2024 Prelims DHS H2 Chem P3 (Ans)

© DHS 2024 9729/03 [Turn over Suggested Solutions DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 CHEMISTRY Paper 3 Free Response Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/03 18 September 2024 2 hours READ THESE INSTRUCTIONS FIRST Write your centre number, index number, name and class at the top of this page. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all questions. Section B Answer one question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Section A 1 20 2 20 3 20 Section B 4 / 5 20 Total 80 This document consists of 22 printed pages.

2 © DHS 2024 9729/03 Section A Answer all the questions in this section. 1 (a) A bomb calorimeter and a Styrofoam cup calorimeter are both tools used in thermodynamics to measure heat changes during a chemical reaction. However, they differ significantly in their design, operation, and the type of reactions they measure. Fig. 1.1 shows a simplified diagram of a bomb calorimeter which can be used for the accurate determination of heat changes during the combustion of alkanes. Fig. 1.1 The combustion process takes place at constant volume and the reaction is initiated via the electrical heating device which ignites the mixture. (i) State one reason why the conditions inside the bomb calorimeter are considered non-standard. [1] The pressure is high inside the bomb calorimeter and hence, is different from standard conditions, which is defined as 1 atm. (ii) Suggest a reason why the Styrofoam cup calorimeter would be unsuitable to be used to measure heat changes during the combustion of alkanes. [1] Styrofoam cup calorimeter would be unsuitable in this case because the combustion reaction involve gases and they would escape from the cup.

3 © DHS 2024 9729/03 [Turn over (b) The heat capacity of a bomb calorimeter, Ccal, is the amount of heat required to raise the temperature of the calorimeter by 1 C, and it can be calculated using a fuel which the heat change is known. In an experiment to calculate Ccal, 2.00 g of liquid hexane is ignited with oxygen gas which is required to be in 20% excess. A temperature change of 12.6 °C is recorded. Under the conditions of the experiment, 1.00 mol of hexane releases 4 254 kJ of energy when combusted. (i) Write an equation for the complete combustion of hexane. [1] CH3(CH2)4CH3(l) + 19 2 O2(g) → 6CO2(g) + 7H2O(l) (ii) Calculate the number of moles of oxygen gas needed to ensure the excess and hence, the volume occupied by this same amount of oxygen gas at a temperature of 298 K and a pressure of 1.00 atm , assuming ideal gas behaviour. [2] Amount of hexane present = 2.00 86.0 = 0.023256 mol Amount of oxygen gas needed to ensure the 20% excess = 0.023256 × 19 2 × 1.2 = 0.26512 mol Let V = volume of O2 gas introduced Using the ideal gas equation, pV = nRT V = nRT P V = 0.26512×8.31×298 101325 V = 0.0064794 m3 ≈ 0.00648 m3 (iii) Calculate the heat capacity of the bomb calorimeter, Ccal, in kJ K–1. [2] Amount of heat energy released = 0.023256 × 4254 = 98.930 kJ Ccal = 98.930 12.6 = 7.85 kJ K–1 (iv) When the experiment is repeated with 2.00 g of butane, the temperature change recorded is 12.3 C. Calculate the heat change per mole of butane combusted. If you were unable to calculate a value for Ccal in (b)(iii), use 6.50 kJ K–1. [2]

4 © DHS 2024 9729/03 Amount of heat released = 7.8516 × 12.3 = 96.575 kJ Amount of butane used = 2.00 58.0 = 0.034483 mol Heat change for butane = 96.575 0.034483 = 2800.7 kJ mol–1 ≈ 2800 kJ mol–1 (c) (i) Halogenoalkanes, R−X, undergo substitution reactions with nucleophiles. Define what a nucleophile is. [1] A nucleophile is an electron pair donor which is attracted to an electron deficient atom or region of low electron density. Table 1.1 shows the melting points of two metal nitrites which have different structure and bonding. Table 1.1 metal nitrite melting point/ C AgNO2 140 KNO2 441 Fig. 1.2 shows a general structure of AgNO2. Fig. 1.2 (ii) Draw a ‘dot-and-cross’ diagram to show the type of bonding present in KNO2. [1] (iii) With consideration of the difference in bonding between KNO 2 and AgNO 2, explain why R−X react with KNO 2 to form alkyl nitrites, R−ONO, while R−X react with AgNO2 to form nitroalkanes, R−NO2, as the major product. [R: alkyl group] [2] KNO2 is predominantly ionic and one of the oxygen atoms have a negative charge. Nucleophilic attack through this negatively charged oxygen atom on the alkyl halides mainly gives RONO. In contrast, AgNO2 is a covalent compound and both O and N atoms carry lone pair of electrons. Since N is less electronegative than O, the lone pair of electrons of N is more readily available for bond formation. Hence, RNO2 is the major product. ● ● ● ● ● X ● X O X X ● N ● ● ● ● K + O ● X X − N O Ag O

5 © DHS 2024 9729/03 [Turn over (d) Carbonylation reactions are chemical reactions in which a carbon monoxide (CO) molecule is incorporated into an organic molecule. Fig. 1.3 shows metal-controlled mono- and double-carbonylation reactions of alkanes with amines to prepare alkyl amides and alkyl α-ketoamides respectively. The choice of the Co or Cu catalyst precursor was the key to producing a switch in the reaction selectivity. [R: alkyl group or H atom] Fig. 1.3 (i) State the type of reaction common to both carbonylation reactions seen in Fig. 1.3. [1] Addition Fig. 1.4 shows a reaction scheme which includes the carbonylation reactions shown in Fig. 1.3. Fig. 1.4 (ii) With reference to Fig. 1.4, draw the structures of • alkane A, • amine B, • compounds C and E. [4] alkyl−H + CO + Co Cu mono-carbonylation double-carbonylation step 1 compound C, C11H15Cl compound D, C12H15N step 2 amine B + CO + alkane A Co Cu compound E

6 © DHS 2024 9729/03 alkane A amine B N H H compound C compound E Cl N O HO (iii) With reference to Fig. 1.4, identify the reagents and conditions for steps 1 and 2. [2] step 1: KCN or NaCN in ethanol, heat under reflux step 2: LiAlH4 in dry ether, room temperature [Total: 20]

7 © DHS 2024 9729/03 [Turn over 2 (a) Describe the reactions, if any, of the chlorides NaCl, AlCl3 and PCl5 with water. Write an equation for any reaction and state the pH of the resultant mixtures.