YIJC 2025 Prelim P3 Suggested Solutions (for exchange)_H2 Chem

©YIJC 9729/03/JC2/PE/2025 [Turn over YISHUN INNOVA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME Suggested Solutions CG INDEX NO CHEMISTRY Paper 3 Free Response Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/03 16 September 2025 2 hours READ THESE INSTRUCTIONS FIRST Write your name, class and index number in the spaces 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. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 28 printed pages. For Examiner’s Use Section A 1 / 17 2 / 18 3 / 25 Section B 4 or 5 / 20 Penalty units significant figures Overall / 80

©YIJC 9729/03/JC2/PE/2025 2 Section A Answer all the questions in the spaces provided. 1 (a) Explain, with the aid of a labelled Boltzmann distribution diagram, the effect on a rate constant of increasing temperature from T1 to T2. [3] When the temperature is increased from T 1 to T 2, the average kinetic energy of the particles increases. As shown from the graph, there is an increase in the fraction of particles with energy equal to or greater than the activation energy, Ea. This result in an increase in the frequency of effective collisions, hence the reaction rate increases. A higher temperature results in an increase in reaction rate and hence a larger rate constant k. energy, E fraction of particles Ea fraction of particles with E  Ea at high T2 fraction of particles with E  Ea at low T1 T1 (lower temperature) T2 (higher temperature) 0

3 ©YIJC 9729/03/JC2/PE/2025 [Turn over (b) Hydrogen bromide, HBr, undergoes addition reaction with alkenes. With but-1-ene, 2-bromobutane is produced rather than 1-bromobutane. (i) Draw a mechanism for this reaction and use it to explain the preferential production of 2-bromobutane. [3] Electrophilic Addition Step 2: 2-bromobutane is preferentially formed over 1-bromobutane as the carbocation intermediate formed is a secondary carbocation / there are more electron-donating alkyl groups in the carbocation intermediate . This disperses the positive charge on the carbocation intermediate, making it more stable as compared to the carbocat