KCSE Past Papers 2021 Chemistry

  KCSE Past Papers 2021 

Chemistry paper 2(233/2)

Chemistry - Paper 2 2021 - 2 Hours

Kenya certificate of Secondary Education

2021 Chemistry paper 2

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

a. 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: i. Compound A (2 marks)

ii. Compound B (2 marks)

b. 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.

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)

c. Figure 1 shows analysis of an alloy containing two metals.

Time  (seconds)	0	30	60	90	120	150	180	210	240
Volume     Of CO2     (cm3)	O	62	92	113	124	130	132	133	133

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)

c. Figure 1 shows analysis of an alloy containing two metals.

i. Give the name of another product formed in step 1. (1 mark)

ii. Write the formula of the complex ion present in the colourless solution obtained in step 2 (1 mark)

iii. 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.

a. Explain how the sulphur used in this process is obtained. (2 marks)

b. Give one advantage of using air in chamber 1 instead of using oxygen gas. (1 mark)

c. Identify substances: (1)

i. M (1 mark)

ii. N (1 mark)

d.i. In chamber 2, drying and purification take place. Give a reason why this is necessary (1 mark)

ii. The reaction in chamber 3 is highly exothermic.

I. Explain why high temperature is required for the reaction in chamber 3. (1 mark)

II. State how heat produced in chamber 3 can be utilised in this process e. Give a reason why this method of manufacture is known as 'contact process". (1 mark)

f. Emission of gases in the sulphuric(VI) acid plant may lead to environmental pollution.

i. State the evidence that could be used to show that the sulphuric(VI) acid plant causes pollution (1 mark)

ii. Explain how the pollution identified in 2(1)(i) can be controlled. (1 mark)

3.a. Chemical reactions occur as a result of collisions of particles. Give a reason why not all collisions are effective. (1 mark)

b. State and explain how the following factors affect the rate of reaction: i. Surface area of reactants. (1 mark)

ii. Pressure. (1 mark)

c. 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.

Time (se conds)	0	30	60	90	120	150	180	210	240
Volume     Of CO2     (cm3)	O	62	92	113	124	130	132	133	133

i. On the grid provided, plot a graph of volume of carbon(IV) oxide (vertical axis) against time (horizontal axis). (3 marks)

ii. Using the graph, determine the rate of reaction at the:

I. 45th second. (1 mark)

II. 105th second. (1 mark)

iii. Give a reason for the differences in the two rates. (1 mark)

iv. 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.a. Sea water contains approximately 3% sodium chloride. Describe how sodium chloride is obtained from sea water. (3 marks)

b. 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)

c. Ammonia is highly soluble in water.

i. Explain how aqueous ammonia is prepared starting with ammonia gas. (2 marks)

ii. On the axes provided, sketch a curve showing how solubility of ammonia gas varies with temperature. (1 mark)

iii. Give a reason for the shape of the curve. (1 mark)

d. Water hardness is due to the presence of magnesium and calcium ions. Explain how these ions get into sources of water. (2 marks)

5.a. Figure 3 shows part of a Periodic Table.

i. Select from the table the most reactive: I. metal. (1 mark)

II. non-metal. (1⁄2 mark)

ii. Select an element with the highest first ionisation energy. (1 mark)

iii.I. Name the method used to obtain argon from its source. (1 mark)

II. Give one industrial use of argon. (1 mark)

iv. Explain each of the following observations:

I. The melting point of lithium is higher than that of potassium. (1 mark)

II. The melting point of chlorine is lower than that of iodine. (1 mark)

v. 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)

b. Use Table 4 to answer the questions that follow.

Properly	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

i. Identify the substance which is a gas at room temperature. Give a reason. (1 mark)

ii. Name the particles responsible for electrical conductivity in substance: IH (1 mark)

I. H II. J (1 mark)

iii. Identify the type of forces that hold the particles together in: I. H (1 mark)

II. Κ (1 mark)

6. Figure 4 shows a flow chart involving reactions of some organic compounds.

a. Write the formula and give the names of compounds: i. X

Name:

Formula:

ii. Y

Name:

Formula:

b. Give the reagents and conditions necessary for carrying out:

i. Step 3. (1 mark)

ii. Step 5. (1 mark)

i. Write an equation for the reaction in step 6. (1 mark)

ii. State the observations made in step 6. (1 mark)

7.a. Using the oxidation numbers of chlorine, explain why the following is a redox reaction. HCIO3(aq) + 5HCl(aq) → 3CI2(g) + 3H2O(I) (2 marks)

b. Use the following standard reduction potentials to answer the questions that follow:

I	Half cell reactions	Eo/V
II	PSO4 (s) + 2e → Pb(s) + SO4'-(aq)	-0.36
III	PBO2(s) + S042- (aq) + 4H+(aq) + 2e → PbSO4 (s) + 2HyO(1)	+1.69
IV	Fe3+(aq) + e → Fe2+(aq)	+0.77
V	Zn2+(aq) + 2e → Zn(s)	-0.76
V	MnO4-' (aq) + 8H+(aq) + 5e -+ Mn2 (aq) + 4HtO(I)	+1.51
Vl	O2g+ 2 H -I- (aq) -- 2e —+ H2O2(aq)	+0.68
VII	Fe2+(aq) -i- 2e → Fe(s)	-0.44
VIII	Ca2+(aq) → 2e  Cu(s)	+0.34

i. The half cells I and II are combined to form an electrochemical cell.

I. Write an equation for the cell reaction. (1 mark)

II. Calculate the e.m.f of the cell. (1 mark)

ii. Draw a labelled diagram for the electrochemical cell formed using half cells III and IV. (3 marks)

iii. State and explain the observations made when a few drops of acidified potassium manganate(VII) are added to hydrogen peroxide. (3 marks)

iv. Coating iron with zinc is a more effective way of corrosion prevention than coating it with copper. Explain. (2 marks)

Questions and Answers

Kenya certificate of Secondary Education

2021 Chemistry paper 2

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

a. Table 1

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:

i. Compound A (2 marks)

• 
  
• Powder
• loss water of crystalization

ii. Compound B (2 marks)

• Dissolve/color loss liquid
• Absorbs water vapour

b. 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)

MgSO4 H2O

Mass(g) 1.20 1.26

Moles 1.20 1.26

120 18 0.07/0.01

0.01/0.01

1 7

MgSO4.7H2O

c. Figure 1 shows analysis of an alloy containing two metals.

Time (se conds)	0	30	60	90	120	150	180	210	240
Volume     Of CO2     (cm3)	O	62	92	113	124	130	132	133	133

i. Give the name of another product formed in step 1. (1 mark)

• Hydrogen gas

ii. Write the formula of the complex ion present in the colourless solution obtained in step 2 (1 mark)

• [Zn(OH)42-

iii. Identify the metals in the alloy, (2 marks)

• Zinc/Zn
• Copper/Cu

2. The flow chart in Figure 2 shows the processes involved in the manufacture of sulphuric(VI) acid.

a. Explain how the sulphur used in this process is obtained. (2 marks)

• Three concentric pipes
• Super heated water through outer pipe
• Hot compressed air through inner pipe
• Molten sulphur through middle pipe

b. Give one advantage of using air in chamber 1 instead of using oxygen gas. (1 mark)

• Air is cheap/Economical/Readily available

c. Identify substances: (1)

i. M (1 mark)

• Concentrated sulphur(VI) acid/ H2SO4

ii. N (1 mark)

• Water/H2O

d.i. In chamber 2, drying and purification take place. Give a reason why this is necessary (1 mark)

• Impurities poisons catalyst/make it inefficient

ii. The reaction in chamber 3 is highly exothermic.

I. Explain why high temperature is required for the reaction in chamber 3. (1 mark)

• Increase rate of reaction
• Effective collisions/Fruitful

II. State how heat produced in chamber 3 can be utilised in this process

• Preheat SO2 & O2/ reactants
• Recycling of heat

e. Give a reason why this method of manufacture is known as 'contact process". (1 mark)

• Reactants come in contact with catalyst

f. Emission of gases in the sulphuric (VI) acid plant may lead to environmental pollution.

i. State the evidence that could be used to show that the sulphuric (VI) acid plant causes pollution (1 mark)

• Rusting of mettalic structures
• Stone structure wearing/ chlorosil
• Death of aquatic life
• Wilting of plants
• Respiratory diseases
• Acid rain loweing pH of soil

ii. Explain how the pollution identified in 2(1)(i) can be controlled. (1 mark)

• Passing through Ca(OH)2/CaO
• 
  
• Scrubbing

3.a. Chemical reactions occur as a result of collisions of particles. Give a reason why not all collisions are effective. (1 mark)

• Particles not possess necessary kinetic energy /activation of energy
• Particles collide in wrong orientation

b. State and explain how the following factors affect the rate of reaction:

i. Surface area of reactants. (1 mark)

• Increases rate of reaction
• Move particles are in contact
• More collision per unit time
• More particles exposed

ii. Pressure. (1 mark)

• Increases rate of reaction
• Increases manner of collision
• Molecules of gaseous reactants close
• Decrease volume/frequency of collision

c. 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.

Time (se conds)	0	30	60	90	120	150	180	210	240
Volume     Of CO2     (cm3)	O	62	92	113	124	130	132	133	133

i. On the grid provided, plot a graph of volume of carbon(IV) oxide (vertical axis) against time (horizontal axis). (3 marks)

ii. Using the graph, determine the rate of reaction at the: I. 45th second. (1 mark)

Tangent 45

Calculations from graph

dy2/dx2 - dy1/dx1

Ans = cm3/sec

II. 105th second. (1 mark)

Tangent 45

Calculations from graph

dy2/dx2 - dy1/dx1

Ans = cm3/sec

iii. Give a reason for the differences in the two rates. (1 mark)

• Rate at 45th is greater than 105th Hence faster rate of reaction/
• Rate at 105 sec is low to reduce reactants

iv. 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").

Moles = 133/24000

Moles ratio 1:1

CaCO3

5.54 x 10-3 x 100 RFM

100 x 133 (over) 24000

Ans = 0.554g

4.a. Sea water contains approximately 3% sodium chloride. Describe how sodium chloride is obtained from sea water. (3 marks)

• Heat/Boil/Evaporate to saturation
• Allow to cool; crystals formed or
• Sea water trapped in pan/shallow pond solid crystallises out

b. 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)

RFM NaCl = 58.5

36.2 x 1000 (over) 100 = 362

362/58.5

6.188M or 6.19M

c. Ammonia is highly soluble in water.

i. Explain how aqueous ammonia is prepared starting with ammonia gas. (2 marks)

• Pass inserted funnel to prevent suck back

ii. On the axes provided, sketch a curve showing how solubility of ammonia gas varies with temperature. (1 mark)

iii. Give a reason for the shape of the curve. (1 mark)

Solubility decreases with increase in temperature becasue the gaseous particles gain energy and escape from solution

d. Water hardness is due to the presence of magnesium and calcium ions. Explain how these ions get into sources of water. (2 marks)

Formation of carbon acid reacts with rocks with Ca & Mg salts leading to Ca2+ and Mg2+

ions

5.a. Figure 3 shows part of a Periodic Table.

i. Select from the table the most reactive: I. metal. (1 mark)

• Cs

II. non-metal. (1⁄2 mark)

• F

ii. Select an element with the highest first ionisation energy. (1 mark)

• HI

I. Name the method used to obtain argon from its source. (1 mark)

• Fractional distillation

II. Give one industrial use of argon. (1 mark)

• Used in flourescent bulbs/lamps

iv. Explain each of the following observations:

I. The melting point of lithium is higher than that of potassium. (1 mark)

• Lithium has stronger metallic bonds than potassium

II. The melting point of chlorine is lower than that of iodine. (1 mark)

• Iodine has more forces than chlorine
• Iodine is stronger/more intermolecular forces

v. 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)

Mg2+; Na+; O2-; N3-;

Protons decreases from Mg to nitrogen hence nuclear attraction decreases from Mg to N

b. Use Table 4 to answer the questions that follow.

Properly	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

i. Identify the substance which is a gas at room temperature.

Give a reason. (1 mark)

K; boiling point below room temperature

ii. Name the particles responsible for electrical conductivity in substance: IH (1 mark)

I. H ions/ mobile ions

II. J (1 mark)

electrons/ delocalized electrons

iii. Identify the type of forces that hold the particles together in:

I. H (1 mark)

electrostatic forces/ ionic bonds

II. Κ (1 mark)

weak van der waals forces/ intermolecular

6. Figure 4 shows a flow chart involving reactions of some organic compounds.

a. Write the formula and give the names of compounds: i. X

Name:

i. Magnesium propoxide

Formula:

(CH3CH2CH2O)2Mg

ii. Y

Name:

Sodium propanoate

Formula:

CH3CH2COONa

b. Give the reagents and conditions necessary for carrying out:

i. Step 3. (1 mark)

H+/ KMnO4/H+K2Cr2O7

Warm/ Heat/ High temperature

ii. Step 5. (1 mark)

i. Write an equation for the reaction in step 6. (1 mark)

CH3CHCH2 + Br2 → CH3CHBr2

ii. State the observations made in step 6. (1 mark)

Brown/Yellow/Orange/decolorized

7.a. Using the oxidation numbers of chlorine, explain why the following is a redox reaction.

HCIO3(aq) + 5HCl(aq) → 3CI2(g) + 3H2O(I) (2 marks)

Oxidatopn of Cl in HClO from +5 to 0 Reduction -1 to 0 - Oxidation

b. Use the following standard reduction potentials to answer the questions that follow:

I	Half cell reactions	Eo/V
II	PSO4 (s) + 2e → Pb(s) + SO4'-(aq)	-0.36
III	PBO2(s) + S042- (aq) + 4H+(aq) + 2e → PbSO4 (s) + 2HyO(1)	+1.69
IV	Fe3+(aq) + e → Fe2+(aq)	+0.77
V	Zn2+(aq) + 2e → Zn(s)	-0.76
V	MnO4-' (aq) + 8H+(aq) + 5e -+ Mn2 (aq) + 4HtO(I)	+1.51
Vl	O2g+ 2 H -I- (aq) -- 2e —+ H2O2(aq)	+0.68
VII	Fe2+(aq) -i- 2e → Fe(s)	-0.44
VIII	Ca2+(aq) → 2e  Cu(s)	+0.34

i. The half cells I and II are combined to form an electrochemical cell.

I. Write an equation for the cell reaction. (1 mark)

PbO2(s) + 2SO22-(aq) + 4H+ + Pb → 2PbSO4(s) +2H2O(I)

II. Calculate the e.m.f of the cell. (1 mark)

+1.69 = 0.36

+ 2.05V

ii. Draw a labelled diagram for the electrochemical cell formed using half cells III and IV. (3 marks)

iii. State and explain the observations made when a few drops of acidified potassium manganate(VII) are added to hydrogen peroxide.(3 marks)

H+/KMnO4 Decolorized/ Purple to colourless Effervescence/ Bubbles of colourless gas H2O2 oxidised to O2 gas/ Production of O2S4 MnO4-

Mn2+

iv. Coating iron with zinc is a more effective way of corrosion prevention than coating it with copper. Explain. (2 marks)

Zinc is more reactive than iron

Iron is more reactive than copper/copper is less reactive than iron