KCSE 2017 Chemistry Paper 3 with Marking Scheme

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INSTRUCTIONS TO CANDIDATES

Answer all questions.
All working MUST be clearly shown where necessary.
Non-programmable silent electronic calculator and KNEC Mathematical tables may be used, except where stated otherwise
Candidates should answer the questions in English

You are provided with:
- Solution A, 0.5 M copper(II) sulphate
- Solid B₁, metal B, powder
- Solid B₂, Iron powder
- Solution C, 0.02 M acidified potassium manganate(VII)
  You are required to determine the:
- Enthalpy change for the displacement reaction between metal B, and copper(II) sulphate.
- Mass of iron that reacts with copper(II) sulphate in the displacement reaction.
  
  PROCEDURE I
  1. 1. Using a pipette and a pipette filler, place 25.0 cm³ of solution A into a 100 ml plastic beaker. Allow to stand for about 1 minute and then measure the temperature of the solution. Record the reading in Table 1 as the initial temperature. Add all of solid B, to the solution. Stir the mixture carefully with the thermometer and measure the highest temperature reached. This will take about 5 minutes. Recor the reading in Table 1 as maximum temperature reached.
      
      Table 1
      
      Maximum temperature reached (°C)
      
      Initial Temperature (°C)
      
      Change in temperature, ΔT₁ (°C)
    2. Calculate the:
      1. number of moles of copper(II) sulphate used. (1 mark)
      2. enthalpy change for the reaction of metal B, with one mole of copper(II) sulphate. (Assume that for the mixture, specific heat capacity = 4.2 Jg⁻¹K⁻¹ and density = 1.0 g cm⁻³) (1 mark)
  2. Repeat procedure I, (a) (i) with all of metal B₂ (iron powder) in place of metal B₁. The maximum temperature is reached after about 8 minutes. Record the temperature readings in Table 2. Retain the mixture for use in PROCEDURE II.
    
    Table 2
    
    Maximum temperature reached (°C)
    
    Initial Temperature (°C)
    
    Change in temperature, ΔT₂ (°C)
  3. Compare the changes in temperature ΔT₁ and ΔT₂ and comment on the differences. (2 marks)
    
    PROCEDURE II
    1. Fill a burette with solution C.
    2. Filter the mixture obtained in procedure I (b) into a 250 ml volumetric flask. Wash the residue with distilled water and add into the flask. Add more distilled water to make up to the mark. Label this as solution B₂
    3. Using a pipette and a pipette filler, place 25.0 cm³ of solution B₂ into a 250 ml conical flask. Titrate solution B₂ with solution C until a permanent pink colour just appears. Record the readings in Table 3.
      
      Repeat step (iii) and complete Table 3.
  4. Table 3
    
    I II III
    
    Final burette reading
    
    Initial burette reading
    
    Volume of solution C used, cm³
    
    (4 marks)
  5. Calculate the average volume of solution C used, (1 mark)
  6. The equation for the reaction between manganate(VII) and iron(II) ions is:
    MnO₄⁻ (aq) +5Fe²⁺(aq) + 8H⁺(aq) → Mn²⁺(aq) + 5Fe³⁺(aq) + 4H₂O(aq)
    Calculate the number of moles of:
    1. potassium manganate(VII) used. (1 mark)
    2. iron (II) ions in 25.0 cm³ solution B (1 mark)
    3. iron that reacted with copper(II) sulphate. (1 mark)
  7. Determine the mass of iron that reacted. (RAM of Fe = 55.8) (1 mark)
You are provided with:
- Solid K
- Aqueous ammonia
- Aqueous sodium sulphate
- Dilute nitric(V) acid
- Wooden splint
  Solid K is suspected to be lead(II) carbonate.
  1. From the reagents provided, select and describe three tests that could be carried out consecutively to confirm if solid K is lead(II) carbonate. Write the tests and expected observations in the places provided.
    1. Test 1 Expected Observations
      
      (1 mark) (1 mark)
    2. Test 2 Expected Observations
      
      (1 mark) (1 mark)
    3. Test 3 Expected Observations
      
      (1 mark) (1 mark)
  2. Carry out the tests described in (a) using solid K and record the observatioons and inferences in the spaces provided.
    1. Test 1
      
      Observations Inferences
      
      (½ mark) (½ mark)
    2. Test 2
      
      Observations Inferences
      
      (1 mark) (2 marks)
    3. Test 3
      
      Observations Inferences
      
      (1 mark) (1 mark)
You are provided with an organic compound solid M. Carry out the following tests. Record the observations and inferences in the spaces provided.
1. Place all of solid M in a boiling tube. Add about 10 cm³ of distilled water and shake. Retain the solution for use in procedure (b) (i), (ii) and (iii).
  
  Observations Inferences
  
  (1 mark) (1 mark)
2. Use about 2 cm³ portions of the mixture in a test tube for tests (i), (ii) and (ill).
  1. To the first portion, add all the solid sodium carbonate provided.
    
    Observations Inferences
    
    (1 mark) (1 mark)
  2. To the second portion, add two drops of acidified potassium manganate(VII) and warm the mixture.
    
    Observations Inferences
    
    (1 mark) (2 marks)
  3. To the third portion, add about 2 cm³ of acidified potassium dichromate(VI). Heat the mixture to boiling and allow to stand for about 2 minutes.
    
    Observations Inferences
    
    (1 mark) (1 mark)

MARKING SCHEME

Table 1

Maximum temperature reached (°C)	43.5
Initial Temperature (°C)	25.0
Change in temperature, ΔT₁ (°C)	18.5

(3 marks)

Complete Table .......... 1 mark
Penalize ½mark for:
- incorrect subtraction;
- maximum temperature less than initial temperature;
- Initial temperature < 10°C or > 40°C.
Use of decimal -------1 mark
- Accept whole numbers or 1 decimal to 0 or .5;
- Accept 2 decimal places for .00,.25.50, 75.
Accuracy----1 mark
Award 1 mark if candidate's initial temperature is within ±2°C of the school value.

1. Moles = 25 x 0.5 ✓½ - penalize fully if any other values apart from 25 and 0.5 are used.
  1000
  = 0.0125 ✓½-If units are used, accept moles/mol not mols. ( 1mark)
2. Enthalpy change = −25 x 4.2x18.5 Jmol⁻¹ ✓½
  0.0125
  = −155,400 Jmol⁻¹ ✓½
  OR
  = − 155.4 kJmol⁻¹
  Penalize ½ mark if sign or correct units Jmol⁻/kJmol⁻ ( 1mark)

Table 2

Maximum temperature reached (°C)	37.5
Initial Temperature (°C)	24.0
Change in temperature, ΔT₂ (°C)	13.5

(3 marks)

Complete table 1mark
Penalize ½ mark for:
- incorrect subtraction;
- maximum temperature less than initial temperature;
- Initial temperature < 10°C or > 40°C.
Use of decimal -------1 mark
- Accept whole numbers or 1 decimal to .0 or .5;
- Accept 2 decimal places for.00,.25.50.75.
Accuracy-------- 1 mark
Award I mark if candidate's initial temperature is within ±2°C of the school value.

ΔT₁ is larger/greater than ΔT₂ ✓1
Metal B₁ is more reactive than metal B, hence greater temperature change. ✓1

Table 3

	I	II	III
Final burette reading	22.50	12.20	24.50
Initial burette reading	10.00	0.00	12.20
Volume of solution C used, cm³	12.50	12.20	12.30

Complete table--------------1 mark
- 3 titration done----- ------------1mark;
- 2 titrations done------- ---------------- ½ mark
- 1 titration done-----------0mark.
Use of decimal -----1 mark
Accept 1or 2 decimal displaces used consistently for 1mark otherwise penalize 1 mark.
Accuracy compared to the school value --------1 mark
Award 1 mark if any value is within ±0.1 of school value if not award ½ mark if any within ±0.2 otherwise award 0 mark
Principles of averaging --1 mark
Values averaged must be within ±0.2 of each other otherwise award 0 mark.
Final answer ---- 1 mark
- Compare candidate' average volume to school value and award as in accuracy.
- If values were wrongly subtracted, compare the corrected ones with school value and award accordingly.

Average volume = 12.20+12.30 ✓½ 12.30+12.50 ✓½
2 2
= 12.25 cm³ ✓½ =12.40 cm³ ✓½
1. Moles of potassium manganate(VII) = 12.25 × 0.02✓½
  1000
  = 2.45 x 10⁻⁴ ✓½
  - penalize mark for wrong units used in each case;
  - number of moles given to at least 4 decimal places unless it works out exactly to less than 4 decimal places otherwise penalize X mark for round off to less than 4 decimal places.
2. Moles of Fe²⁺ = 5 x 2.45 x 10⁻⁴ ✓½
  = 1.225 x 10⁻³✓½
3. Moles of Fe²⁺ that reacted with Cu²⁺ = 1.225x10⁻³ x ²⁵⁰/₂₅ ✓½
  = 1.225 x 10⁻²✓½
Mass of iron that reacted = 1.225 x 10⁻² × 55.8g✓½
= 0.68✓½
- penalize ½ mark for wrong units or missing units.
- penalize ½ mark if mass of iron reacted is greater than 1.5g (because 1.5g of iron was used);
- penalize ½ mark if average value is used for R.A.M apart from 55.8.

i	Test 1	Expected Observations
	To solid K in a boiling tube, add about 10cm dilute nitric(V) acid. Retain mixture for tests 2&3. ✓½ Test any gas produced using a burning splint ✓½	Effervescence / bubbles of gas or fizzing. ✓½ colourless gas extinguishes a burning splint. ✓½ Reject : Fizzling, sizzling/hissing
	(1 mark)	(1 mark)
ii	Test 2	Expected Observations
	To about 2cm³ of mixture, add aqueous ammonia dropwise until in excess	White precipitate insoluble in excess.
	(1 mark)	(1 mark)
iii	Test 3	Expected Observations
	To about 2cm³ of mixture add 2 drops of aqueous sodium sulphate.	White precipitate.
	(1 mark)	(1 mark)

For tests 2 and 3 reject:

White / clear solution;
White precipitate soluble in excess.
NOTE: The order is important HNO3(aq), followed by NH3(aq) and lastly Na2SO4(aq). If Na₂SO₄ done before OH then it will suggest Ba²⁺General Note on 2(a)
1. The order in the note above is very important hence mark the first order and reject fully (award 0 mark) where the tests follow any other order.
2. If tests 2 & 3 are interchanged the 2(b) can only be marked put 2b (ii), there being no need for 2b (ii) because absence of Pb²⁺ will already have been identified at 2b (ii).

i	Test 1
	Observations	Inferences
	Effervescence, colourless gas extinguishes burning splint.	CO₃²⁻ present. -Accept CO₃²⁻ written in words; -Award 0 mark if contradicting ion is mentioned
	(½ mark)	(½ mark)
ii	Test 2
	Observations	Inferences
	White precipitate insoluble in excess.	Mg²⁺, Pb²⁺ present. -ammonia is not expected to precipitate Ca²⁺ions (weak base) -If K was a carbonate aluminium carbonate does not exist.
	(1 mark)	(2 marks)
iii	Test 3
	Observations	Inferences
	No white precipitate.	Pb²⁺ absent OR Mg²⁺ present
	(1 mark)	(1 mark)

Observations Inferences

Dissolves to form a colourless solution. Soluble salt/polar compound.

(1 mark) (1 mark)

i	Observations	Inferences
	Dissolves, NO effervescence/ No gas bubbles/ No fizzing	-COOH absent RCOOH OR H⁺/H₂O⁺ for (½mark)
	(1 mark)	(1 mark)
ii	Observations	Inferences
	Purple potassium manganate(VII) is decolourised / turns colourless.	, R-OH present.
	(1 mark)	(2 marks)
iii	Observations	Inferences
	Colour changes from orange to green.	R-OH present.
	(1 mark)	(1 mark)

KCSE 2017 Chemistry Paper 3 with Marking Scheme

MARKING SCHEME

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Observations	Inferences
Dissolves to form a colourless solution.	Soluble salt/polar compound.
(1 mark)	(1 mark)