chlorine and sodium hydroxide in the paper industry.
      1. Chlorine  (1 mark)
      2. Sodium hydroxide (1 mark)
  7.  
    1.  
      1. Give the formulae of two ionic compounds that can be used to prepare lead(II) sulphide salt. (I mark)
      2. Two moles of aqueous ammonia reacted with one mole of phosphoric(V) acid. Write an equation for the reaction that took place. (1 mark)
    2. Solid copper(II) sulphate is available either as anhydrous or hydrated salt. Figure 4 shows enthalpy changes involved when water is added to each solid.
      CHEMP27
      1. Calculate the enthalpy change for the process:
        CHEMP27B (1 mark)
      2. Describe how each of the following can be prepared starting with aqueous copper(II) sulphate.
        1. hydrated copper(II) sulphate. (2 marks)
        2. anhydrous copper(II) sulphate.(1 mark)
    3. Aluminium hydroxide is used as an antacid.
      1. Name another compound that is used as an antacid. (1 mark)
      2. The concentration of hydrochloric acid in the stomach is 0.01 M. If an antacid containing aluminium hydroxide is used, calculate the mass of the antacid required to neutralise 100.0 cm3 of the stomach acid (Al=27.0; O 16.0; H = 1.0). (3 marks)

MARKING SCHEME ;

  1.  
    1. Aluminium and phosphorus form oxides with general formula M,O,. Complete Table 1 by writing the properties of the oxides.
      Property  AΙ2Ο3  P2O3
       Structure  Giant ionic   Simple Molecule Murcular
       Bonding  ionic   covalent 
       Acid/base character  Amphotic   Acidic
    2. The grid in Figure 1 shows part of the Periodic Table. Use it to answer the questions  that follow.
      CHEMP2ms1b
      1. Give the total number of elements that can be placed in:
        1. period I         2 elements   (1/2marks)
        2. period 5       18 elements (1⁄2 mark)
      2. Place each of the following elements in the grid:
        1. Element X, whose atomic number is 14(1 mark)
        2. Element Y, with the highest first ionisation energy(1 mark)
        3. Element 2, with the lowest first ionisation energy(1 mark)
        4. Element L, whose ion L2- has electron arrangement 2.8(1 mark)
        5. Element D, whose ion D2 has electron arrangement 2.8.8(1 mark)
        6. VI. Element Q, a halogen with the highest atomic radius(1 mark)
        7. Element R, a period 3 element that exists as a monoatomic gas (1 mark)
  2.  The general formula of alkanels is C,H2+OH
    1. Draw the structure and give the name of the alkanol with n-5
      StructureCHEMP2ms2a
      Name  Pentan-1/ Pentanol
    2. Table 2 gives the boiling points of some alkanels 
       n  Boiling point C
       2  78.5
       3  97.2
       4  117.0
                                                                                                                                                                                                                            (2 marks)
      1. On the grid provided, draw the graph of boiling point against number of carbon atoms, n. (3 marks)
        chemP2pMS2b
      2.  From the graph, determine the boiling point of the alkanol with n = 5. (1mark)
        • 136°C
      3. The boiling point of the alkanol with n = 2 is much higher than that of butane. Explain (C= 12.0; H = 1.0; O= 16.0). (2 marks)
        • Ethanol has hydrogen bonds between its molecules , while butane the weak varder walls forces between its molecules
    3. Alkanols are used as fuel.
      1. Give another use of alkanols. (1mark)
        • Manufacture of: - Synthetic fibres.  /alcoholic drinks
        • As solvents in manufacture of bugs.
      2. Write an equation for the combustion of the alkanol with n = 2. (I mark)
        • CH3CH2OH + 3O2  →  2CO2 + 3H2O
      3. Use the bond energies in Table 3 to calculate the enthalpy change of combustion of the alkanol with n =2.(3 marks)
         Bond  Energy kJ/mol
         C-C  348
         C-H  412
         C-O  360
         O-H  463
         0=0  496
         C=O  743
        CHEMP2QN2C
        Bonds broken
        (5x412)+348+ 360 + 463 + (3x416)
        2060 + 708+463+1488
        =+471945
        Bouds formed
        (4×743) + 6(463)
        -5750 kJ .
        ΔHc Ethanol =+4719+-5750/2
        =-1031kj/mol
  3.  
    1. Use the following cell notation to answer the questions that follow:
      Al(s)/Al3+ (aq) // Pb2+(aq)/Pb(s); E = +1.53 V
      1. State what the symbol /represents.. (1 mark)
        Phase boundary/ interthase between the electrode and electrolyte.
      2. Write the equation for the cell reaction. (1 mark)
        2 Al(s) + 3Pb2+ →  2Al3+ + 3Pb(s)
      3. Given that E value for Pb2+(aq)/ Pb(s) is -0.13V calculate the E value for Al3+ (aq)/Al(s). (2 marks)
        1-53 = -0:13-x
        X=-0.13-1.53 = -1.66 
      4. State one use of electrochemical cells.(I mark)
        Making b. Henies
        Source of electrical energy
        Making day cells, car batteries, other batteries, etc.
    2. Figure 2 shows a cell used to electrolyse water.
      CHEMP23B
      1. State why it is necessary to add dilute sulphuric(VI) acid to the water. (1 mark)
        • To ionise the water  OR  To make water an electrolyte 
      2. State the electade at which oxygen is produced and give a reason. (1 mark)
        -B
        • Its the anode/OH- ions migate to anode
      3. Write an equation for the formation of oxygen. (1 mark)
        4OH-(aq)  →  2H2O(l) + O2(g) + 4e- 
      4. After electrolysing the water for 88 seconds, the volume of oxygen gas collected was 23.0cm3. Determine the:
        1. volume of hydrogen gas collected (1mark)
          vol ratio  O2 :H2 
                           1 :2
          Vel of H2 = 23x2 = 46 cm3.
        2. amount of current used(1 Faraday =96500C)
          MGV = 24000
          mol  of 02 =23/24000  
          1mol 02 = 4 faradays
          9.6 × 10-4 = I × 88/4×96500 =4.2A
  4.  
    1. State and explain how a catalyst affects:
      1. rate of a reaction     (2 marks) 
        • Catalyst increase the rate of a reaction.
        • It lowers the activation energy of the reactants.
      2. yield of the products  (1 mark)
        • No effect on the field
    2. Rates of reactions are measured using various methods. In the decomposition of hydrogen peroxide, the rate is measured by recording the volume of oxygen gas produced with time.
      Other than measuring volume of gas produced, describe a method that can be used to measure the rates of each of the following reactions.
      1. Sodium thiosulphate with hydrochloric acid.
        S2O32- (aq) + 2H+ (aq)  →  S(s)+ SO2(g) + H2O(l)  (2marks)
        • React sodium this Sulphite With hydrochloric acid   in a beaker  placed on a paper with a cross  , record time it takes for the cross to disappear
      2. Acidified potassium manganate(VII) with iron(II) sulphate.
        MnO4(aq) + 5Fe2+(aq) + 8H-(aq)    →  Mn(aq) + 5Fe3+(aq) + 4H2O(l) (1 mark)
        Measure time it takes for the burple colour of KMnO/H+ it to change. colourless
    3. In an experiment, the rate of decomposition of 50 cm3 of hydrogen peroxide in the presence of manganese(IV) oxide was measured. Figure 3 shows a graph of the results obtained.
      CHEMP24C
      1. Write an equation for the reaction. (1 mark)
        27202 (aq)  MnO2→   2H2O(l) + O2(g)
      2. Using the graph, determine the maximum number of moles of oxygen produced. (Molar gas volume at room temperature and pressure = 24dm3). (2 marks)
        Volume  of  O2 produced. = 112CM3 
        Molec of O2 =112/24000 
        =0.00467mol/h.
        (4·67X10-3)
      3. Calculate the concentration in moles per litre of hydrogen peroxide. (2 marks)
        Mole ratio 0.2  : H2O2  
                            1  :  2
        moles of H2O2 = 0.00467×2
                                =0.00934
        mole in 1000cm³ = 0.00934× 1000/50  
                                    = 0.1868m
      4. Determine the rate of decomposition at the 18th second. (1 mark)
        Tangent at 18th sec
        Rate = Δ-1/Δx 
                   = 120 -80/30-10  =40/20  =2cm³/s
      5. State and explain one factor that would increase the rate of decomposition of 50cm3 of the hydrogen peroxide.(1 mark)
        • Heat increase in temperature increases the Kinetic energy of  the molecules. OR
        • increase in concentration of H2O2 which increase rate of effentive collision's
  5.  
    1. Explain how concentrated sulphuric(VI) acid can be prepared from sulphur(VI) oxide gas and distilled water. (2 marks)
      • Bibble SO3 far in cold distilled water using an inverted funnell The reaction of S03 with water is highly exothermic
    2. Concentrated sulphuric(VI) acid acts as a dehydrating and as a drying agent.
      1. Give an example of a gas that can be dried using concentrated sulphuric(VI) acid. (1 mark)
        All gases except NH3 gas and H2S gas
      2. Complete the following equations to show how concentrated sulphuric(VI) acid acts as a dehydrating agent.
        1. C12HO22HO11  → 12C + 11H2O  (1mark)
        2. CoCl26H2O  →  CoCl2 + 6H2O  mark)
      3. State the observations made when concentrated sulphuric(VI) acid dehydrates: (1 mark)
        1. C12H22O11 
          A black solid brown solid formed Valid/brown (1 mark)
        2. CoCl26H2O
          Pink crystals changes to blue powder
    3. State the properties of concentrated sulphuric(VI) acid which are illustrated by the following reactions:
      1. S(s) + 2H2SO4(l)   →   3502(g)+2H2O(l)  (1 mark)
        Oxidation oxidising agent
      2. NANO5(s)  + H2SO4(l)   →  HNO3(aq)  + NaHSO4(s)  (1 mark)
        Volatility/ has low volatility compared to HNO3
    4. When a mixture of 5 cm' ethanol, 1 cm' concentrated sulphuric(VI) acid and 5 cm3 ethanoic acid was heated in a beaker, a pleasant smelling compound was formed.
      1. state the role of the concentrated sulphuric(VI) acid (1 mark)
        Catalyst catalyses the reaction / behydrating agent
      2. write the formula of the pleasant smelling compound. (I mark)
        CH3COO2CH5
  6. Various types of cells are used to electrolyse concentrated sodium chloride. One of them is the mercury cell.
    1.  Name another type of cell used (1 mark).
      • The diaphragm cell // nembrave cell
    2. The mercury cell uses titanium or graphite as anode and mercury as cathode. State why steel is not used for the anode.
      • Iron in steel  reacts with chlorine to form  FeCl3  OR  Steel eat with chlorine present at anode
    3. At the anode, chloride ions and not hydroxide ions are oxidised Give a reason. (1 mark)
      • Concentration of cl- lons is higher than thatof OH- lons
    4. Describe using equations, how sodium hydroxide and hydrogen are produced in the cell. (3 marks)
      1. Na+(l) + e-  →   Na(l) 
      2. Na(l) + Hg(l)  →  NaHg(l) 
      3. 2NaHg(l)  +  2H2O    →  2NaOH(aq) + H2(g) + 2Hg(l)
    5. Give two reasons why it is necessary to recycle the mercury used in the cell. (2 marks)
      • Mecury is expensive
      • mecury is poisonous
    6. The products of electrolysis of concentrated sodium chloride find extensive use in industries. State the role of chlorine and sodium hydroxide in the paper industry.
      1. Chlorine       (1 mark)
        • Chlorine used to breach word
      2. Sodium hydroxide   (1mark)
        • Sodium hydroxide used to digest/ decompose break down-the wood.
  7.  
    1.  
      1. Give the formulae of two ionic compounds that can be used to prepare lead(II) sulphide salt.  (1 mark)
        • Pb (N03)2 / Pb (C2H3O2)2/2
        • Na2 S / Aqueous H2S.
      2. Two moles of aqueous ammonia reacted with one mole of phosphoric(V) acid. Write an equation for the reaction that took place. (1 mark)
        • 2NH3(aq) + H3PO4(aq)   →   (NH4)2 HP04.
    2. Solid copper(II) sulphate is available either as anhydrous or hydrated salt. Figure 4 shows enthalpy changes involved when water is added to each solid.
      CHEMP27
      1. Calculate the enthalpy change for the process: 
        CHEMP27B (1 mark)
        • ΔΗ=(-156.0)-(12.0)
            =-168K jMol
      2. Describe how each of the following can be prepared starting with aqucous copper(II) sulphate.
        1. hydrated copper(II) sulphate. (2 marks)
          • Heat aqueous copper (ii) sulphate to saturation
          • Allow saturated solution to cool to crystalise'z
        2. anhydrous copper(II) sulphate.  (I mark)
          • Heat queous Cuso4 to dryness
    3. Aluminium hydroxide is used as an antacid.
      1. Name another compound that is used as an antacid. (1 mark)
        • Magnesium hydroxide.
      2. The concentration of hydrochloric acid in the stomach is 0.01 M. If an antacid containing aluminium hydroxide is used, calculate the mass of the antacid required to neutralise 100.0 cm3 of the stomach acid (Al=27.0; O 16.0; H 1.0). (3 marks)
        Mole ratio of  Al(OH)3 : HCl = 1:3 ~?2
        RFM of Al(OH)g  • 274+481+3 =78
        Moles of the acid =   100 × 0.01/1000.  = 0.001
        Moles of Al(OH)3  = 0.001/3  =0.000333
        Mass of  A1(OH)2 =  0.00033 x78
                        = 0.02574g

 

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