KCSE 2025 Chemistry Paper 2 (PDF)

233/2 · CHEMISTRY (Theory) · Paper 2 · Nov. 2025 · 2 hours

Instructions: Answer all the questions in the spaces provided. Non-programmable silent electronic calculators and KNEC mathematical tables may be used. All working must be clearly shown where necessary.

1. Table 1 shows part of the periodic table of elements.

Part of the periodic table: period 3 showing Mg, then (after the transition block) Al, Si, P, S, Cl.

(a) Explain each of the following: (i) the atomic radius of the elements decreases from magnesium to chlorine; (1 mark) (ii) the ionisation energy of the elements increases from magnesium to chlorine. (1 mark)
(b) Explain each of the following in terms of structure and bonding: (i) the melting point of aluminium is higher than that of magnesium; (1 mark) (ii) the melting point of phosphorus is higher than that of chlorine. (1 mark)
(c) Complete Table 2 by giving the formula of the oxides of the elements and their acid–base character. (4 marks)

Element	Mg	Al	Si	P
Formula of oxide
Acid–base character

(d) Sulphur forms the chloride S₂Cl₂. (i) Name the type of bond in the chloride. (1 mark) (ii) Determine the oxidation number of sulphur in the chloride. (1 mark)
(e) Silicon tetrachloride fumes in air. Write an equation for the reaction. (1 mark)
(f) Silicon is a metalloid. (i) State what is meant by the term metalloid. (1 mark) (ii) State one use of silicon as a metalloid. (1 mark)

2. (a) State the names of the following compounds: (i) $\text{CH}_3(\text{CH}_2)_5\text{CH}_3$ ; (1 mark) (ii) $\text{CH}_3(\text{CH}_2)_3\text{C}{\equiv}\text{CH}$ . (1 mark)
(b) Figure 1 shows the extraction of copper from copper pyrites, CuFeS₂.

Figure 1: copper pyrites and air into chamber 1 (gas C out); Cu₂S + FeO into chamber 2 with substance B (slag out); Cu₂S into chamber 3 with air (gas C and impure copper out).

(i) Identify gas C. (1 mark)
(ii) Write an equation for the reaction that takes place in chamber 1. (1 mark)
(iii) Identify substance B. (1 mark)
(iv) Give the formula of the main compound in the slag. (1 mark)
(v) Write equations for the two reactions that take place in chamber 3. (1 mark)
(vi) Describe the process that is used to purify the impure copper. (2 marks)
(c) When left exposed for a long time in air, copper coins turn green. Explain this observation. (1 mark)

3. (a) State what is meant by the standard enthalpy of formation of a compound. (1 mark)
(b) Table 3 gives the enthalpies of formation of some compounds.

Compound	CO₂	H₂O	C₃H₈
$\Delta H_f^\circ$ (kJ mol⁻¹)	−393	−286	−104

(i) Write the thermochemical equations for the formation of: I. CO₂; (1 mark) II. H₂O. (1 mark)
(ii) Study the energy level diagram in Figure 2 and answer the questions using the data in Table 3.

Figure 2: 3C(s) + 4H₂(g) at the top; ΔH₁ down to C₃H₈(g); ΔH₃ from C₃H₈(g) down to 3CO₂(g) + 4H₂O(l); ΔH₂ directly from the top to the bottom level.

I. Give the value of ΔH₁. (1 mark) II. Calculate the values of ΔH₂ (2 marks) and ΔH₃. (1 mark)
(iii) When a sample of butane (C₄H₁₀) was burned, the heat produced raised the temperature of 500 g of water by 35 °C. (Specific heat capacity of water = 4.2 J g⁻¹ K⁻¹) Calculate the: I. heat change for the reaction; (1 mark) II. mass of butane that was burned, given that the heat of combustion of one mole of butane is −2880 kJ mol⁻¹. (C = 12.0, H = 1.0) (2 marks)

4. (a) Describe how a sample of chlorine gas can be prepared using the following compounds: solid sodium chloride, solid manganese(IV) oxide and concentrated sulphuric(VI) acid. (3 marks)
(b) Figure 3 is a flowchart showing some reactions of chlorine.

Figure 3: chlorine gas reacts with iron (step I) to give compound C; with a reagent (step II) to give NaClO₃; and with phosphorus (step III) to give compounds D + E.

(i) Give the name of compound C formed in step I. (1 mark)
(ii) State the reagent and conditions necessary for the reaction in step II. (1 mark)
(iii) Two compounds are formed in the reaction in step III. Give the formula of each of the two compounds D and E. (2 marks)
(c) Write an equation for the reaction that takes place when chlorine gas reacts with aqueous hydrogen sulphide. (1 mark)
(d) State the observation made when: (i) chlorine gas is bubbled into aqueous potassium iodide; (1 mark) (ii) moist red litmus paper is placed in a gas jar of chlorine gas. (1 mark)
(e) Write an ionic equation for the reaction that takes place when aqueous lead(II) nitrate is added to aqueous potassium chloride. (1 mark)
(f) State the reason why chlorine is used in water treatment. (1 mark)

5. (a) Figure 5 shows the Down's cell used in the extraction of sodium from molten sodium chloride.

Figure 5: Down's cell — a central electrode B under a hood leading to the chlorine outlet; a ring electrode A around it; a steel gauze diaphragm between them; sodium collected and run off at the side; sodium chloride fed in at the top.

(i) Identify the anode and cathode in the cell. Give a reason. (2 marks)
(ii) State the purpose of the steel gauze diaphragm. (1 mark)
(b) Table 4 shows the melting point and boiling point of sodium and sodium chloride.

Substance	Melting point (°C)	Boiling point (°C)
Sodium	97.8	890
Sodium chloride	801	1467

(i) Given that the Down's cell is operated at 600 °C, state how this temperature is achieved. (2 marks)
(ii) State the advantage of operating at 600 °C. (1 mark)
(iii) State the physical state in which sodium is obtained from the cell. Give a reason. (2 marks)
(c) Starting with 1000 kg of sodium chloride, calculate the mass of sodium produced. (Na = 23.0; Cl = 35.5) (2 marks)
(d) Explain how a sample of solid sodium should be stored in the laboratory. (2 marks)
(e) State one industrial use of sodium. (1 mark)

6. The products of electrolysis depend on the conditions used.
(a) (i) State the conditions if the products of electrolysis of aqueous copper(II) sulphate are: I. oxygen and copper; (1 mark) II. copper. (1 mark)
(ii) During electrolysis of an electrolyte containing aqueous A³⁺ ions, 19.4 g of A were obtained when a current of 15 amperes was passed through it for 2 hours. I. Write the half equation for the reaction that took place. (1 mark) II. Determine the relative atomic mass of A. (1 Faraday = 96 500 coulombs) (2 marks)
(b) Study the following cell representation and answer the questions that follow: $\text{Ni}(s)\,|\,\text{Ni}^{2+}(aq)\,||\,\text{Ag}^+(aq)\,|\,\text{Ag}(s)$ , $E^\circ_{cell} = +1.05$ V.
(i) State what the symbol || represents in the diagram. (1 mark)
(ii) Write an equation for the reaction that occurs in the cell. (1 mark)
(iii) Given that the $E^\circ$ of the Ag⁺(aq)/Ag(s) half-cell is +0.80 V, calculate the $E^\circ$ of the Ni²⁺(aq)/Ni(s) half-cell. (1 mark)
(iv) State how the e.m.f. of a cell can be used to predict whether a reaction is feasible. (1 mark)
(c) Corrosion of a metal involves formation of an oxide layer on the metal surface. Explain why corrosion of aluminium is protective while that of iron is destructive. (2 marks)

7. (a) State and explain how the rate of the reaction $A(g) + B(g) \rightarrow AB(g)$ is affected by an increase in: (i) pressure; (2 marks) (ii) temperature. (2 marks)
(b) The rate of reaction between ethanoic acid and copper(II) carbonate can be monitored by measuring the volume of gas produced. The results obtained using 75.0 cm³ of 0.1 M ethanoic acid and excess copper(II) carbonate are shown in Table 5.

Time (s)	0	5	15	25	35	45	55	70	80
Volume of gas (cm³)	0	20	41	50	56	59	61	63	63

(i) Write an equation for the reaction that took place. (1 mark)
(ii) Give a reason why excess copper(II) carbonate was used. (1 mark)
(iii) On the grid provided, draw a graph of the results obtained. (3 marks)
(iv) Use the graph to determine the rate of reaction at: I. 10 seconds; (1 mark) II. 40 seconds. (1 mark)
(v) Give a reason for the difference between the two rates calculated in (iv). (1 mark)

KCSE 2025 Chemistry Paper 2

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