2025 H1 Chem 8873 Prelim P2 MARK SCHEME CJC

CANDIDATE NAME CLASS 2T CHEMISTRY 8873/02 Paper 2 Structured Questions 29 August 2025 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. 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. Section A Answer all the questions. Section B Answer one question. The use of an approved scientific calculator is expected where appropriate. You are reminded of the need for good English and clear presentation in your answers. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part of the question. This document consists of 25 printed pages and 1 blank page Section A Q1 5 Q2 6 Q3 15 Q4 16 Q5 6 Q6 12 Section B Q7 20 Q8 20 Paper 2 (67%) 80 Paper 1 (33%) 30 Percentage Grade Catholic Junior College JC2 Preliminary Examinations Higher 1

2 8873/02/CJC JC2 Preliminary Examinations 2025 Section A Answer all the questions in this section in the spaces provided. 1(a) The hydrogen carbonate ion, HCO3‒, has both acidic and basic properties. State the conjugate acid and conjugate base of HCO3‒. Conjugate acid: ................................ ................................ ................................ ............. Conjugate base: ................................ ................................ ................................ ........ [1] (b) In the human body, it is important for blood to be maintained at a pH between 7.35 and 7.45. The main buffering agent in blood plasma is the H2CO3(aq) / HCO3–(aq) system. During a vigorous physical workout, the muscles will produce an acid which will be transported by the blood to the liver to be broken down. (i) Define the term buffer solution. ................................ ................................ ................................ .............................. ................................ ................................ ................................ .......................... [1] (ii) Briefly explain the effect physical workout will have on the pH of blood if there is no buffer present. ................................ ................................ ................................ .............................. ................................ ................................ ................................ .............................. ................................ ................................ ................................ .............................. ................................ ................................ ................................ .......................... [2] (iii) Write an equation to explain how the H2CO3(aq) / HCO3–(aq) buffer system helps to maintain the pH of blood during a physical workout. ................................ ................................ ................................ .............................. ................................ ................................ ................................ .......................... [1] [Total: 5] When blood becomes more acidic, HCO3¯ + H+ → CO2 + H2O Blood will become more acidic due to production of acid, resulting in the pH of blood falling below 7.35. A buffer solution is one that can maintain a fairly constant pH when small amount of acid or base is added to it. H2CO3 CO32‒

3 8873/02 CJC JC2 Preliminary Examinations 2025 [Turn over 2 A student used the apparatus shown in Fig. 2.1 to find the enthalpy change of combustion of butan-1-ol, Hc(butan-1-ol). Fig. 2.1 temperature of water before heating = 25.0 °C temperature of water after heating = 83.7 °C mass of spirit burner and butan-1-ol before heating = 80.44 g mass of spirit burner and butan-1-ol after heating = 79.70 g The specific heat capacity of water is 4.18 J g‒1 K‒1. CH3CH2CH2CH2OH(l) + 6O2(g) → 4CO2(g) + 5H2O(l) Hc(butan-1-ol). (a) From the data provided above, calculate the number of moles of butan -1-ol that reacted and hence determine the enthalpy change of combustion of butan-1-ol. [3] beaker 100 cm3 of water spirit burner butan-1-ol wick thermometer clamp mass of butan-1-ol reacted = 80.44 – 79.70 = 0.74 g No. of moles of butan-1-ol reacted = 0.74 / 74.0 = 0.0100 mol amount of heat absorbed by water = mcT = 100 x 4.18 x (83.7 – 25.0) or (100 x 4.18 x 58.7) = 24.54 kJ enthalpy change of combustion of butan-1-ol = –24.54/ 0.0100 = –2450 kJ mol-1 (3sf)

4 8873/02/CJC JC2 Preliminary Examinations 2025 (b) CH3CH2CH2CH2OH(l) + 6O2(g) → 4CO2(g) + 5H2O(l) Hc(butan-1-ol). Complete the table 2.1 below by using the bond energies given in the Data Booklet to calculate another value for the standard enthalpy change of combustion of butan-1-ol. Table 2.1 [2] (c) Suggest a reason for the discrepancy between the two values calculated in (a) and (b). ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ......................... [1] [Total: 6] Bonds broken: Bonds formed: 3 C-C 3 x 350 8 C=O 8 x 805 9 C-H 9 x 410 10 O-H 10 (460) 1 C-O 1 x 360 1 O-H 1 x 460 6 O=O 6 x 496 Energy required to break bonds = (3 x 350) + (9 x 410) + (1 x 360) + (1 x 460) + (6 x 496) = 8536 kJ Energy released from bonds formed = (8 x 805) + 10 x 460 = 11 040 kJ HcӨ of butan-1-ol = -11040 + 8536 = –2500 kJ mol–1 3sf Heat loss to the surrounding hence the temperature rise was lower. OR The bond energies given in the data booklet are only averages and would not apply to the exact compounds in the reaction. Or The HcӨ calculated applies for the reactants and products in the gaseous phase but the standard enthalpy change of combustion is defined in terms of the formation of liquid H2O rather than gaseous H2O.

5 8873/02 CJC JC2 Preliminary Examinations 2025 [Turn over 3(a) Propanone reacts with iodine in an aqueous solution of sulfuric acid as shown below. CH3COCH3(aq) + I2(aq) → CH3COCH2I(aq) + H+ + I‒(aq) The rate of this reaction was investigated by withdrawing samples from the reacti on mixture, 'quenching' by adding the sample to a large volume of ice -cold water, then measuring the amount of iodine in the sample. (i) Why is it necessary to quench the sample before measuring the amount of iodine? ................................ ................................ ................................ .............................. ................................ ................................ ................................ .............................. ................................ ................................ ................................ .............................. ................................ ................................ ................................ ......................... [1] Separate experiments were carried out to determine the initial rates of reaction and the results obtained are given in the table below. The rate equation for the reactio