2022 EJC Prelims Paper 3 (solution with comments)

© EJC [Turn Over a EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2022 General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CIVICS GROUP 2 1 – INDEX NUMBER CHEMISTRY Paper 3 Free Response 9729/03 21 September 2022 2 hours Candidates answer on the Question Paper Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, civics group and registration number 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. 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. For Examiner’s Use Section A 1 / 20 2 / 20 3 / 20 Section B 4 / 20 5 / 20 Total / 80 This document consists of 32 printed pages.

2 © EJC 9729/03/J2Prelims/22 Section A Answer all the questions in this section. 1 (a) Elements in Period 3 of the Periodic Table form chlorides and oxides with varying properties. (i) Equal amounts of MgC l2 and PC l5 dissolve in equal volumes of water to form solutions with pH 6.5 and 1.0 respectively. Construct two equations to account for the different pH values. [2] Comments: Many students forgot that the hydrolysis of the hydrated magnesium ion is a reversible reaction since it is only partial hydrolysis. (ii) Explain why the first ionisation energies of aluminium and sulfur are lower than the element before it. [2] Comments: Many students stated that the p orbital of Al i s higher in energy than the s orbital, without stating the principal quantum number. This makes it ambiguous, because it is only true if it is 3p vs 3s. If it was 2p vs 3s, then the 3s would be of higher energy. Thus, it is important to specify the principal quantum shell. Some students thought that the valence shell of Al and Mg contains 2p and 2s orbitals. (iii) Element L is an element in Period 3 . L forms a chloride with a simple molecular structure but an oxide with a giant molecular structure. Identify element L and construct an equation for the reaction of its chloride with NaOH(aq). [2] Comments: This was very badly attempted. One of the most common errors for this question was seeing HCl appear in the products. There is no chemical reaction known to mankind where adding a strong aqueous base, such as NaOH, will result in the formation of a strong aqueous acid, such as HCl. If this could happen, then