KCSE 2018 Chemistry Paper 3 with Marking Scheme

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QUESTIONS

  1. You are provided with:
    • 0.30 g solid A, magnesium metal 
    • Hydrochloric acid, solution B 
    • 0.15 sodium carbonate, solution C 
    • Methyl orange indicator
      You are required to de:ermine the:
    • Enthalpy change, ΔH per mole, of the reaction between magnesium metal and excess hydrocholoric acid. 
    • Concentration in moles per litre of hydrochloric acid, solution B.

PROCEDURE 1

  1. Using a burette, measure 50.0 cm3 of solution B and place it in a 100 ml plastic beaker.
  2. Measure the temperature of solution B in the beaker after every 30 eccends and record it in Table 1.
  3. At the 90th second, add all of the solid A provided into the beaker, stir with the thermometer and continue measuring and recording the temperature after every 20 seconds. Complete Table 1. Retain the mixture in the beaker for use in procedure II.

Table 1

Time (seconds)  0  30  60  90  120  150  180 210 240 270
Temperature(ºC)       X            

(3 marks)

  1. Plot a graph of temperature (vertical axis) against time on the grid provided.(3 marks)
    graph paper
  2. Determine the change in temperature, AT, for the reaction. Show the working on the graph.
    ΔT = ............................................................................(1 mark)
  3. Calculate the heat change, in joules, for the reaction. Assume that for the solution, specific heat capacity is 4.2Jg-1K-1 and density is 1.0gcm-3. (2 marks)
  4. The relative atomic mass of magnesium is 24.0. Calculate the enthalpy change, ΔH, of the reaction per mole of magnesium. Indicate the sign of ΔH. (1 mark)

PROCEDURE II

  1. Fill a clean burette with the 0.15M sodium carbonate, solution C.
  2. Place all of the mixture in the beaker from procedure I into a 250 ml volumetric flask.
    Add distilled water to the mark and shake thoroughly. Label the mixture as solution D.
  3. Using a pipette filler, pipette 25.0cm3 of solution D into a 250 ml conical flask and add 2 drops of methyl orange indicator.
  4. Titrate solution D in the conical flask with the sodium carbonate, solution C and record the readings in Table 2.
  5. Repeat steps (iii) and (iv) and complete Table 2

Table 2

  II  III 
Final burette reading       
Initial burette reading      
Volume of solution D used (cm3)      

(3 marks)

  1. Determine the average volume of the 0.15M sodium carbonate, solution C, used.(1 mark)
  2. Calculate the number of moles of:
    1. sodium carbonate used.(1 mark)
    2. hydrochloric acid in the 25.0 cm3 of solution D.(1 mark)
    3. hydrochloric acid in the 250 cm3 of solution D.(1 mark)
    4. hydrochloric acid that reacted with magnesium metal.(1 mark)
    5. total number of moles of hydrochloric acid in the 50.0cm3, solution B. (1 mark)
  3. Determine the concentration of hydrochloric acid in moles per litre, in solution B.(1 mark)

2.You are provided with solid E. Carry out the following tests and record the observations and inferences in the spaces provided.

  1. Place about one-third of solid E in a dry test-tube. Heat the solid strongly and test any gas with both blue and red litmus papers.
    Observations Inferences 
       
       
       
    (2 mark) (1 mark)
  2. Place the remaining amount of solid E in a boiling tube. Add about 15 cm3 of distilled water and shake. Divide the mixture into four test tubes each containing about 2 cm3.
    1. To the first portion, add three or four drops of dilute hydrochloric acid.
      Observations Inferences 
         
         
      (1 mark) (2 mark)
    2. To the second portion, add two or three drops of aqueous barium nitrate.
      Observations Inferences 
         
      (1/2 mark) (1/2 mark)
    3.  To the third portion, add aqueous sodium hydroxide dropwise until in excess.
      Observations Inferences 
         
         
      (1 mark) (1 mark)
    4. To the fourth portion, add aqueous ammonia dropwise until in excess.
      Observations Inferences 
         
         
      (1 mark) (2 mark)

3. You are provided with solid F. Carry out the following tests and record the observations and inferences in the spaces provided.

  1. Place about one-third of solid F on a clean metallic spatula and burn it in a Bunsen burner flame.
    Observations Inferences 
       
       
    (1 mark) (1 mark)
  2. Place the remaining amount of solid F in a boiling tube. Add about 10 cm3 water and shake. Use the mixture for tests (i) to (iii) below.
    Observations Inferences 
       
    (1/2 mark) (1/2 mark)
    1. Using about 2 cm3 of the mixture in a test-tube, determine the pH using universal indicator paper and chart.
      Observations Inferences 
         
         
      (1 mark) (1 mark)
    2. To about 2 cm3 of the mixture in a test tube, add two or three drops of acidified potassium manganate(VII).
      Observations Inferences 
         
         
      (1 mark) (1 mark)
    3. To about 2cm3 of the mixture in a test tube add two or three drops of bromine
      Observations Inferences 
         
         
      (1 mark) (1 mark)

MARKING SCHEME

Table 1

Time (seconds)  0  30  60  90  120  150  180 210 240 270
Temperature(ºC) 22.0 22.0 22.0  X 43.0 47.5  48.0  47.0  46.5  46.0 
  • Complete table(1 mark)
  • Use of decimals (Either whole numbers, one decimal place as 0,5 (1 mark)
  • Accuracy (1/mark)
  • Trends - constant readings from t=0 sec to t=60sec, continuous rise in temperature readings from t=120 sec to maximum followed by a drop - (1/2 mark)

Graph
completed graph

  • Scale (1/2 mark)
  • Labeling of axes (1/2 mark)
  • Plotting (1 mark)
  • Curve / lines (1 mark)

b). On graph paper,
ΔT = (50.0 - 22.0°C) = 28.0°C

c). Heat change =  4.2 x 50 x 28.0 Joules
= 5880 Joules

d). Moles of magnesium = 0.30 = 0.0125
                                        24
Enthalpy change. ΔH = -5880 x 1J mol-1
                                  0.0125 
= - 470400 J mol-1 OR - 470.4 kJ mol-1
-ve sign must be shown otherwise penalize

Table 2

  II  III 
Final burette reading  24.80 33.50 41.50
Initial burette reading 1.00 10.00 18.00
Volume of solution D used (cm3) 23.80 23.50 23.50
  • Complete table with three titrations (1 mark)
  • Use of decimals ( 1 or 2) consistently (1 mark)
  • For 2 decimal places, the second decimal should be 0 or 5 (1 mark)
  • Accuracy (any one value compared to the school value that is within 0.10 cmor 0.20 cm3 of the school value)
  • Principles of averaging - average of volumes that are within ± 0.10cm3 to 0.20 cm3 from each other.
  • Final accuracy-should be within 0.10 cm3 to 0.20 cm3 of the school value.

a). Average volume of solution C used
= 23.50 +23.50 cm3
           2
= 23.50 cm3

b).

  1. Moles of sodium carbonate = 0.15 x average titre
                                                         1000
    = 23.50 x 0.15
             1000 
    = 3.525 x 10-3
  2. Reaction ratio is 1 mole Na2CO2 : 2 moles HCI
    Moles of hydrochloric acid in 25.0cm3 = 2 x  Answer in b(i)
    = 2 x 3.525 x 10-3
    =7.05 x 10-3
  3. Moles of hydrochloric acid in 250 cm3 =  Answer in b(ii) x 250
                                                                            25
    -10 x 7.05 x 10-3
    = 0.0705
  4. Mg + 2HCI → Mg Cl2 + H2
    Reaction ratio is Mg: HCI=1:2
    Moles of magnesium = 0.0125
    Moles of hydrochloric acid = 2 x 0.0125
    = 0.0257
  5. Total number of moles of hydrochloric acid in 50 cm3 of solution B
    = Answer in b(iii) + answer in b(iv)
    = 0.0705 +0.025
    = 0.0955

c). Concentration of hydrochloric acid in moles per litre of solution B
= Answer in b(v) x 1000
                 50
= 0.0955 x 1000  Moles per litres
          50
= 1.91 mol dm-3

2.You are provided with solid E. Carry out the following tests and record the observations and inferences in the spaces provided.

  1. Place about one-third of solid E in a dry test-tube. Heat the solid strongly and test any gas with both blue and red litmus papers.
    Observations Inferences 
    Colourless liquid condenses the on cooler parts of the test-tube. 
    Red litmus turned blue 
    Blue litmus remains blue.
    hydrated compound of ammonia salt
    (NH+4)present
    (2 mark) (1 mark)
  2. Place the remaining amount of solid E in a boiling tube. Add about 15 cm3 of distilled water and shake. Divide the mixture into four test tubes each containing about 2 cm3.
    1. To the first portion, add three or four drops of dilute hydrochloric acid.
      Observations Inferences 
      no effervescence CO2-3 absent
      Pb2+ absent
      no white precipitate
      (1 mark) (2 mark)
    2. To the second portion, add two or three drops of aqueous barium nitrate.
      Observations Inferences 
      white  precipitate SO2-4
      (1/2 mark) (1/2 mark)
    3.  To the third portion, add aqueous sodium hydroxide dropwise until in excess.
      Observations Inferences 
      white precipitate which dissolves in excess aquesous sodium hydroxide Zn2+ or Al3+ present
      (1 mark) (1 mark)
    4. To the fourth portion, add aqueous ammonia dropwise until in excess.
      Observations Inferences 
      white precipitate insoluble in excess aquesous ammonia Al3+
      (1 mark) (2 mark)

3. You are provided with solid F. Carry out the following tests and record the observations and inferences in the spaces provided.

  1. Place about one-third of solid F on a clean metallic spatula and burn it in a Bunsen burner flame.
    Observations Inferences 
    burns with a yellow smoky/sooty flame unsaturated organic compound
    or
    long-chain organic compound
    (1 mark) (1 mark)
  2. Place the remaining amount of solid F in a boiling tube. Add about 10 cm3 water and shake. Use the mixture for tests (i) to (iii) below.
    Observations Inferences 
    dissolves forming a colourless solution polar compound
    (1/2 mark) (1/2 mark)
    1. Using about 2 cm3 of the mixture in a test-tube, determine the pH using universal indicator paper and chart.
      Observations Inferences 
      pH = 2
      (accept Ph  value from 1 -3 ) 
      reject pH range
      acidic compound probably carboxylic acid
      (1 mark) (1 mark)
    2. To about 2 cm3 of the mixture in a test tube, add two or three drops of acidified potassium manganate(VII).
      Observations Inferences 
      Acidified potassium manganate (VII) decolourised / purple potassium manganate (VII) changes to colourless. q3 ahgfdOR
      Alkene / alkyne or alkanol present
      (1 mark) (1 mark)
    3. To about 2cm3 of the mixture in a test tube add two or three drops of bromine
      Observations Inferences 
      Bromine water decolourized / Yellow or
      brown colour of bromine turns colourless
      q3 2 jafhda
      OR
      Alkene OR alkyne present
      (1 mark) (1 mark)

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