KCSE 2016 Chemistry Paper 2 Questions with Marking Scheme

1. Use the information in the table below to answer the questions that follow. The letters do not represent the actual symbols of the elements.
   

   Element	Atomic Number	Melting Point (0C)
   R	11	97.8
   S	12	650.0
   T	15	44.0
   U	17	-102
   V	18	-189
   W	19	64.0

   
   
   1. Give reasons why the melting point of:
      1. S is higher than that of R; (1 mark)
      2. V is lower than that of U. (2 marks)
   2. How does the reactivity of W with Chlorine compare with that of R with chlorine? (2 marks) Explain.
   3. Write an equation for the reaction between T and excess oxygen. (1 mark)
   4. When 1.15g of R were reacted with water, 600cm3 of gas was produced. Determine the relative atomic mass of R. (Molar gas volume = 24 000 cm3). (3 marks)
   5. Give one use of element V. (1 mark)
2.  
   1. Describe the process by which Nitrogen is obtained from air on a large scale. (4 marks)
   2. Study the flow chart below and answer the questions that follow.
      
      
      1. Identify gas J.(1 mark)
      2. Using oxidation numbers, show that ammonia is the reducing agent in step (VI)(2 marks)
      3. Write the equation for the reaction that occurs in step (V). (1 mark)
      4. Give one use of ammonium nitrate. (1 mark)
   3. The table below shows the observations made when aqueous ammonia was added to cations of elements E, F and G until in excess.
      

      Cation of	Addition of few drops of aqueous ammonia	Addition of excess aqueous ammonia
      E	White precipitate	Insoluble
      F	No precipitate	No precipitate
      G	White precipitate	Dissolves

      1. Select the cation that is likely to be Zn²⁺ (1 mark)
      2. Given that the formula of the cation of element E is E 2+ , write the ionic equation for the reaction between E2+(aq) and aqueous ammonia. (1 mark)
3.  
   1. Methanol is manufactured from carbon (IV) oxide and hydrogen gas according to the equation:
      CO₂ (g) + 3H₂(g) ⇌ CH₃OH(g) + H₂O(g)
      The reaction is carried out in the presence of a chromium catalyst at 700K and 30kPa. Under these conditions, equilibrium is reached when 2% of the carbon (IV) oxide is converted to methanol
      1. How does the rate of the forward reaction compare with that of the reverse reaction when 2% of the carbon (IV) oxide is converted to methanol?(1 mk) 
      2. Explain how each of the following would affect the yield of methanol: 
         • Reduction in pressure (2mks)
         • Using a more efficient catalyst (2mks)
      3. If the reaction is carried out at 500K and 30kPa, the percentage of carbon (IV) oxide converted to methanol is higher than 2% 
         • what is the sign of ΔH for the reaction? Give a reason (2mks)
         • Explain why in practice the reaction is carried out at 700K but NOT at 500K (1mk)
   2. Hydrogen peroxide decomposes according to the following equation
      2H₂O₂(aq) →2H₂O(l) + O₂ (g)
      In an experiment, the rate of decomposition of hydrogen peroxide was found to be 6.0 x 10^-8 mol dm^-3 S^-1.
      1. Calculate the number of moles per dm3 of hydrogen peroxide that had decomposed within the first 2 minutes (2mks)
      2. In another experiment, the rate of decomposition was found to be 1.8 x 10^{- 7} mol dm^-3S^-1. The difference in the two rates could have been caused by addition of a catalyst. State, giving reasons, one other factor that may have caused the difference in two rates of decomposition (2 marks)
4.  
   1. The set up below can be used to produce sodium hydroxide by electolysing brine.
      
      
      1. Identify gas Y. (1mk)
      2. Describe how aqueous sodium hydroxide is formed in setup above.(2mks)
      3. One of the uses of sodium hydroxide is in manufacture of soaps. State one other use of sodium hydroxide. (1mk)
   2. Study the information given in the table below and answer the questions that follow.
      
      
      1. Construct an electrochemical cell that will produce the highest emf. (3mks)
      2. Calculate the emf of the cell constructed in (i) above. (2mks)
      3. Why is it not advisable to store a solution containing E+ ions in the container made of H? (2mks)
5. The diagram below represents a set up of an electrolytic cell that can be used in the production of aluminium.
   
   
   1. On the diagram, label the anode.
   2. Write the equation for the reaction at the anode.(1 mark)
      
   3. Give a reason why the electrolytic process is not carried out below 950°C. (1 mark)
   4. Give a reason why the production of aluminium is not carried out using reduction process (1 mark)
   5. Give two reasons why only the aluminium ions are discharged. (2 marks)
   6. State two properties of duralumin that makes it suitable for use in aircraft industry.(2 marks)
   7. Name two environmental effects caused by extraction of aluminium. (2 marks)
6.  
   1. Draw the structural formula for all the isomers of C₂H₃CL₃ (2marks)
      
   2. Describe two chemical tests that can be used to distinguish between ethene and ethane. (4marks)
   3. The following scheme represents various reactions starting with propan-1-ol. Use it to answer the questions that follow.
      
      
      1. Name one substance that can be used in step I. (1mark)
      2. Give the general formula of X. (1 mark)
      3. Write the equation for the reaction in step IV. (1mark)
      4. Calculate the mass of propan-I-ol which when burnt completely in air at room temperature and pressure would produce 18dm3 of gas. (C = 12.0; O = 16.0; H = 1.0; Molar gas volume = 24dm³) (3marks)
7.  
   1. Write an equation to show the effect of heat on the nitrate of: (2 marks)
      1. Potassium
      2. Silver
   2. The table below gives information about elements Ai, A2, A3, and A4
      

      Elements	Atomic  Number	Atomic Radius (nm)	Atomic Radius (nm)
      A1	3	0.134	0.074
      A2	5	0.090	0.012
      A3	13	0.143	0.050
      A4	17	0.099	0.181

      1. In which period of the periodic table is element A2? Give a reason. (2 marks
      2. Explain why the atomic radius of:
         • Ai is greater than that of A2;
         • A4 is smaller than its ionic radius. (2 marks)
      3. Select the element which is in the same group as A3.(1 mark)
      4. Using dots (•) and crosses(x) to represent outermost electrons, draw a diagramto show the bonding in the compound formed when Ai reacts with A4. (1 mark)

MARKING SCHEME

1. Use the information in the table below to answer the questions that follow. The letters do not represent the actual symbols of the elements.
   

   Element	Atomic Number	Melting Point (0C)
   R	11	97.8
   S	12	650.0
   T	15	44.0
   U	17	-102
   V	18	-189
   W	19	64.0

   
   
   1. Give reasons why the melting point of:
      1. S is higher than that of R; (1 mark)
         • Metalic bonds in S are stronger than in R
      2. V is lower than that of U. (2 marks)
         • V is monoatomic while U is diatomic
         • The Van der Waals force in V are weaker than in U
   2. How does the reactivity of W with Chlorine compare with that of R with chlorine? (2 marks) Explain.
      • W reacts more vigorously
      • It is easier to lose the odur/valence electrons in W than in T
      • W is more electropositive than R
      • Reactivating of groups elements. Increases down the group
   3. Write an equation for the reaction between T and excess oxygen. (1 mark)
      
      • 4T(s) + 5O₂ (g)→ 2T₂O₅(g)
   4. When 1.15g of R were reacted with water, 600cm3 of gas was produced. Determine the relative atomic mass of R. (Molar gas volume = 24 000 cm3). (3 marks)
      2R(s) + 2H₂O (l) 2ROH (aq) + H₂ (g)
      Moles of gas = 600/24000 = 0.025 moles
      Moles of R = 2 x 0.025
      = 0.05moles
      RAM = 1.15/0.09 = 23
   5. Give one use of element V. (1 mark)
      • Used in florescent bulbs / lumbs
      • Used in arc welding
      • Used in fire extinguishers
      • Used as a preservative in museum
2.  
   1. Describe the process by which Nitrogen is obtained from air on a large scale. (4 marks)
      • Filter the air/ electrostatic precipitation/ Purify the air
      • Pass air through NaOH in KOH to remove CO₂
      • Cool to remove to remove water vapour
      • Cool the remaining gases from a liquid air
      • Perform fractural distillation of liquid ai
      • Nitrogen is collected at – 196⁰ C
   2. Study the flow chart below and answer the questions that follow.
      
      
      1. Identify gas J.(1 mark)
         • Nitrogen II Oxide (NO)
      2. Using oxidation numbers, show that ammonia is the reducing agent in step (VI)(2 marks)
         • 4NH₃ (g) + 3 CUO →2N₂ (g) + 3H₂O(l) + 3 Cu
         • Oxidation no of N in ammonia increases from -3 to 0
         • Oxidation number of reducing agent increases
         • Oxidation number Cu decreases from + 2 to O hence an oxidizing agent Ammonia is a reducing agent
      3. Write the equation for the reaction that occurs in step (V). (1 mark)
         • NH₄NO₃(s) or (aq) →N₂O (g) + 2H₂O (g or l)
      4. Give one use of ammonium nitrate. (1 mark)
         • Fertilizer
         • Explosive
   3. The table below shows the observations made when aqueous ammonia was added to cations of elements E, F and G until in excess.
      

      Cation of	Addition of few drops of aqueous ammonia	Addition of excess aqueous ammonia
      E	White precipitate	Insoluble
      F	No precipitate	No precipitate
      G	White precipitate	Dissolves

      1. Select the cation that is likely to be Zn²⁺ (1 mark)
         • G or G²⁺
      2. Given that the formula of the cation of element E is E 2+ , write the ionic equation for the reaction between E2+(aq) and aqueous ammonia. (1 mark)
         • E₂+(ag) + 2OH^-(ag) → E (OH)₂(s)
3.  
   1. Methanol is manufactured from carbon (IV) oxide and hydrogen gas according to the equation:
      CO₂ (g) + 3H₂(g) ⇌ CH₃OH(g) + H₂O(g)
      The reaction is carried out in the presence of a chromium catalyst at 700K and 30kPa. Under these conditions, equilibrium is reached when 2% of the carbon (IV) oxide is converted to methanol
      1. How does the rate of the forward reaction compare with that of the reverse reaction when 2% of the carbon (IV) oxide is converted to methanol?(1 mk) 
         • Rate of forward reaction is equal to tare of backward reaction
      2. Explain how each of the following would affect the yield of methanol: 
         • Reduction in pressure (2 mks)
           • Production will reduce since equilibrium will shift backward so as to raise the pressure.
         • Using a more efficient catalyst (2 mks)
           • No change in amount of methanol since a catalyst will help reaction to come to equilibrium
      3. If the reaction is carried out at 500K and 30kPa, the percentage of carbon (IV) oxide converted to methanol is higher than 2% 
         • what is the sign of ΔH for the reaction? Give a reason (2 mks)
           • Negative: the reaction is exothermic since it requires low temperature to be fast.
         • Explain why in practice the reaction is carried out at 700K but NOT at 500K (1 mk)
           • To ensure that the reacting particles posses more activation energy
   2. Hydrogen peroxide decomposes according to the following equation
      2H₂O₂(aq) →2H₂O(l) + O₂ (g)
      In an experiment, the rate of decomposition of hydrogen peroxide was found to be 6.0 x 10^-8 mol dm^-3 S^-1.
      1. Calculate the number of moles per dm3 of hydrogen peroxide that had decomposed within the first 2 minutes (2 mks)
         • No. of seconds = 2 x 60 = 120 Sec
           Moles of H₂0₂ decomposed
           = 120 x 6.0 x 10-8 = 7.20 x 10^-6
      2. In another experiment, the rate of decomposition was found to be 1.8 x 10^{- 7} mol dm^-3S^-1. The difference in the two rates could have been caused by addition of a catalyst. State, giving reasons, one other factor that may have caused the difference in two rates of decomposition (2 marks)
         • Concentration of H₂O₂ may be higher since concentration increases the rate of reaction.
4.  
   1. The set up below can be used to produce sodium hydroxide by electolysing brine.
      
      
      1. Identify gas Y. (1mk)
         • Chlorine or Cl
      2. Describe how aqueous sodium hydroxide is formed in setup above.(2mks)
         • Na⁺ and H⁺ migrate to the cathode. The H⁺ ions are preferentially discharged liberating hydrogen gas. Cl^- and OH^- migrate to anode. The Cl^-2 are preferentially discharged liberating Cl₂ gas. Na⁺ migrate to anode through the membrane , they combine with OH to from NaOH
      3. One of the uses of sodium hydroxide is in manufacture of soaps. State one other use of sodium hydroxide. (1mk)
         • Glass making
         • Separating components of air.
         • Manufacture of soda lime, NaoCl and NaClO₃
         • Purification of bauxite
         • Used in science lab.
   2. Study the information given in the table below and answer the questions that follow.
      
      
      1. Construct an electrochemical cell that will produce the highest emf. (3mks)
         
      2. Calculate the emf of the cell constructed in (i) above. (2mks)
         • E = Ered – Eox = + 0.80- (-2087) = + 3.67v
      3. Why is it not advisable to store a solution containing E+ ions in the container made of H? (2mks)
         • H has a high tendancy to lose electrons hence will react with the solution by displacing E+ ions
5. The diagram below represents a set up of an electrolytic cell that can be used in the production of aluminium.
   
   
   1. On the diagram, label the anode.
      
   2. Write the equation for the reaction at the anode.(1 mark)
      • 20₂-(1)→0₂(g)+4e^-
   3. Give a reason why the electrolytic process is not carried out below 950°C. (1 mark)
      • Aluminium oxide would solidify hence the ions will not be free to conduct current.
   4. Give a reason why the production of aluminium is not carried out using reduction process (1 mark)
      • The common reducing agents such as carbon cannot reduce aluminium oxide.
   5. Give two reasons why only the aluminium ions are discharged. (2 marks)
      • They are in higher concentration than Na+ from cryolite
      • Aluminium is lower than sodium in electrochemical series.
   6. State two properties of duralumin that makes it suitable for use in aircraft industry.(2 marks)
      • It is stronger than pure aluminium
      • It is light
   7. Name two environmental effects caused by extraction of aluminium. (2 marks)
      • It causes land degradation due to mining the ores from deep down from the earth
      • Carbon (IV) Oxide produced cause global warming.
6.  
   1. Draw the structural formula for all the isomers of C₂H₃CL₃ (2marks)
      
   2. Describe two chemical tests that can be used to distinguish between ethene and ethane. (4 marks)
      • Add a few drops of acidified potassium dichromate with ethane the solution changes from orange to green while in ethane the solution remains orange
      • Add a few drops of acidified potassium manganate with ethane solution changes from purple to clourless while in ethane the solution remains purple
   3. The following scheme represents various reactions starting with propan-1-ol. Use it to answer the questions that follow.
      
      
      1. Name one substance that can be used in step I. (1mark)
         • Concentrated sulphuric (VI) acid / Al₂O₃/ concentrated phosphoric (V)
      2. Give the general formula of X. (1 mark)
         
      3. Write the equation for the reaction in step IV. (1mark)
         • Na₂CO₃(aq) + 2CH₃CH₂COOH(aq) → 2CH₃CH₂COONa(aq) + H₂O(l) + CO₂(g)
      4. Calculate the mass of propan-I-ol which when burnt completely in air at room temperature and pressure would produce 18dm3 of gas. (C = 12.0; O = 16.0; H = 1.0; Molar gas volume = 24dm³) (3 marks)
         • 2C₃H₇OH(l) + 9O₂(g) → 6CO₂(g) + 8H₂O(l)
7.  
   1. Write an equation to show the effect of heat on the nitrate of: (2 marks)
      1. Potassium
      2. Silver
         

          

   2. The table below gives information about elements Ai, A2, A3, and A4
      

      Elements	Atomic  Number	Atomic Radius (nm)	Atomic Radius (nm)
      A1	3	0.134	0.074
      A2	5	0.090	0.012
      A3	13	0.143	0.050
      A4	17	0.099	0.181

      
      
      1. In which period of the periodic table is element A2? Give a reason. (2 marks)
         • Period 2, two energy levels
      2. Explain why the atomic radius of:
         • A1 is greater than that of A2;
           • A2 has greater atomic number than A1
           • A2 has greater nucleus charge than A1
           • A2 has more protons than A1
           • Therefore across the period from left to right nuclear charge, exert greater pull on electrons hence reduction in size.
         • A4 is smaller than its ionic radius. (2 marks)
           • A4 gains electrons, incoming electron is repelled by existing electrons then electrons cloud increases.
      3. Select the element which is in the same group as A3.(1 mark)
         • A2
      4. Using dots (•) and crosses(x) to represent outermost electrons, draw a diagramto show the bonding in the compound formed when A1 reacts with A4. (1 mark)