ACSI Paper 3 Prelim (Student Version) Answer

YEAR 6 PRELIMINARY EXAMINATION 2023 INTERNATIONAL BACCALAUREATE DIPLOMA PROGRAMME CHEMISTRY HIGHER LEVEL PAPER 3 Tuesday 12 th September 2023 1 hour 15 minutes INSTRUCTIONS TO CANDIDATES  Do not open this examination paper until instructed to do so.  Write your candidate session number in the box above.  A calculator is required for this paper.  A copy of the Chemistry Data Booklet is required for this paper.  Write your answers in the boxes provided.  If you use additional sheets of paper for your answer, attach them to the booklet. Indicate the question number clearly on these sheets.  All drawings must be in ink. For examiner’s use Qn 1 /6 Qn 2 /9 Qn 3 /4 Qn 4 /6 Qn 5 /5 Qn 6 /6 Qn 7 /5 Qn 8 /4 Wrong s.f. /units Total /45 ________________________________________________________________________ This question paper consists of 15 printed pages including this cover page. Name Candidate Number

2 Section A Answer all questions. Answers must be written in the answer boxes provided. 1. Molecules that have a different arrangement as a result of bond rotation are called conformers. Conformers can be represented by the Newman projection. A Newman projection views the carbon-carbon bond directly end- on and represents the two carbon atoms by a circle. Bonds attached to the front carbon are represented by lines to the centre of the circle, and bonds attached to the rear carbon are represented by lines to the edge of the circle. Ethane can have two conformers as a result of the bond rotation: staggered and eclipsed. Using the Newman projection, the angle between the C-H bonds of the front carbon and the C-H bonds on the back carbon is known as the dihedral angle. Butane can have four types of conformers as a result of bond rotation. Table 1 shows the four conformers of butane. The gauche conformer occurs when the dihedral angle between the methyl groups are 60o apart and the anti conformer occurs when the dihedral angle is 180o apart. (This question continues on the following page)

3 (Question 1 continued) An equilibrium between the gauche 1 and the staggered (anti) conformer can be achieved: ⇌ The equilibrium constant, Kc, between the gauche 1 and the staggered (anti) conformer at 298 K can be calculated by the equation given below:  3630 = RT ln Kc R = molar gas constant in J mol1 K1 T = temperature in Kelvins (a) Which of the two conformer of butane is the most stable? Explain your answer. [1] Anti conformer is most stable as the two bulky groups are furthest from each other / it has the lowest potential energy / Kc > 1 / G < 0 so forward reaction is more spontaneous. Marker comments: Generally well done. A good handful of candidates interpreted the question wrongly, not recognising that the question is asking for one out of the tw