KCSE 2018 Chemistry Paper 3 with Marking Scheme

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

(3 marks)

1. Plot a graph of temperature (vertical axis) against time on the grid provided.(3 marks)
   
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

(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

• Complete table(1 mark)
• Use of decimals (Either whole numbers, one decimal place as 0,5 (1 mark)
• Accuracy (¹/₂ 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

• 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

• 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 cm³ or 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.	OR 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	OR Alkene OR alkyne present
      (1 mark)	(1 mark)