KNEC KCSE Past Papers 2018 Chemistry Paper 3 (233/3)

KNEC KCSE Past Papers 2018 Chemistry Paper 3 (233/3)

Kenya Certificate of Secondary Education

Practical

2018 Chemistry paper 3

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 determine the:

• Enthalpy change, AH per mole, of the reaction between magnesium metal and excess hydrocholoric acid.

• Concentration in moles per litre of hydrochloric acid, solution B.

Procedure I

(i) Using a burette, measure 50.0 cm³ of solution B and place it in a 100 ml plastic beaker.

(ii) Measure the temperature of solution B in the beaker after every 30 seconds and record it in Table 1.

(iii) 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 30 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°C				x

(3 marks)

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

 

(b) Determine the change in temperature, AT, for the reaction. Show the working on the graph.

AT . (1 mark)

(c) Calculate the heat change, in joules, for the reaction. Assume that for the solution, specific heat capacity is 4.2 Jg K 1 and density is 1.0gcm 3. (2 marks)

(d) The relative atomic mass of magnesium is 24.0. Calculate the enthalpy change, AH, of the reaction per mole of magnesium. Indicate the sign of AH. (1 mark)

Procedure H

(i) Fill a clean burette with the 0.15M sodium carbonate, solution C.

(ii) Place all of the mixture in the beaker from procedure I into a 250ml volumetric flask. Add distilled water to the mark and shake thoroughly. Label the mixture as solution D.

(iii) Using a pipette filler, pipette 25.0 cm³ of solution D into a 250 ml conical flask and add 2 drops of methyl orange indicator.

(iv) Titrate solution D in the conical flask with the sodium carbonate, solution C and record the readings in Table 2.

(v) Repeat steps (iii) and (iv) and complete Table 2.

Table 2

Observations	Inferences

(a) Determine the average volume of the 0.15M sodium carbonate, solution C, used. (1 mark)

(b) Calculate the number of moles of:

(i) sodium carbonate used.

(ii) hydrochloric acid in the 25.0 cm³ of solution D.

(iii) hydrochloric acid in the 250 cm³ of solution D.

(iv) hydrochloric acid that reacted with magnesium metal.(1 mark)

(v) total number of moles of hydrochloric acid in the 50.0cm³, solution B. (1 mark)

(c) Determine the concentration of hydrochloric aeid 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.

(a) 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

(b) 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 .

(i) To the first portion, add three or four drops of dilute hydrochloric acid (1 mark)

(ii) To the second portion, add two or three drops of aqueous barium nitrate.

 

Observations	Inferences

(iii) To the third portion, add aqueous sodium hydroxide dropwise until in excess.

Observations	Inferences

(iv) To the fourth portion, add aqueous ammonia dropwise until in excess.

Observations	Inferences

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

(a) Place about one-third of solid F on a clean metallic spatula and burn it in a Bunsen burner flame.

 

Observations	Inferences

(b) Place the remaining amount of solid F in a boiling tube. Add about 10 cm³ of distilled water and shake. Use the mixture for tests (i) to (iii) below.

 

Observations	Inferences

(i) Using about 2 cm³ of the mixture in a test-tube, determine the pH using universal indicator paper and chart.

 

Observations	Inferences

(ii) To about 2 cm³ of the mixture in a test tube, add two or three drops of acidified potassium manganate(VII).

 

Observations	Inferences

(iii) To about 2 cm³ of the mixture in a test-tube add two or three drops of bromine

Observations	Inferences

Practical Answers

2018 Chemistry paper 3

1.

Time,s	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 —————-½

• 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 ½

a).

• Scale——-(½ mark)

• Labeling of axes—–(½ mark)

On graph paper

b).△T=(50.0 – 22.0)^oc =28.0^o

• Plotting (1 mark)

• Curve / lines (1 mark)

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

= 5880 Jou1es

d).Moles of magnesium = 30 0.0125/24

Enthalpy change △H= —5880/0.0125 x 1 J mol^-1

= – 470400 J mol^-1 OR – 470.4 kJ mol_-1

-ve sign must be shown otherwise penalize

 

	I	II	III
Final brette reading	24.80	33.50	41.50
Initial burette reading	1.00	10.00	18.00
Volume of solution C used cm<sub>3</sub>	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

• Accuracy ( any one value compared to the school value that is within 0.10 cmm or 0.20 cm³ of the school value) (1 mark)

• Principles of averaging — average of volumes that are within -L 0.10cm 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 cmcm³

b).Moles of sodium carbonate = 0.15average titre/1000

23.50 x 0.15/ 1000

— 3.525 x10^-3

(ii) Reaction ratio is 1 mole Na₂ COC₃ : 2 moles HCl

Moles of hydrochloric acid in 25.0cm’ = 2 x Answer in b(i) =2×3.525 x 10^-3

= 7.05 x10 ^-3

(nf) Moles of hydrochloric acid in 250 cm° = Answer in b(ii) x 250 2

— 0.0705

(iv) Mg * 2HCl—+ Mg Cl, + H₂

Reaction ratio is Mg : HCP = 1 : 2

Moles of magnesium — 0.0125

Moles of hydrochloric acid = 2 x 0.0125 M

= 0 025

(v) 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

Concentration of hydrochloric acid in moles per litre of solution B Answer in b(v) x i000 50

0.0955 x1000 Moles per litre

= 1.91 mol dm^-3

 

Observations	Inferences
(a) Colourless liquid condenses the on cooler parts of the test-tube. Red litmus turned blue Blue litmus remains blue (2marks)	Hydrated compound of ammonium salt (NH ) present (1marks)
(b) No effervescence (i) No white precipitate	CO-2 3 * and SOA’ absent Pb2+absent
(ii) White precipitate	SO2-4
(iii) White precipitate which dissolves in excess aqueous sodium hydroxide	Zn2+  or Al3+ present
(iv) White precipltate insoluble in excess aqueous ammonia	Zn2+ or Al 3+
3.(a)Bums with a yellow smoky / sooty flame	Unsaturated organic compound OR – long-chain organic compound
(b) Dissolves forming a colourless solution.	Polar compound
(i) pH = 2 (accept pH value from 1 to 3) Reject pH range	Acidic compound probably carboxylic acid
(ii) Acidified potassium manganate (VII) decolourised / purple potassium manganate (VII) changes to colourless.	Alkene / alkyne or alkanol present
Bromine  water decolourized / Yellow or brown colour of bromine turns colourless	Alkene OR alkyne present