CHEMISTRY PAPER 2 - 2020 KCSE PREDICTION SET 1 (QUESTIONS AND ANSWERS)

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INSTRUCTIONS TO CANDIDATES.

  • Answer ALL the questions in this question paper
  • Mathematical tables and silent electronic calculators may be used.
  • All working MUST be clearly shown where necessary.
  1.  
    1. Sodium -20 is a radio isotope and decays by beta emission as shown in the equation below.
      lanjetchemp2q1
      1. Define the term radio isotope.                                                                             (1mk)
      2. Find the value of a and b.                                                                                                (1mk)
      3. Identify the actual symbol of element x.                                                              (1mk)
    2. The half life of sodium - 20 is 0.3 seconds. P grams of sodium - 20 decays to 6 grams in 0.9 seconds.
      1. Calculate the initial mass P, of the isotope.                                                           (2mks)
      2. With reference to sodium - 20, give one use of radio isotopes in medicine.           (1mk)
    3. In the Down’s process, (used to manufacture sodium) a certain salt is added to lower the melting point of sodium chloride from 8000c to 6000
      1. Name the salt that is added.                                                                                    (1mk)
      2. State why it is necessary to lower the temperatures.                                               (1mk)
    4. Explain why aqueous sodium chloride is not suitable as an electrolyte for the manufacture of sodium in Down’s process.       (2mks)
    5. Sodium metal reacts with air to form two oxides; give the formulae of the two oxides. (1mk)
    6. Why is sodium used in nuclear reactors?                   (1mk)
  1. The grid below shows part of the periodic table. Use it to answer the question that follows. (Letters are not actual symbols of elements.)
    lanjetchemp2q2
    1. Write the electronics configuration of the following element.          (1mk)
      1. E
      2. L
    2. Give the formula of one stable ion with an electron arrangement of 2:8 which is(1mk)
      1. Negatively charged.
      2. Positively charged
    3. The oxide of H reacts with both hydrochloric acid and sodium hydroxide to form a salt. What is the nature of the oxide?    (1mk)
    4. Identify the most reactive non-metal, give a reason.   (1mk)
    5. Explain the following observations.
      1. Atomic radius decreases from H to J.    (1mk)
      2. Melting point of J is higher than that of F.  (1mk)
    6. When a piece of element G is placed on water, it melts and a hissing sound is produced as it moves on water.
      1. Explain these observations.                                                                       (2mks)
      2. Write a chemical equation between element G and water.                       (1mk)
    7. 60cm3 of gas E diffused through a porous partition in 50 seconds. How long would it take 60cm3 of sulphur IV oxide gas to diffuse through the same partition, under the same conditions?                                      (3mks)
      (0=16, S=32)
  2.  
    1. Complete the table below to show the differences between the two types of detergents                            (2mks)

      Detergent

      Type of detergent

      Effect on hard water

      C17H35COONa

         

      R-OSO3Na

         
    2. Study the flow chart below and answer the questions that follows
      lanjetchemp2q3b
      1. Names the processes in steps    (1mk)
        I           -
        IV        -
      2. State the reagent and condition necessary for steps II     (1mk)
        Reagent           -
        Condition        -
      3. Identify substances     (1mk)                                                                                      
        A
        B
      4. State the observation in step III.             (1mk)
      5. Write the equation for the reaction in step IV.       (1mk)
    3. A polymer has the following structure.
      lanjetchemp2q3c
      1. A sample of this polymer is found to have a molecular mass of 750. Determine the number of monomers is the polymer.  (Cl=35.5, C=12, H=1)          (2mks)          
      2. State why the polymer above should not be thrown away.                                               (1mk)
  3. The flow chart below represents preparation and properties of oxygen gas. Study it and answer the questions that follow.
    lanjetchemp2q4
    1. Name the following    (2mks)
      1. Solid K
      2. Gas P
      3. Solid Q
      4. Solution R
    2. Write equation leading to the formation of;
      1. Solid Q and solution R     (1mk)
      2. Gas P.                           (1mk)
    3.  
      1. What observation is made when excess ammonia solution is added to solution M. (1mk)
      2. Write an ionic equation for the reaction in C (i) above.   (1mk)
    4. 68g of hot copper metal completely reacted with oxygen gas. Calculate the volume of oxygen gas used (molar gas volume= 24dm3 Cu=63.5)                                                 (3mks)
    5. State two uses of oxygen gas.                                                                                    (2mks)
  1.  
    1. In an experiment to determine the molar heat of reaction when magnesium displaces copper, 0.15g of magnesium powder were added to 25.0cmof 2M copper II chloride solution. The temperature of copper II chloride solution was 250c while that of the mixture was 430c.
      1. Other than increase in temperature, state and explain the observation made during the reaction.  (3mks)
      2. Calculate the heat change during the reaction
        (S.H.C of solution=4.2j/g/k, density of solution=1g/cm3)      (2mks)
      3. Determine the molar heat of displacement of copper by magnesium.   (2mks)
      4. Write the ionic equation for the reaction.      (1mk)
      5. Sketch an energy level diagram for the reaction.    (2mks)
    2.  
      1. Use the reduction potentials below to draw a labeled diagram of an electrochemical cell that can be constructed to measure the electromotive force between magnesium and copper. (2mks)
                                Mg2++2e-→ Mg(s)        EӨ=-2.38V
                                Cu2++2e-→Cu(s)         EӨ=+0.34v
      2. Calculate the EӨ value for the cell above.      (1mk)
  1.  
    1. Chlorine can be prepared using the following reagents; concentrated hydrochloric acid, potassium manganate VII and concentrated sulphuric VI acid.
      1. What is the role of each of the following?
        Potassium manganate VII.                     (1mk)
        Concentrated sulphuric VI acid.              (1mk)
      2. Name the bleaching agent formed when chlorine gas is passed through cold, dilute sodium hydroxide solution.   (1mk)
      3. Name one other use of the compound formed in (ii) above other than bleaching.      (1mk)
    2. State and explain observations that would be made if a moist blue litmus paper was placed in a gas jar full of chlorine gas.       (2mks)
    3. 1.9g of magnesium chloride was dissolved in distilled water. Silver nitrate solution was added until in excess. Calculate the mass of silver nitrate that was used for complete reaction.      (3mks)
      (RMM MgCl2=95, N=14, O=16, Ag=108)
    4. Other than bleaching, state two other uses of chlorine gas.         (2mks)
  1. The table below gives the solubility of ammonium phosphate at the stated temperatures.

    Temp  0C

    10

    18

    26

    34

    42

    50

    Solubility g/100g H2O

    21

    28

    38.5

    51

    68.5

    98

    1. Plot a graph of solubility of ammonium phosphate against temperature.                   (3mks)
      lanjetchemp2q7
    2. Use the graph to;
      1. Determine the solubility of ammonium phosphate at 17 0c.                           (1mk)
      2. Determine the molar concentration of ammonium phosphate at 17 0c.          (2mks)
        (N=14, H=1, P=31, 0=16)
      3. State two uses of solubility curves.      (2mks)
      4. Use an equation to explain how temporary water hardness is removed.        (1mk)
    3. Study the information in the table below and answer the question that follows.
      A mixture containing 90g of sodium carbonate and 72g of lead II nitrate in 100g of water at 600c was cooled to 45 0c.

      Salt

      Solubility(g/100g H2O) at

       

      45C

      60C

       i.   Na2CO3

      35

      80

       ii.  Pb(NO3)2

      77

      101


      1. Identify the salt that crystallized out.                        (1mk)
      2. Calculate the mass of the salt that crystallized out.     (1mk)


MARKING SCHEME

  1.  
    1. Sodium -20 is a radio isotope and decays by beta emission as shown in the equation below.
      1. Define the term radio isotope. (1mk)
        • These are atoms of an element with the same atomic number but different mass number that undergoes a spontaneous disintegration to form a stable nuclide.
      2. Find the value of a and b. (1mrk)
        a=20
        b=12
      3. Identify the actual symbol of x. (1mrk)
        Mg
    2. The half life of sodium -20 is 0.3 seconds. P grammes of sodium -20 decays to 6 grammes in 0.9 seconds.
      1. Calculate the initial mass P, of the isotope. (2mks)
        •  
          lanjetms1b
      2. With reference to sodium -20, give one use of radio isotopes in medicine. (1mrk)
        • To diagnose diseases through x-ray.
        • To treat cancer- radiotherapy.
        • To sterilize surgical apparatus.
    3. In the dawns process, (used to manufacture sodium) a certain salt is added to lower the melting point of sodium chloride from 8000c to 6000c.
      1. Name the salt that is added. (1mrk)
        • Calcium chloride.
      2. State why it is necessary to lower the temperatures. (1mrk)
        • To lower the cost of production.
    4. Explain why aqueous sodium chloride is not suitable as an electrolyte for the manufacture of sodium in Downs process. (2mks)
      • Hydrogen gas will be produced instead of sodium metal
    5. Sodium metal reacts with air to form two oxides; give the formulae of the two oxides. (1mrk)
      • Na2O
      • Na2O2
    6. Why is sodium used in nuclear reacts? (1mrk)
      • It is a better coolant.
  1. The grid below shows part of the periodic table. Use it to answer the question that follows. (Letters are not actual symbols of elements.)
    1. Write the electronics configuration of the following element. (1mrk)
      1. E :           2.6                                                       (½mk)
      2. L :           2.8.8.2                                                 (½mk)
    2. Give the formula of one stable ion with an electron arrangement of 2:8 which is;                                                                                                       (1mrk)
      1. Negatively charged.
        • E2-
      2. Positively charged
        • Gor H3+
    3. The oxide of H reacts with both hydrochloric acid and sodium hydroxide to form a salt. What is the nature of the oxide? (1mk)
      • Amphoteric.
    4. Identify the most reactive non-metal, give a reason. (1mk)
      • F - Has the smallest atomic radius, thus easily gains/ has a strong nuclear force thus easily gains.
    5. Explain the following observation.
      1. Atomic radius decreases from H to J. (1mk)
        • Due to increase in nuclear force across the period.
      2. Melting point of J is higher than that of F. (1mk)
        • J has a strong intermolecular force of attraction compared to F.
    6. When a piece of element G is placed on water, it melts and a hissing sound is produced as it moves on water.
      1. Explain these observations. (2mks)
        • Melts on water since the reaction is exothermic
        • A hissing sound is due to production of hydrogen gas.
      2. Write a chemical equation between element G and water. (1mk)
         2G(s) + 2H20(l)→ 2GOH(aq) + H2(g)
    7. 60cm3 of gas E diffused through a porous partition in 50 seconds. How long would it take 60cm3 of sulphur IV oxide gas to diffuse through the same partition, under the same conditions?       (0=16, s=32) (3mks)
      lanjetms 2g
  1.  
    1. Complete the table below to show the differences between the two types of detergents. (2mks)

      Detergent

      Types of detergent

      Effects on hard water

      C17 H35 COONa

      Soapy

      Does not leather easily 

      R-OSO3Na

      Soapless

      Lathers easily

    2. Study the flow chart below and answer the questions that follow.
      1. Names the in steps; (1mk)
        •  I           :           Fermentation
        • IV        :           Esterification
      2. State the reagent and condition necessary for steps ii (1mk)
        • Reagent           :           Concentrated sulphuric VI acid
        • Condition        :           Temperature of 1800c
      3. Identify substances; (1mk)
        • A         :          Ethanoic acid/  CH3COOH
        • B         :           Ethylethanoate/ CHCOOCH2CH3
      4. State the observation in step iii. (1mk)
        • Burns with a blue flame
      5. Write the equation for the reaction in step IV. (1mk)
        • CH3COOH(aq) + CH3CH2OH(l) mkmkmlCH3COOCH2CH3(aq) + H20(l)
    3. A polymer has the following structure.
      1. A sample of this polymer is found to have a molecular mass of 750. Determine the number of monomers is the polymer. (Cl=35.5, d=12, h=1)     (2mks)
        • RMM of monomer= 62.5(1mk)        No. of monomers =750/62.5 =12 monomers (1 mk)
      2. State why the polymer above should not be thrown away. (1mk)
        • It is non- biodegradable, thus pollutes the environment.
  1. The flow chart below represents preparation and properties of oxygen gas. Study it and answer the questions that follow.
    1. Name the following (2mks)
      • Solid K -           Sodium peroxide.
      • Gas P -           Sulphur IV oxide.
      • Solid Q -           Barium sulphate.
      • Solution R -           Copper II nitrate.
    2. Write equation leading to the formation of;
      1. Solid Q and solution R (1mk)
        • CuSO4(aq) + Ba(NO3)2(aq)   →  Cu(NO3)2(aq) +   BaSO4(s)
      2. Gas P. (1mk)
        • S(s)+O2(g) → SO2(g)
    3.  
      1. What observation is made when excess ammonia solution is added to solution M. (1mk)
        • Deep blue solution is formed.
      2. Write an ionic equation for the reaction in (i) above. (1mk)
        • CU2+(aq)+4NH(aq) →   [CU(NH3)4]2+(aq)
    4. 6g of hot copper metal completely reacted with oxygen gas. Calculate the volume of oxygen gas used (molar gas volume= 24dm3 cu=63.5) (3mks)
      2CU(S)+O→ 2CUO(s)
      Moles of CU= (1.68)/(63.5)
      =0.0265 moles
      Moles of O2 FROM RATIO 2:1
      2=0.0265
      1= ?
      1 × 0.0265 =0.0133 moles.
             2
      1 mole of O2 = 24dm3
      0.0133 = ?
      0.0133 × 24
            1
      =0.3192dm3 or 319.2 cm3
    5. State two uses of oxygen gas. (2mks)
      • Used in hospital by patients with breathing difficulties.
      • Used in welding and cutting of metals.
      • Used by deep sea divers and mountain climbers.
      • Used to remove impurities in steel making
  1.  
    1. in an experiment, to determine the molar heat of reaction when magnesium displaces copper, 0.15g of magnesium powder were added to 25.0cmof 2m copper ii chloride solution. The temperature of copper ii chloride solution was 250c while that of the mixture was 430
      1. Other than increase in temperature, state and explain the observation made during the reaction. (3mks)
        • Blue color of solution fades.
        • Brown solid is deposited.
        • Copper ions are discharged/ displaced to form copper metal and magnesium chloride.
      2. Calculate the heat change during the reaction (S.H.C of solution=4.2s/g/k. density of solution=1g/cm3. (2mks)
        H =25x4.2x18
        =1890J
      3. Determine the molar heat of displacement of copper by magnesium. (2mks)
        nnnnnnjjjj33333
      4. Write the ionic equation for the reaction. (1mk)
        • Mg (s) + Cu2+(aq)   →     Mg2+(aq)   +   Cu(s)
      5. Sketch an energy level diagram for the reaction.            (2mks)
        • a sketch of exothermic reaction
    2.  
      1. use the reduction potentials below to draw a labeled diagram of an electrochemical cell that can be constructed to measure the electromotive force between magnesium and copper. (2mks)
        Mg2++2e→  mg(s)       EӨ=-2.38V
        CU2++2e→ cu(s)         EӨ=+0.34v
        dddgrgtfyhju
      2. Calculate the EӨ value for the cell above. (1mk)
        EӨ= Ered- Eoxid
        (+0.34)- (-2038v)
        =+2.72v
  1.  
    1. Chlorine can be prepared using the following reagent; concentrated hydrochloric acid, potassium manganate vii and concentrated sulphuric VI acid.
      1. What is the role of each of the following?
        Potassium manganate vii :          to oxidize Hc                                     (1mk)
        Concentrated sulphuric VI acid :  to dry the gas                          (1mk)
      2. Name the bleaching agent formed when chlorine gas is passed through cold, dilute sodium hydroxide solution. (1mk)
        • Sodium chlorate 1/ sodium hypochlorite.
      3. Name one other use of the compound formed in (ii) above than bleaching. (1mk)
        • Used as an antiseptic.
    2. State and explain observation that would be made if a moist blue litmus paper was placed in a gas jar full of chlorine gas. (2mks)
      • It turns red then bleaches.
      • The moist blue litmus turns red due to the presence of hydrochloric acid, it is then bleached due to the presence of chloric I acid.
    3. 1.9g of magnesium chloride was dissolved in distilled water. Silver nitrate solution was added until in excess. Calculate the mass of silver nitrate that was used for complete reaction. (3mks)
      (RMM mgcl2=95, N=14, O=16, Ag=108)
      Moles of MgCl2 =1.9=0.02
                                  95
      MgCl2(aq)+2AgNO3(aq)Mg(NO3)(aq)+2AgCl(aq)
         1                  2
      Moles of AgNO3 from ratio 1:2
      =0.02 x 2 = 0.04 moles
             1
      RFM of AgNO3= 170
      Mass of AgNO3=170X0.04
      =6.8g
    4. Other than bleaching state two other uses of chlorine gas. (2mks)
      • Manufacture of Hcl
      • Manufacture of pvc (accept any 2, each 1 mk)
      • Manufacture of insecticides
      • Manufacture of antiseptic
  1. The table below gives the solubility of ammonium phosphate at the stated temperatures.
    1. Plot a graph of solubility of ammonium phosphate against temperature.            (3mks)
      • Scale 1mk
      • Axes 1mk
      • Plot 1mk
    2. Use the graph to;
      1. Determine the solubility of ammonium phosphate at 170c. (1mk)
        • 26g/100g H2O
      2. Determine the molar concentration of ammonium phosphate at 170c. (2mks)
        (N=14, H=1, P=31, 0=16)
        Rmm(NH4)3PO4 =155
        Moles of (NH4)3PO4 =26/155
                                            =0.1677moles
        0.1677= 100cm3
        X         =1000cm3
        X = 1000 x 0.1677
                       100
        X = 1.677mol/l
      3. State two uses of solubility curves.
        • To separate soluble salts.
        • Extract salts from seas/lakes
      4. Use an equation to explain how temporary water hardness is removed. (1mk)
        • Ca(HCO3)2(aq) → CaCO3(aq)+H2O(l)+CO2(g)
        • Mg(HCO3)2(aq) → MgCO3+H2O(l)+CO2(g)
    3. Study the information in the table below and answer the question that follows.

      Salt

      Solubility (g/100g H2O) at

       

      450C

      600C

             I.            Na2CO3

      35

      80

          II.            Pb(NO3)2

      77

      101
      A mixture containing 90g of sodium carbonate and 72g of lead ii nitrate in 100g of water at 600 c was cooled to 450
      1. Identify the salt that crystallized out. (1mk)
        • Sodium carbonate
      2. Calculate the mass of the salt that crystallized out. (1mk)
        600c -           80g
        450c     -      -35g
                             45g crystallized out

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