Chemistry Questions and Answers - Form 4 Mid-term Exams Term 1 2022

QUESTIONS

Instructions:
Answer all the questions in the spaces provided

1.                      
   1. Define a soluble base. (1mk)
   2. Aqueous solutions of 2M ethanoic acid and 2M nitric (v) acid were tested for electrical conductivity. Which solution is a better conductor of electricity? Explain. (2mks)
2.                      
   1. Explain why it is not advisable to prepare a sample of carbon (IV) oxide using lead (II) carbonate and dilute sulphuric (VI) acid. (2mks)
   2. State a method that can be used to collect dry carbon (IV) oxide gas. Give a reason. (1mk)
3. The following are formulae of organic compounds. Use the formular to answer the questions that follow.
   CH₃CH₂CH₂OH;
   CH₃COOH;
   CH₃CH₂CH₂CH₃;
   CH₃CCCH₃;
   Select;
   1.                      
      1. Two compounds which when reacted together produce a pleasant smelling compound. (1mk)
      2. Name the compound formed in (i) above. (1mk)
      3. Select an unsaturated hydrocarbon.
   2. Name the compound selected in a (iii) above. (1mk)
4.                    
   1. State Boyle’s law. (1mk)
   2. Explain why a balloon filled with helium gas deflates faster than a balloon of the same size filled with argon gas. (2mks)
5. 30.0cm³ of aqueous sodium hydroxide containing 8.0g per litre of sodium hydroxide were completely neutralized by 0.294g of a dibasic acid. Determine the relative formula mass of the dibasic acid. (Na=23, O=16, H=1) (3mks)
6. Study the flow chart in figure below and answer the questions that follow.
   
   Gas N forms a white suspension with aqueous calcium hydroxide.
   1. Name the anion present in the potassium salt. (1mk)
   2. Write an ionic equation for the formation of solid M. (1mk)
   3. Give the uses of gas N. (2mks)
7. Element U has atomic number 12 while element V has atomic number 16.
   1. Using dot (.) and cross(x) diagram show bonding in the two elements. (1mk)
   2. State the bond type in the compound formed in (a) above. Explain. (2mks)
8. When ethane gas is compressed at a high temperature, a solid is formed.
   1. Give the name of the solid. (1mk)
   2. Explain why it is not advisable to allow the solid accumulate in the environment. (2mks)
9. In the Harber process, nitrogen reacts with hydrogen according to the following equation.
   3H₂(g) + N₂(g) 2NH₃(g) ; H= -92KJmol^-1
   1. What would be the effect of adding a catalyst on the position of the equilibrium? (1mk)
   2. What would be the effect of increasing the pressure to the system on the position of the equilibrium? Explain. (2mks)
   3. Explain why it is not advisable to use temperatures higher that 773K in the haber process. (2mks)
10. You are provided with solid potassium hydrogen carbonate. Describe how a solid sample of potassium nitrate can be prepared. (2mks)
11.                  
    1. Define the term molar heat of displacement. (1mk)
    2. The following ionic equation represents the reaction between Zinc metal and an aqueous solution of copper ions.
       Zn(s) + Cu²⁺(aq) Zn²⁺(aq) + Cu(s) H =-ve
       Draw an energy level diagrams to represent the reaction. (2mks)
12. Study the setup below and answer the questions that follow.
    
    
    1. Name the substance that was collected in test tube A. (1mk)
    2. Write an equation for the reaction which occurs in tube B: in
       1. In the first few minutes of the experiment. (1mk)
       2. After a long time. (1mk)
    3. Explain the equations in (b) above. (2mks)
    4. Give a suitable conclusion for the experiment in the set up. (1mk)
13. Explain why a solution of sodium chloride conducts electricity while that of sugar does not. (2mks)
14. Explain why commercial indicators are preferred to flower extracts as acid-base indicators. (2mks)
15. (NH₄)₂HPO₄ is a fertilizer used by farmers to boost their crop production.
    1. Calculate the mass of phosphorous in a 20kg packet of (NH₄)₂HPO₄ (N=14, H=1, P=31, O=16) (2mks)
    2. State one advantage of this fertilizer, (NH4)2HPO4 over urea CO(NH2)2 (1mk)
16. Name the technique used to separate coloured substances in green leaves. (1mk)

MARKING SCHEME

1.                              
   1. Define a soluble base. (1mk)
      • A substance that dissociates in water to produce hydrocide ions as the only negative ions.
   2. Aqueous solutions of 2M ethanoic acid and 2M nitric (v) acid were tested for electrical conductivity. Which solution is a better conductor of electricity? Explain. (2mks)
      • Nitric acid
        It is a strong acid hence dissociates completely to produce many hydrogen ions.
2.                  
   1. Explain why it is not advisable to prepare a sample of carbon (IV) oxide using lead (II) carbonate and dilute sulphuric (VI) acid. (2mks)
      • The reaction starts but soon stops.
      • It is the insoluble calcium sulphate produced which form a coating on the surface of the calcium carbonate.
   2. State a method that can be used to collect dry carbon (IV) oxide gas. Give a reason. (1mk)
      • Downwards delivery/upward displacement of air
      • Co₂ is denser that air
        Or
      • Using a gas syringe the gas produced exerts pressure on the piston then pushes it.
3. The following are formulae of organic compounds. Use the formular to answer the questions that follow.
   CH₃CH₂CH₂OH;
   CH₃COOH;
   CH₃CH₂CH₂CH₃;
   CH₃CCCH₃;
   Select;
   1.                  
      1. Two compounds which when reacted together produce a pleasant smelling compound. (1mk)
         • CH₃CH₂CH₂OH and CH₃COOH
      2. Name the compound formed in (i) above. (1mk)
         • propylethanoate
      3. Select an unsaturated hydrocarbon.
         • CH₃CCCH₃
   2. Name the compound selected in a (iii) above. (1mk)
      • But-2-yne
        NB; don’t mark if not selected in a (ii) above
4.                  
   1. State Boyle’s law. (1mk)
      • The volume of fixed mass of gas is inversely proportional to its pressure at a constant temperature.
   2. Explain why a balloon filled with helium gas deflates faster than a balloon of the same size filled with argon gas. (2mks)
      • helium is less dense that argon; hence it diffuses out faster than argon.
        Rej; deflates for diffusion
5. 30.0cm³ of aqueous sodium hydroxide containing 8.0g per litre of sodium hydroxide were completely neutralized by 0.294g of a dibasic acid. Determine the relative formula mass of the dibasic acid. (Na=23, O=16, H=1) (3mks)
   
6. Study the flow chart in figure below and answer the questions that follow.
   
   Gas N forms a white suspension with aqueous calcium hydroxide.
   1. Name the anion present in the potassium salt. (1mk)
      • Carbonate/CO₃^2- or sulphite ion, SO₃^2-
   2. Write an ionic equation for the formation of solid M. (1mk)
      • Ba²⁺+CO₃^2- → BaCO₃(s)
        Or
      • Ba²⁺(aq)+ SO₃^2-(aq) BaSO₃(s)
   3. Give the uses of gas N. (2mks)
      • If CO₂
      • Making soft drinks/aerated drinks
      • Cloud seeding
      • In refrigeration
      • In extinguishing fires
      • Making baking powder
      • If SO₂
      • As a fumigant
      • Manufacture of H₂SO₄
      • Bleaching agent
      • A preservative
7. Element U has atomic number 12 while element V has atomic number 16.
   1. Using dot (.) and cross(x) diagram show bonding in the two elements. (1mk)
   2. State the bond type in the compound formed in (a) above. Explain. (2mks)
      • Ionic/electrovalent bond
      • There is complete transfer of electrons from the metal V to the non-metal V.
8. When ethane gas is compressed at a high temperature, a solid is formed.
   1. Give the name of the solid. (1mk)
      • Polythene/polyethene
   2. Explain why it is not advisable to allow the solid accumulate in the environment. (2mks)
      • Non-biodegradable hence pollutes the environment.
        Accept does not decompose for non-biodegradable.
9. In the Harber process, nitrogen reacts with hydrogen according to the following equation.
   3H₂(g) + N₂(g) → 2NH₃(g) ; H= -92KJmol^-1
   1. What would be the effect of adding a catalyst on the position of the equilibrium?
      • No effect/has no effect on position or equilibrium
   2. What would be the effect of increasing the pressure to the system on the position of the equilibrium? Explain. (2mks)
      • Equilibrium shifts to the right
      • Increase in pressure favour the side with fewer molecules.
   3. Explain why it is not advisable to use temperatures higher that 773K in the haber process. (2mks)
      • Forward reaction is exothermic, excessive temperatures would favour backward reaction, it lowers the yield.
10. You are provided with solid potassium hydrogen carbonate. Describe how a solid sample of potassium nitrate can be prepared. (2mks)
    • Measure a certain volume of dilute nitric (V) acid and place it in a beaker.
    • Add potassium hydrogen carbonate little by little as the mixture is stirred until effervescence stops.
    • Filter the excess KHCO₃
    • Evaporate the solution to saturation and allow it to cool for crystals to form.
    • Dry the crystals between filter papers.
11.              
    1. Define the term molar heat of displacement. (1mk)
       • The heat evolved/ the enthalpy change when one mole of a substance is displaced from its ions in solution.
    2. The following ionic equation represents the reaction between Zinc metal and an aqueous solution of copper ions.
       Zn(s) + Cu²⁺(aq) →  Zn²⁺(aq) + Cu(s) H =-ve
       Draw an energy level diagrams to represent the reaction. (2mks)
12. Study the setup below and answer the questions that follow.
    1. Name the substance that was collected in test tube A. (1mk)
       • Water
    2. Write an equation for the reaction which occurs in tube B: in
       1. In the first few minutes of the experiment. (1mk)
          • CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s)+H₂O(l)
       2. After a long time. (1mk)
          • CaCO₃(s) + Ca(OH)₂(aq) → CaCO₃(s) +H₂O(l)
    3. Explain the equations in (b) above. (2mks)
       • Carbon (IV) oxide reacts with calcium hydroxide (lime water) to form a white precipitate after a short while; then the precipitate dissolves to form a colourless solution when more carbon (IV) oxide is passed through it due to the formation of the soluble Ca(HCO₃)₂.
    4. Give a suitable conclusion for the experiment in the set up. (1mk)
       • Burning candle produces water and CO₂/ candle contains carbon and hydrogen/ candle is a compound of carbon and hydrogen/candle is a compound of carbon and hydrogen.
13. Explain why a solution of sodium chloride conducts electricity while that of sugar does not. (2mks)
    • A solution of NaCl contains free ions/ mobile ions that are free to conduct electricity while sugar contains molecule/no charged ions/ mobile ions/free ions hence cannot conduct electricity.
14. Explain why commercial indicators are preferred to flower extracts as acid-base indicators. (2mks)
    • The composition of commercial indicators remain constant hence gives consistent results while the composition of flower extracts changes with time giving inconsistent results.
15. (NH₄)₂HPO₄ is a fertilizer used by farmers to boost their crop production.
    1. Calculate the mass of phosphorous in a 20kg packet of (NH₄)₂HPO₄ (N=14, H=1, P=31, O=16) (2mks)
       • RFM of (NH₄)₂ HPO₄ = (14x2) + (9x1) +31 (16x4)
         = 28+9+31+64
         = 132
         31g or P = 132
         20,000g of P=?
         = 20,000 x 132
                      31
         = 4697g/ 4.697kg
         Or
       • ²⁰/₁₃₂ x 31
         = 4.697kg
    2. State one advantage of this fertilizer, (NH₄)₂HPO₄ over urea CO(NH₂)₂ (1mk)
       • (NH₄)₂ HPO₄ has two nutrients available for crops; nitrogen and phosphorus, while urea has only one
16. Name the technique used to separate coloured substances in green leaves. (1mk)
    • Chromatography