SAJC 2025 Dec Holiday Assignment Students

1 St Andrew’s Junior College JC1 H2 Chemistry 2025 Dec Holidays Assignment Name : …………………………………………………… Date : …………………….. Class : 25S……… Time : 1 hour Instructions: • Complete under timed exam conditions (2 min per MCQ, 1.6 min per mark for structured) • Self-mark MCQ, Structured Q1 and Q2. Answers on google drive. • Submit the assignment to your tutor during the first tutorial in 2026. Section A – Multiple Choice Questions [10 marks] For each question, there are four possible answers. Choose the one you consider correct and write your answer in the boxes below. Question 1 2 3 4 5 Answer Question 6 7 8 9 10 Answer 1 10 cm3 of a gaseous hydrocarbon, CxHy, was exploded with an excess of oxygen. There was a contraction of 30 cm3. When the resulting gaseous mixture was treated with aqueous sodium hydroxide, there was a further contraction of 40 cm3. Given that all gas volumes were measured at r.t.p., what is the molecular formula of the hydrocarbon? A C3H8 B C3H10 C C4H8 D C4H10 35

2 2 What is the proton number of an element that has four unpaired electrons in its ground state? A 16 B 22 C 24 D 26 3 The boiling points of pent–1–ene and 2,2–dimethylpropane are 30 C and 9 C respectively. Which statements help to account for this difference in boiling points? 1 The molecule of pent–1–ene is less compact than that of 2,2–dimethylpropane. 2 The covalent bonds in pent–1–ene are stronger than those in 2,2–dimethylpropane. 3 Pent–1–ene is a polar molecule while 2,2–dimethylpropane is a non–polar molecule. 4 The intermolecular forces of attraction in pent–1–ene are more extensive than those in 2,2–dimethylpropane. A 1, 2 and 4 only B 2 and 3 only C 1 and 4 only D 3 and 4 only 4 A 5.0 dm3 sample of neon gas at a pressure of 200 kPa and a 2.0 dm 3 sample of helium gas at a pressure of 500 kPa are introduced into a 2.5 dm3 vessel at constant temperature. What is the total pressure in the vessel? A 500 kPa B 700 kPa C 800 kPa D 1000 kPa

3 5 The oxides and chlorides of Mg, Al, Si and P are added separately to excess water. Which of the following shows the correct trends of the pH of the resultant solutions? A B pH of its oxide in water pH of its chloride in water .P .Si .Al .Mg pH of its oxide in water pH of its chloride in water .P .Si .Al .Mg C D pH of its oxide in water pH of its chloride in water .P .Si .Al .Mg pH of its oxide in water pH of its chloride in water .P .Si .Al .Mg 6 Which of the following enthalpy changes is exothermic? A Mg2+(g) + 2Cl–(g) → MgCl2(aq) B Al2O3(s) → 2Al3+(g) + 3O2–(g) C O–(g) + e– → O2–(g) D Br2(g) → 2Br•(g)

4 7 The reaction between NO and Br2 is proposed to proceed via the following mechanism: Step 1: NO + Br2 ⇌ NOBr2 (fast) Step 2: NOBr2 + NO → 2NOBr (slow) Which statements are correct? 1 NOBr2 is a transition state. 2 The rate equation for this reaction is rate = k[NOBr2][NO]. 3 NOBr2 is a radical. A 1, 2 and 3 B 1 and 2 only C 2 and 3 only D 3 only 8 Given the following reaction in a sealed vessel: 2A(g) + B(g) ⇌ C(g) + D(g) H > 0 Which change in conditions increases both the rate of reaction and yield of the products? 1 add a suitable catalyst and add a suitable solvent to dissolve C only 2 decrease the volume of the vessel 3 increase pressure of vessel by adding an inert gas at constant volume 4 increase temperature of the vessel A 1, 2 and 4 only B 2 and 3 only C 1 and 3 only D 2 and 4 only

5 9 1,1–dimethylcyclopentane undergoes free radical substitution with limited bromine in the presence of ultraviolet light. A mixture of mono-brominated products is obtained. 1,1–dimethylcyclopentane What is the total number of mono–brominated products (ignore stereoisomers) obtained? A 2 B 3 C 4 D 6 10 Cholecalciferol is a type of vitamin D that is made by the skin when exposed to sunlight. It can also be found in the flesh of fatty fish and fish liver oils. OH cholecalciferol Which statements about cholecalciferol are correct? 1 There is a maximum of 128 stereoisomers. 2 The carbon bonded to oxygen atom is an electrophilic site. 3 1 mol of cholecalciferol produces 3 mol of carbon dioxide when heated with acidified KMnO4. A 2 only B 1 and 2 only C 1 and 3 only D 1, 2 and 3

6 Section B – Structured Questions [25 marks] 1 (a) Silicon is the 14th element in the Periodic Table. (i) State the full electronic configuration of a silicon atom. [1] (ii) On the axes provided in Fig. 1.1, s ketch the first six successive ionisation energies of silicon against the number of electrons removed. Fig. 1.1 Explain the general shape and significant change (if any) in your sketch. [3] number of electrons removed ionisation energy / kJ mol−1 1 2 3 4 5 6

7 (iii) Sketch and label clearly all the occupied orbitals of a Si4+ ion. [2] (c) Use of the Data Booklet is relevant to this question. In an experiment, KMnO4 oxidises X2+(aq) in acidic medium, while itself is reduced to Mn2+. 25.0 cm 3 of 0.300 mol dm –3 X2+(aq) was found to require 26.30 cm 3 of 0.0570 mol dm–3 KMnO4 for complete oxidation. Determine the final oxidation state of X. [2] [Total: 8 – complete within 13 min]

8 2 (a) Data concerning CaCl2, at 298 K, are given in Table 5.1 below. Table 5.1 Standard enthalpy change of formation, CaCl2 / kJ mol–1 –796 Standard enthalpy change of hydration of Ca2+ / kJ mol–1 –1650 Standard enthalpy change of hydration of Cl– / kJ mol–1 –363 Lattice energy of CaCl2 / kJ mol–1 –2258 (i) Write an equation for the standard enthalpy change of formation of CaCl2. [1] (ii) Calculate the standard enthalpy change of solution of CaCl2 at 298 K. [1] (iii) The theoretical enthalpy change of solution of CaCl2 is –81 kJ mol–1. Using this theoretical value, calculate the final temperature of the solution when 5.50 g of CaCl2 is dissolved in 60.0 cm3 of water at 25.0 °C. [3] [Total: 5 – complete within 8 min]

9 3 Many chemical reactions such as the reaction between peroxodisulfate, S2O82−, and iodide ions occur very slowly at room temperature. S2O82− + 2I− → 2SO42− + I2 H < 0 One way to speed up the rate of reaction is to use a homogeneous catalyst. (a) (i) Explain why the reaction between S2O82− and I− has a high activation energy. [1] A solution of iron(II) ions can be used to catalyse the reaction between S2O82−and I−. A two−step reaction mechanism is proposed. The first step is shown below. Step 1: 2Fe2+(aq) + S2O82⁻(aq) → 2Fe3⁺(aq) + 2SO42⁻(aq) (ii) Write a balanced equation, including state symbols, for the second step of the reaction mechanism. [1] (b) A kinetics study was conducted on the reaction between S2O82−and I− to determine the rate equation. Varying volumes of water, 2.50 mol dm −3 sodium peroxodisulfate, Na 2S2O8, and 0.05 mol dm −3 potassium iodide, K I, were added to a mixture containing sodium thiosulfate and starch indicator. As the reaction of S 2O82−and I− proceeds, the iodine produced will be consumed by the thiosulfate. When all the thiosulfate has reacted, the remaining iodine will react with the starch indicator, forming a blue−black complex. The rate of reaction is determined by the time taken for the blue-black colouration to appear. The results for experiments 1 to 3 are shown in Table 3.1.

10 Table 3