Chemistry Paper 2 Questions and Answers - KCSE 2021 Past Papers

QUESTIONS

1. Table 1 gives the properties of two compounds, A and B.
      
   1. Table 1
      

      A	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. 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.
   
   
   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)
         
      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)
         
      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.
      
      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
      	H	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.
   
   
   1. Write the formula and give the names of compounds:
      1. X 
         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.
      HCIO_3(aq) + 5HCl_(aq) → 3CI_2(g) + 3H₂O_(I) (2 marks)
   2. Use the following standard reduction potentials to answer the questions that follow:
      

      	Half cell reactions	Eº/V
      I	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
      V	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)₄^2-
      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.     
         
      2.      
         1. Tangent 45
            Calculations from graph
            dy₂ - dy₁ 
            dx₂ - dx₁ 
            Ans = cm³/sec
         2. Tangent 105
            Calculations from graph
            dy₂ - dy₁ 
            dx₂ - dx₁ 
            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
         CaCO₃ 
         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.      
         
      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+; O^2-; N^3-; 
         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. PbO_2(s) + 2SO₄^2-_(aq) + 4H⁺ + Pb → 2PbSO_4(s) +2H₂O_(I) 
         2. +1.69 = 0.36
            + 2.05V
      2.    
         
      3. H⁺/KM_nO₄ Decolorized/ Purple to colourless
         Effervescence/ Bubbles of colourless gas
         H₂O₂ oxidised to O₂ gas/ Production of O₂S₄
         MnO^4- 
         Mn²⁺ 
      4. Zinc is more reactive than iron
         Iron is more reactive than copper/copper is less reactive than iron