Chem pp1 contains short answered questions across all the topics from form 1 to form 4. Chem PP1 does not concentrate on drawing questions exhaustive from a single topic. Actually it is chem pp2 paper 2 that exhaust almost the whole topic in a single question. Chemistry paper 2 needs a deep understanding of any given topic from form 1 to form four.

Chemistry Paper 1 KCSE 2024 Marking Scheme

1. Name the apparatus that can be used to:

(a) Measure volume of solutions more accurately than a measuring cylinder. Burette

(b) Lower a burning piece of magnesium ribbon into a gas jar. Deflagrating spoon

(c) Store and keep substances free from moisture. Desiccator

2. A radioactive isotope decays by either emission of alpha (α) or beta (β) particles.

(a) Explain why alpha particles have a higher ionizing power compared to beta particles.

Alpha particles have a charge of 2+, large mass thus move slowly making them in contact with target atoms for a longer time/½ while beta particles are less charged, lighter thus move faster/½

(b) Explain how alpha and beta particles can be distinguished in terms of their penetrating power. (1 mark)

Alpha particles have low penetrating power compared to beta particles.

Alpha particles cannot penetrate a sheet of paper while beta particles can but beta particles can be stopped by a sheet of aluminium foil.

3. Three bottles containing zinc nitrate, aluminum nitrate or lead (II) nitrate have their labels missing. Describe a chemical test that can be carried out using aqueous sodium sulphate and aqueous ammonia to label the bottles correctly.

Dissolve a sample of each salt in distilled water in a boiling tube.

Divide each sample into two portions in test tubes.

To first portion of each add aqueous ammonia dropwise till in excess, white precipitate soluble in excess indicate presence of Zn²⁺.

To second portion of each add three drops of sodium sulphate solution white precipitate shows presence of Pb²⁺ while no white precipitate indicate presence of Al³⁺.

4. Consider the following equilibrium reaction for a mixture of gases in a gas syringe

2NO₂(g) ⇌ N₂O₄(g); ΔH = -ve

(brown) (yellow)

(a) Give a reason why the enthalpy change is negative, exothermic reaction. (1 mark)

The products have a lower energy than the reactants / 1

(b) State and explain the observation made when the pressure in the syringe is increased

(1 mark)

Observation

Yellow colour intensifies / 1

Explanation:

The equilibrium shifts to the right to reduce the pressure / 1

5. Ammonia gas can be prepared using ammonium sulphate and sodium hydroxide.

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

(NH₄)₂SO₄(s) + 2NaOH(s) → 2NH₃(g) + Na₂SO₄(s) + 2H₂O(l) √1

(b) Draw a well labelled diagram of a set - up of the apparatus that can be used to prepare and collect a sample of dry ammonia. (2 marks)

6. Some properties of four metals and their oxides are shown in table 1.

Table 1

(a) Arrange the metals in order of their reactivity starting with the most reactive.

(1 mark)

C, B, A, D

→ Decreasing reactivity

(b) State the observation that would be made when metal B is heated with:

(i) Oxide of D

(1 mark)

Black solid changes to brown/1

(ii) Oxide of C

(1 mark)

White solid remains white / 1

7. In an experiment to determine the water of crystallisation in sodium carbonate,

Na₂CO₃.XH₂O, the data in table 2 was obtained

Table 2

Calculate the;

(a) Mass of dry salt after heating;

67.00 - 57.20 = 9.8g

(b) Mass of water;

78.60 - 67.00 = 11.60g

(c) Value of x (2 marks)

Moles of sodium carbonate = 9.8g / 106 = 0.09245 mol

Mole of water = 11.6 / 18 = 0.6444 mol

Mole ratio Na₂CO₃ : H₂O = 1 : 6.97 ≈ 1 : 7, hence x = 7

8. A polymer was formed using compound M which has the following structure.

a) Name compound M

Phenyl ethene

b) Draw a section of the polymer showing three repeat units.

c) Give one advantage of using this polymer over natural polymer.

It can be moulded into desired shapes easily

9. Figure 1 shows a set - up that was used to investigate the products of a burning candle.

(a) State and explain

i) The observations made in the U - tube (1 mark)

Observation: Blue anhydrous cobalt (II) chloride changes to pink

Explanation: Candle wax produces water and carbon (IV) oxide when it burns in air, water combines with blue anhydrous cobalt (II) chloride to form pink hydrated cobalt (II) chloride

ii) How the pH of distilled water changes with time (1 mark)

Observation: pH decreases from pH = 7.0 to pH = 6.0

Explanation: Carbon (IV) oxide from burning candle wax dissolves in the distilled forming weak carbonic acid

(b) State the role of the pump. (1 mark)

To create a suction force that sucks the gaseous products into the apparatus

10. In an experiment to determine the molar heat of neutralisation, 100cm³ of 1.0M sodium hydroxide was added to 100cm³ of 1.0M hydrochloric acid. The data obtained is shown in table 3 below.

Table 3

(a) Calculate the:

(i) Heat change (1 mark) ΔH = 200g × 4.2 Jg⁻¹K⁻¹ × 6.1K = 5124J

(ii) Molar heat of neutralisation of hydrochloric acid. (1 mark)

(Specific heat capacity of the solution = 4.2 Jg⁻¹deg⁻¹, density of the solution = 1 g/cm³)

(ii) Molar heat of neutralisation of hydrochloric acid. (1 mark) (Specific heat capacity of the solution = 4.2 Jg⁻¹deg⁻¹, density of the solution = 1.0 g/cm³)

Mol of HCl reacting = 1 x 0.1 = 0.1 mol

But 0.1 mol HCl produces 0.1 mol water (mole ratio HCl : H₂O = 1 : 1).

If 0.1 mol water liberates 5.124 kJ of heat, then

1 mol of water will liberate 5.124 kJ ÷ 0.1 mol = 51.24 kJ

Hence molar heat of neutralisation = -51.24 kJ/mol

(b) The molar heat of neutralisation obtained in 10(a)(ii) differs from that obtained using aqueous ammonia and hydrochloric acid. Explain (1 mark)

Aqueous ammonia is a weak alkali hence some of the heat energy produced is used to dissociate ammonia solution

11. Iron is extracted from its ore in a blast furnace. The raw materials used are: iron ore, coke, air and limestone.

(a) State the role of each of the following in the extraction process.

(i) Limestone (1 mark)

Decomposes to CaO which react with silica impurities to form slag

(ii) Coke (1 mark)

Burns to produce carbon (IV) oxide which is then reduced to carbon (II) oxide used as the main reducing agent

(b) State and explain why the iron obtained is not suitable for making bridges.

It is brittle due to high carbon content and has low tensile strength

12. Table 4 shows some properties of three substances. Complete the table by filling in the types of forces and structures in each substance.

Table 4

13. Describe how the presence of sulphite ions in aqueous sodium sulphite can be confirmed using:

(a) aqueous barium nitrate and dilute nitric (V) acid;

Add 1cm³ of barium nitrate solution followed by 5cm³ of dilute nitric (V) acid to a sample of sodium sulphite solution in a test tube. Effervescence and white precipitate soluble upon addition of nitric (V) acid confirms the presence of sulphite ions.

(b) acidified potassium dichromate (VI) (1 mark)

Add three drops of orange acidified potassium dichromate (VI) to a second sample of sodium sulphite solution. Orange acidified potassium dichromate (VI) changes to green. This confirms presence of sulphite ions.

14. Use the bond energies in Table 5 to calculate the enthalpy change for the combustion of ethene.

C₂H₄(g) + 3O₂(g) → 2CO₂(g) + 2H₂O(l)

Table 5

H₂C=CH₂ + 3(O=O) → 2(O=C=O) + 2(H-O-H)

ΔH = energy needed to break bonds + energy released during bond formation

= (4 x 496) + 612 + (3 x 412) - [(148 x 6) x 2 + (2 x 926)]

= 3832 - 4824 = -992 kJ/mol

15. Hydrogen gas can be prepared by electrolysis of acidified water.

(a) Explain why the water is acidified before electrolysis. (1 mark)

To increase ionisation of water since water is a weak electrolyte

(b) Calculate the volume of hydrogen gas produced when a current of 12 amperes is used for 2 hours. ((Faraday=96,500) coulombs, volume of 1 mole of \((\underline{gas}=24,000cm^{3})\)

(2 marks)

\(2H^{+}(aq)+2e^{-}\rightarrow H_{2}(g)\)

Q = It = 12 x 2 x 60 x 60 = 86400 coulombs

1 Faraday = 96,500 coulombs, then 2 Faraday = 193,000c

193,000c deposit 24,000cm³ of H₂(g)

86,400c will deposit 86,400 x 24,000 / 193,000 = 10,744.04cm³ H₂(g)

16. Ethanol can be prepared from glucose (C₆H₁₂O₆) according to the following equation:

C₆H₁₂O₆(aq) ⟶ 2C₂H₅OH(aq) + 2CO₂(g) 30°C

a) Explain why temperatures more than 30°C are not used. (1 mark)

Enzymes in yeast cells are denatured at temperatures above 30°C

hence no fermentation can occur

(b) The concentration of ethanol in the product is about 12%. State how the concentration of ethanol in the product can be increased. (1 mark)

By carrying out fractional distillation of the crude solution

(c) State one use of ethanol in hospitals

used as an antiseptic in specific concentrations

17. The following steps are followed when carrying out paper chromatography of a dye.

1. The solvent is placed in a beaker.

2. A baseline is drawn with a pencil on the paper.

3. The sample is spotted on the baseline.

4. The paper is carefully placed in the beaker with the baseline above the surface of the solvent.

5. The beaker is covered and allowed to stand for some time.

Give a reason for the following:

(a) The baseline is drawn with a pencil.

Graphite in pencil does not dissolve in the solvent which may interfere with the separation

(b) The paper is carefully placed in the beaker with the baseline above the surface of the solvent.

To prevent the sample from dissolving in the solvent and spreading below the baseline.

(c) The beaker is covered.

To prevent the solvent from vaporising since most solvents are volatile.

18. 6.12g of hydrogen peroxide were dissolved in 100cm³ of water. The peroxide was allowed to decompose in the presence of a catalyst and oxygen gas produced collected.

\(2H_{2}O_{2}(aq)\rightarrow2H_{2}O(l)+O_{2}(g)\)

(a) Name a suitable catalyst for the reaction. (1 mark)

Manganese (IV) oxide

(b) Calculate the:

(i) Number of moles of hydrogen peroxide in the solution. ((H=1.0 O=16.0))(1 mark)

6.12g / 34g/mol = 0.18 moles

(ii) Volume of oxygen gas produced. (volume of 1 mole of gas = 24,000cm³) (1 mark)

mole ratio H₂O₂ : O₂(g) = 2 : 1

moles of oxygen = 0.18 x 1 / 2 = 0.09 moles

volume of oxygen = 24,000 x 0.09 = 2,160cm³

19. Describe how a sample of ammonium nitrate can be prepared starting with ammonia as the only nitrogen containing compound.

Dissolve 1200cm³ of ammonia gas in 25cm³ distilled water in a beaker to make 2.0M ammonia solution.

Add 12.5cm³ of 2.0M lead (II) nitrate solution to the ammonia solution to precipitate lead (II) hydroxide.

Filter to obtain lead (II) hydroxide as residue while ammonium nitrate as filtrate.

Heat the filtrate to saturation and allow it to cool to form crystals of ammonium nitrate.

20. The set up in Figure 2 is used to prepare carbon (II) oxide from ethanedioic acid. The equation for the reaction is:

H₂C₂O₄(s) → CO(g) + CO₂(g) + H₂O(l)

(a) Identify X and Y and give the role of each

(i) X: Concentrated sulphuric (VI) acid

Role: Act as a dehydrating agent

(ii) Y: Concentrated sodium hydroxide solution

Role: Absorbs carbon (IV) oxide

(b) State a property of the gas that allows it to be collected as shown:

It is insoluble in warm water

21. 2.0g of hydrogen gas was reacted with 2.0g of oxygen gas to form water. (H=1.0; O=16.0)

Calculate the:

(a) Mass of water produced;

\(2H_{2}(g)+O_{2}(g)\rightarrow2H_{2}O(l)\)

Mole of \(H_{2}(g)=2/2=1 mol\); moles of \(O_{2}(g)=2/32=0.0625 mol\)

Moles of water = 0.0625 x 2 = 0.125 mol

Mass of water = 0.125 x 18 = 2.25g

22. Activation energy is the energy required to initiate a reaction.

(a) Explain why reactions differ in their activation energies.

Bond energies of reacting particles differ

Sketch on the same axis, the curve obtained if the reaction was carried out without a catalyst.

Give a reason.

Catalysts lower activation energy by providing an alternative pathway for the reaction

23. (a) Complete table 6 by filling the missing information for phosphorous and iron. (2 marks)