KCSE Past Papers 2021 Chemistry paper 1(233/1)

Chemistry - Paper 1 2021 - 2 Hours

Kenya certificate of Secondary Education

2021 Chemistry paper 1

1.a. Draw a labelled diagram showing the atomic structure of 24 12 mg. (2 marks)

b. The atomic number of phosphorus is 15. Draw a dot (•) and cross (x) diagram for the compound formed when phosphorus reacts with chlorine, atomic number 17. (1 mark)

2.a. State the condition under which a Bunsen burner produces a luminous flame. (1 mark)

b. Write an equation for the reaction that takes place in a luminous flame assuming the laboratory gas is butane. (1 mark)

c. One of the regions in the non-luminous flame is the unburnt gas region. Describe how the presence of this region can be shown using a wooden splint. (1 mark)

3.a. The elements sodium, magnesium and aluminium belong to group I, II and III respectively.Select the element with the highest electrical conductivity and give a reason. (1 mark)

b. Complete Table 1 to show the products of electrolysis for concentrated sodium chloride and molten sodium chloride.

4. A small piece of sodium metal was placed in a beaker containing pure water revision.

a. State two observations made during the reaction. (1 mark)

b. State and explain another observation made when a drop of phenolphthalein is added to the mixture in the beaker. (1 mark)

c. Explain why it is not advisable to carry out this experiment using potassium metal. (1 mark)

5. Describe how a pure sample of copper(II) nitrate crystals can be prepared using recycled copper wire. (3 marks)

6. The following apparatus and chemicals are used to investigate the percentage of air used when iron rusts: iron filings, 100 ml measuring cylinder, trough and water.

a. Draw a setup of the experiment. (2 marks)

b. Write an expression to show how the percentage of air used is calculated at the end of the experiment (1 mark)

7. Figure I shows a graph of atomic radius of some group I and group II elements.

a. Explain why the atomic radius of sodium is higher than that of:

i. lithium (1 mark)

ii. magnesium (1 mark)

b. Predict the atomic radius of calcium. (1 mark)

8. Compound D with formula, C3H4 was reacted with excess hydrogen chloride gas.

a. Give the name of compound D. (1 mark)

b. Draw two possible structures of the products formed. (2 marks)

9. Study the setup in Figure 2 and answer the questions that follow.

a. State the precaution that should be taken in carrying out the experiment. Give a reason. (1 mark)

b. State the observations made in the boiling tube. (2 marks)

10. Consider the following reaction:

N2(g) + 3H2(g) ⇌ 2NH3(g)

The enthalpy change is 92.4 kJ per mole of nitrogen.

a. Give the enthalpy change per mole of ammonia. (1 mark)

b. State and explain how each of the following affects the yield of ammonia:

i. Increase in temperature. (1 mark)

ii. Finely divided iron. (1 mark)

11. Study the flow chart in Figure 3 and answer the questions that follow.

a. Identify solid E. (1 mark)

b. Name the type of reaction that takes place in chamber 1. (1 mark)

c. Write an equation for the reaction that takes place in chamber 2. (1 mark)

12. Compounds H and J have the following structures.

a. Give the names of:

i. Compound H. (1 mark)

ii. Compound J. (1 mark)

b. State the conditions necessary for H and J to react. (1 mark)

13. Rhombic sulphur is one of the allotropes of sulphur

a. Draw the structure of rhombic sulphur. (1 mark)

b. Describe the observations made when rhombic sulphur is heated from room temperature until it boils. (1 mark)

14. The molar enthalpy of solution for potassium sulphate (K,SO) is +23.8 kJ.

a. On the axes provided, draw a labelled energy level diagram for the dissolution process of potassium sulphate in water. (2 marks)

b. Calculate the enthalpy change when 5:12 gof potassium sulphate is completely dissolved in water (K = 39:0;5=32,0; 0 - 16.0)(1 mark)

15.a. State Gay-Lussac's law. (1 mark)

b. 180 cm3 of nitrogen(II) oxide gas was reacted with 400 cm3 of oxygen gas.

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

ii. Calculate the total volume of the gases at the end of the reaction. (3 marks)

16. Describe how the setup in Figure 4 can be used to distinguish between 50.0 cm of 0.2M hydrochloric acid and 50.0 cm of 0.2 M ethanoic acid using pieces of 6 m length of magnesium ribbon and a stop watch. (3 marks)

17. Describe how dilute nitrie(V) acid and blue litmus papers can be used to distinguish between solid

samples of sodium carbonate and sodium sulphite. (3 marks)

18.a. Describe how propanone can be used to extract a pure sample of sunflower oil. (2 marks)

b. State why sodium hydroxide solution is not suitable for the extraction of sunflower oil. (1 mark)

19. 31.5 cm3 of concentrated nitric(V) acid was diluted to 500 cm3. 10.0 cm3 of the dilute acid required 25.0 cm3 of 0.4M sodium hydroxide for neutralisation.

a. Calculate concentration of the:

i. dilute acid. (1 mark)

ii. concentrated acid. (1 mark)

20. Figure 5 shows part of a radioactive decay series.

a. Write a nuclear equation for the formation of nuclide K from nuclide X. (1 mark)

b. The half-life of nuclide X is 47 minutes. Determine the percentage of nuclide X that remains after 188 minutes. (2 marks)

21. Aluminium is extracted from aluminium oxide by electrolysis.

a. Other than the cost of electricity, give another reason why this method is expensive. (1 mark)

b. Calculate the mass of aluminium obtained when a current of 20A is used for 5 hours. (1 Faraday - 96500 C; Al - 27.0)(2 marks)

22. Explain each of the following observations:

a. Articles made of copper turn green when left exposed in air over a long period of time. (1 mark)

b. Addition of aqueous ammonia to a solution containing copper(II)ions produces a deep blue solution (1 mark)

23.a. State what is meant by relative atomic mass of an element. (1 mark)

Let RAM 7 x be n	C	X
RAM	12	N	96.4 = 4 n 0.3
% mass	3.6 12	96.4 n	n = 96.4 1.2
		96.4 n
Ratio	1	4	= 80.3

First row is a table header How to use it? b. A compound of carbon and element X with formula, CX, contains 3.6% carbon by mass.

Calculate the relative atomic mass of X. (2 marks)

24. Carbon(II) oxide can be prepared by dehydration of ethanedioic acid

a. Complete the following equation to show the reaction that takes place. (1 mark) H20204

b. Name another reagent that can be used to prepare carbon(II) oxide by dehydration (1 mark)

25. Figure 6 shows an incomplete diagram of a setup for laboratory preparation of nitrogen gas.

a. Complete the setup in Figure 6 to show how nitrogen gas can be collected. (2 marks)

b. The nitrogen prepared using this setup is purer than that obtained from air. Give a reason(1 mark)

26. Hydrazine, is used as a fuel in rockets. Using the bond energies in Table 2, calculate the enthalpy

change for combustion of hydrazine.

N₂H4 (1) + O₂(g) → N₂(g) + 2H₂O(g)

Table 2

Bond	Bond Energy kJ/mol
N-H	388
N-N	163
O=O	496
N≡N	944
O-H	463

27.a. Table 3 gives the standard reduction potentials of some group VII elements. Table 3

State and explain the reactions that take place when aqueous bromine is added to a sample of sea water containing both chloride and iodide ions. (2 marks)

Reduction equations	Eo/V
CI2 + 2e→2CI-	+1.36
Br₂ +2e→2Br-	+1.07
I2 + 2e→21-	+0.54

Questions and Answers

Kenya certificate of Secondary Education

2021 Chemistry paper 1

1.a. Draw a labelled diagram showing the atomic structure of 24 12 mg. (2 marks)

b. The atomic number of phosphorus is 15. Draw a dot (•) and cross (x) diagram for the compound formed when phosphorus reacts with chlorine, atomic number 17. (1 mark)

2.a. State the condition under which a Bunsen burner produces a luminous flame. (1 mark)

When airhole/collar is closed or fully closed

b. Write an equation for the reaction that takes place in a luminous flame assuming the laboratory gas is butane. (1 mark)

b. CH4(g) + 4O2(g) → C(s) + 3CO(g) + CO(g) +5H2O(I)

CH4(g) + 4O2(g) → C(s) + CO(g) + CO(g) +5H2O(I)

c. One of the regions in the non-luminous flame is the unburnt gas region. Describe how the presence of this region can be shown using a wooden splint. (1 mark)

3.a. The elements sodium, magnesium and aluminium belong to group I, II and III respectively.

Select the element with the highest electrical conductivity and give a reason. (1 mark)

When airhole/collar is closed or fully closed

b. Complete Table 1 to show the products of electrolysis for concentrated sodium chloride and molten sodium chloride.

CH4(g) + 4O2(g) → C(s) + 3CO(g) + CO(g) +5H2O(I)

CH4(g) + 4O2(g) → C(s) + CO(g) + CO(g) +5H2O(I)

4. A small piece of sodium metal was placed in a beaker containing pure water revision.

a. State two observations made during the reaction. (1 mark)

The piece of metal darts/floats
Melts into silvery ball
Production of effervescence/hissing sound
The beaker becomes warm

b. State and explain another observation made when a drop of phenolphthalein is added to the mixture in the beaker. (1 mark)

Solution turns pink because sodium hydroxide/alkaline solution is formed

c. Explain why it is not advisable to carry out this experiment using potassium metal. (1 mark)

Potassium reacts explosively with water/ more vigorously/ more violent

5. Describe how a pure sample of copper(II) nitrate crystals can be prepared using recycled copper wire. (3 marks)

Heat the copper wire in air to form copper
Add excess copper(II) oxide to dilute nitric acid
Filter to remove unreacted copper(II) oxide
Heat the resulting solution to saturation
Allow it to cool to form crystals
Dry/filter the crystals

6. The following apparatus and chemicals are used to investigate the percentage of air used when iron rusts: iron filings, 100 ml measuring cylinder, trough and water.

a. Draw a setup of the experiment. (2 marks)

b. Write an expression to show how the percentage of air used is calculated at the end of the experiment (1 mark)

Initial height of air column - Final height of air column/Initial height of air column

Initial height of water - Final height of water/Initial height of water

7. Figure I shows a graph of atomic radius of some group I and group II elements.

a. Explain why the atomic radius of sodium is higher than that of:

i. lithium (1 mark)

Na = 2.8.1

Li = 2.1

Sodium has 3 energy levels while lithium has two

Li = 2

Na = 2.8.1

ii. magnesium (1 mark)

Mg = 2.8.2

Na = 2.8.1

The effective nuclear charge is higher in magnesium than sodium. Mg has a higher number of protons

b. Predict the atomic radius of calcium. (1 mark)

208± 2 Ithout showing on the graph Extrapolate to 20 on x-axis and mark to value

8. Compound D with formula, C3H4 was reacted with excess hydrogen chloride gas.

a. Give the name of compound D. (1 mark)

Propyne
prop 1 -yne
prop -1,2-iodene

b. Draw two possible structures of the products formed. (2 marks)

9. Study the setup in Figure 2 and answer the questions that follow.

a. State the precaution that should be taken in carrying out the experiment. Give a reason. (1 mark)

The experiment should be carried out in a fume chamber out in open since carbon(II) oxide is poisonous

b. State the observations made in the boiling tube. (2 marks)

A white percipitate is formed which dissolves to form a colourless solution 10. Consider the following reaction:

N2(g) + 3H2(g) ⇌ 2NH3(g)

The enthalpy change is 92.4 kJ per mole of nitrogen.

a. Give the enthalpy change per mole of ammonia. (1 mark)

- 92.4/2 ⇒ =46.2 KJmol-1 b. State and explain how each of the following affects the yield of ammonia:

i. Increase in temperature. (1 mark)

i. It lowers the yield of ammonia since the forward reaction is exothermic or backward reaction is endothermic

ii. Finely divided iron. (1 mark)

ii. No effect

A catalyst has no effect on the position of the equilibrium

11. Study the flow chart in Figure 3 and answer the questions that follow.

a. Identify solid E. (1 mark)

Potassium magnate VII or maganese(iv) oxide lead(vi) oxide

b. Name the type of reaction that takes place in chamber 1. (1 mark)

Red ox/oxidation

c. Write an equation for the reaction that takes place in chamber 2. (1 mark)

CI2(g) + 2NaOH → NaCl(aq) + NaOCl(aq) + H2O

12. Compounds H and J have the following structures.

. Give the names of:

i. Compound H. (1 mark)

3-methylpentanol/ 3-methylpentanol

ii. Compound J. (1 mark)

Butanoic acid

b. State the conditions necessary for H and J to react. (1 mark)

Concentrated sulphuric VI acid/ sulphuric acid

Warm/heat/ temperature between 3-6oC 13. Rhombic sulphur is one of the allotropes of sulphur

a. Draw the structure of rhombic sulphur. (1 mark)

b. Describe the observations made when rhombic sulphur is heated from room temperature until it boils. (1 mark)

Yellow solid forms amber liquid
As the temperature increases the liquid becomes darker and vicious
Then it turns dark red/brown and less vicious

14. The molar enthalpy of solution for potassium sulphate (K,SO) is +23.8 kJ.

a. On the axes provided, draw a labelled energy level diagram for the dissolution process of potassium sulphate in water. (2 marks)

b. Calculate the enthalpy change when 5:12 gof potassium sulphate is completely dissolved in water (K = 39:0;5=32,0; 0 - 16.0)(1 mark)

RFM of K2SO2 = 174

moles of K2SO2 =5.22/174 = 0.03

ΔH = 0.03 x 23.8 = 0.714KJ

15.a. State Gay-Lussac's law. (1 mark)

When gases react, they do so in volumes that bear simple ratios to one another and to the products if gaseous at constant temperature and pressure

b. 180 cm3 of nitrogen(II) oxide gas was reacted with 400 cm3 of oxygen gas.

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

2NO(g) + O2(g) → 2NO2(g)

ii. Calculate the total volume of the gases at the end of the reaction. (3 marks)

2NO + O2 → 2NO2

using ratio

Volume of oxygen =180 x 1/2 = 90cm3

Volume of oxygen unreacted = 400 - 90

= 310

Volume of NO2 = 18cm3

Total volume = 310 + 180

= 490cm3

Any contact between
Mg and hydrochloric acid
Mg and ethanoic acid
Using a stopwatch to show the difference
Conclusion - HCl takes a shorter time
Put a 6cm Mg ribbon in conical flask and add 50cm3
HCl. Using a stopwatch, record the
volume of gases collected at a time inferral e.g. 15cm3
Repeat the experiment using 50cm3 of ethanoic acid
More/higher volume of gas will be collected when HCl is used than ethanoic acid at same inferral of time OR
The reaction will take a shorter time to completion when HCl is used than when ethanoic acid used 17. Describe how dilute nitrie(V) acid and blue litmus papers can be used to distinguish between solid samples of sodium carbonate and sodium sulphite. (3 marks)
Place blue items at the mouth of test tubes
Both turn red
Thereafter one of them is bleached
The sample that produces bleaching on the litmus is sodium sulphite 18.a. Describe how propanone can be used to extract a pure sample of sunflower oil. (2 marks)
Crush the sunflower seeds using motar and pestle
Add propane and stir
Decant
Leave the extract on sunlight for propane to evaporate leaving oil behind

b. State why sodium hydroxide solution is not suitable for the extraction of sunflower oil. (1 mark)

It will react with oil to form soap

19. 31.5 cm3 of concentrated nitric(V) acid was diluted to 500 cm3. 10.0 cm3 of the dilute acid required 25.0 cm3 of 0.4M sodium hydroxide for neutralisation.

a. Calculate concentration of the:

i. dilute acid. (1 mark)

Moles of NaOH = 0.4 x 25 = 0.01/1000

Moles of HNO3 = 0.01

Molarity of HNO3 = 0.01 x 1000/10

ii. concentrated acid. (1 mark)

C1V1 = C2V2 1 x 500/31.5 = 15.9M

20. Figure 5 shows part of a radioactive decay series.

a. Write a nuclear equation for the formation of nuclide K from nuclide X. (1 mark)

Q = It

= 20 x 5 x 60 = 360000

moles = 360000 = 1.244moles

3 x 96500

mass = 1.244 x 27

= 33.588g

b. The half-life of nuclide X is 47 minutes. Determine the percentage of nuclide X that remains after 188 minutes. (2 marks)

21. Aluminium is extracted from aluminium oxide by electrolysis.

a. Other than the cost of electricity, give another reason why this method is expensive. (1 mark)

The graphite anode has to be replaced periodically

b. Calculate the mass of aluminium obtained when a current of 20A is used for 5 hours. (1 Faraday - 96500 C; Al - 27.0)(2 marks)

22. Explain each of the following observations:

a. Articles made of copper turn green when left exposed in air over a long period of time. (1 mark)

Due to formation of copper(II) carbonate. Since copper reacts with carbon(IV) oxide/CO2 b. Addition of aqueous ammonia to a solution containing copper(II)ions produces a deep blue solution (1 mark)
Due to formation of complex ion of tetramine copper(II) ions OR due to formation of tetramine copper (II) ions

23.a. State what is meant by relative atomic mass of an element. (1 mark)

Is the mass of one atom of an element compound to the mass of carbon - 12

Let RAM 7 x be n	C	X
RAM	12	N	96.4 = 4 n 0.3
% mass	3.6 12	96.4 n	n = 96.4 1.2
		96.4 n
Ratio	1	4	= 80.3

First row is a table header How to use it? b. A compound of carbon and element X with formula, CX, contains 3.6% carbon by mass.

Calculate the relative atomic mass of X. (2 marks)

24. Carbon(II) oxide can be prepared by dehydration of ethanedioic acid

a. Complete the following equation to show the reaction that takes place. (1 mark) H20204

H2C2O4 → CO(g) + CO2(g) + H2O(I)

b. Name another reagent that can be used to prepare carbon(II) oxide by dehydration (1 mark)

Methanoic
Sodium methanate

25. Figure 6 shows an incomplete diagram of a setup for laboratory preparation of nitrogen gas.

a. Complete the setup in Figure 6 to show how nitrogen gas can be collected. (2 marks)

b. The nitrogen prepared using this setup is purer than that obtained from air. Give a reason(1 mark)

Bond	Bond Energy kJ/mol
N-H	388
N-N	163
O=O	496
N≡N	944
O-H	463

It has impurities such as noble gases

26. Hydrazine, is used as a fuel in rockets. Using the bond energies in Table 2, calculate the enthalpy change for combustion of hydrazine.

N₂H4 (1) + O₂(g) → N₂(g) + 2H₂O(g)

Table 2

Bond	Bond Energy kJ/mol
N-H	388
N-N	163
O=O	496
N≡N	944
O-H	463

N2H4 + O2 → N2 + H2O

Bonds broken

4 x 388 = 1552

1 x 163 = 163

1 x 496 = 496

= 2211

Bonds formed

1 x 944 = 944

2 x 463 = 1852

= -2796

Enthalpy of combination = -2796 + 2211

= 585KJmol-

27.a. Table 3 gives the standard reduction potentials of some group VII elements. Table 3

State and explain the reactions that take place when aqueous bromine is added to a sample of sea water containing both chloride and iodide ions. (2 marks)

a. Br2(aq) + 2I-aq) → 2Br-(aq) + I2(aq)

Br2(aq) + 2CI- → No reaction

Bromine will oxidize iodide ions to iodine since it has more positive Eθ
Bromine will not displace chlorine since Eθ for Cl- is more positive

Reduction equations	Eo/V
CI2 + 2e→2CI-	+1.36
Br₂ +2e→2Br-	+1.07
I2 + 2e→21-	+0.54

b. Give a reason why potassium iodide is added to table salt

Potassium iodide is a source of iodine is needed to regulate functioning of thyroid gland

Xaxa Digital

KCSE Past Papers 2021 Chemistry paper 1(233/1)