CJC 2022 JC2 H2 CHEM PRELIM P3 MS__Examiners Comments

1 9729/03/CJC JC2 Preliminary Examination 2022 [Turn over CANDIDATE NAME CLASS 2T CHEMISTRY 9729/03 Paper 3 Free Response 13 September 2022 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. 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 bookl et. The question number must be clearly shown. Section A Answer all the questions. Section B Answer one question. The use of an approved scientific calculat or is expected, where appropriate. A Data Booklet is provided. At the end of 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 Q1 /15 Q2 /21 Q3 /24 Section B Q4 /20 OR Q5 /20 TOTAL 80 Catholic Junior College JC2 Preliminary Examinations Higher 2 MARK SCHEME & EXAMINER’S COMMENTS

2 9729/03/CJC JC2 Preliminary Examination 2022 Section A Answer all the questions in this section. 1 Chromium is a hard, steel-grey metal with a lustrous appearance. It is valued for its high corrosion resistance and hardness and is commonly used to manufacture alloys such as steel. Chromium plating is sometimes used to give a polished mirror finish to steel. Chromium compounds are also often used as pigments, known as chrome yellow. (a) The following sequence of reactions involving chromium illustrates many of the characteristics properties of transition metals. (i) Solutions of transition metals are frequently coloured. With reference to [Cr(H2O)6]2+,explain fully why it forms a blue solution. [2] .………………………………………………………………………………… .………………………………………………………………………………… .………………………………………………………………………………… .………………………………………………………………………………… .………………………………………………………………………………… .………………………………………………………………………………… .………………………………………………………………………………… orange solution H2O2(aq) yellow solution chromium metal dilute H2SO4 [Cr(H2O)6]2+ blue solution [Cr(H2O)6]3+ green solution grey-green precipitate [Cr(OH)6]3─ dark green solution air NaOH(aq) In the presence of H2O ligands, the partially filled degenerate d-orbitals of Cr2+ ion are split into 2 groups of non-degenerate d orbitals with a small energy gap. In the presence of visible light, d electron in a d orbital of lower energy absorbs orange light and is promot