2023 H2 Chem Prelim P1 Worked Solutions

1 9729/01 CJC JC2 Preliminary Examination 2023 [Turn over CANDIDATE NAME CLASS 2T CHEMISTRY 9729/01 Paper 1 Multiple Choice 15 September 2023 1 hour Additional Materials: Multiple Choice Answer Sheet Data Booklet READ THESE INSTRUCTIONS FIRST Write in soft pencil. Do not use staples, paper clips, glue or correction fluid. Write your name, class and NRIC/FIN number on the Answer Sheet in the spaces provided. There are thirty questions on this paper. Answer all questions. For each question there are four possible answers A, B, C and D. Choose the one you consider correct and record your choice in soft pencil on the separate Answer Sheet. Read the instructions on the Answer Sheet very carefully. Each correct answer will score one mark. A mark will not be deducted for a wrong answer. Any rough working should be done in this booklet. The use of an approved scientific calculator is expected, where appropriate. This document consists of 16 printed pages. WORKED SOLUTIONS Catholic Junior College JC 2 Preliminary Examinations Higher 2

2 9729/01 CJC JC2 Preliminary Examination 2023 1 What volume of air is required for the complete combustion of 1.0 dm 3 of octane vapour, C8H18 (g), in a car engine? (Air contains 20% oxygen by volume.) A 2.5 dm3 B 12.5 dm3 C 62.5 dm3 D 125.0 dm3 2 The ionisation energies, in kJ mol –1, of three elements are given in the table. 1st ionisation energy 2nd ionisation energy 3rd ionisation energy Ne 2080 3950 6150 Na 494 4560 6940 Mg 736 1450 7740 Which statement(s) about these ionisation energies is/are correct? 1 Ne has the greatest 1st ionisation energy of the three elements because its electrons experience the most interelectronic repulsion. 2 Mg has the lowest 2nd ionisation energy of the three elements because the electron being removed experiences the greatest shielding effect. 3 There is a large increase from the 2nd to 3rd ionisation energy of Na because the electron is being removed from the next inner subshell. A 1 and 2 only B 2 only C 1 and 3 only D 3 only Topic: Mole Concept Combustion of octane: C8H18 + 𝟐𝟐𝟐𝟐 𝟐𝟐 O2  8CO2 + 9H2O Volume of O2 needed = 1 x 𝟐𝟐𝟐𝟐 𝟐𝟐 = 12.5 dm3 Volume of air needed = 𝟏𝟏𝟏𝟏𝟏𝟏 𝟐𝟐𝟏𝟏 × 𝟏𝟏𝟐𝟐. 𝟐𝟐 = 62.5 dm3 Answer: C Topic: Atomic Structure Electronic configuration of Ne: 1s22s22p6; Na: 1s22s22p62s1; Mg: 1s22s22p62s2 Statement 1 – Wrong. Ionization energy (IE) depends on nuclear charge, distance of electron from nucleus, and shielding effect. Out of the three elements, Ne has the smallest nuclear charge, but the electron being removed is from the 2nd principal quantum shell compared to Na and Mg (3rd principal quantum shell), and experiences less shielding effect, hence greatest attraction from nucleus, and greatest amount of energy required to remove it. Statement 2 – Correct.