Prelim Paper 2 - Suggested Solutions_ACJC H2 Chem

1 ACJC2025 9729/02/Prelim/2025 [Turn over Anglo-Chinese Junior College JC2 Preliminary Examinations Higher 2 CANDIDATE NAME SUGGESTED SOLUTIONS FORM CLASS TUTORIAL CLASS INDEX NUMBER CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/02 27 August 2025 2 hours READ THESE INSTRUCTIONS FIRST Write your index number and name 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, graphs or rough working. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiners’ use only 1 / 7 2 / 12 3 / 10 4 / 11 5 / 11 6 / 16 7 / 8 Total / 75 This document consists of 19 printed pages.

2 ACJC2025 9729/02/Prelim/2025 [Turn over 1 When ammonium dichromate( VI) is added gradually to molten ammonium thiocyanate, Reinecke’s salt is formed. It has the formula NH 4[Cr(SCN)x(NH3)y] and the following composition by mass: Cr 15.5 %; S 38.15 %; N 29.2 %. (a) Calculate the values of x and y in the above formula. Given Cr is 15.5% of total Therefore Mr = 100 X 52.0 /15.5 = 335.5 For S, 38.15 / 100 x 335.5 = 128 nS =128 / 32.1 = 4 x = 4 Therefore NH4[Cr (SCN)4(NH3)y] = 335.5 Therefore 18 + 52 + 4 x 58.1 + 17y = 335.5 Therefore 17y = 33.5 y = 2 [2] (b) Suggest a shape for the complex anion. octahedral …………………………………………………………………………………………………...[1] (c) Draw two possible structures for the anion and state the type of isomerism it exhibits. cis-trans isomerism …………………………………………………………………………………………………...[2] (d) Linkage isomerism is a form of constitutional isomerism in which certain coordination compounds have the same composition but differ in which atom of the ligand is bonded to the metal. Examples of linkage isomers are violet-colored [(NH3)5Co-SCN]2+ (S being the donor atom) and the orange [(NH3)5Co-NCS]2+ (N being the donor atom). Draw the dot -and-cross diagrams of NCS ˉ and SCNˉ. In each diagram, underline the donor atom. [2] [Total: 7]

3 ACJC2025 9729/02/Prelim/2025 [Turn