MI Prelim H2 Chem P3 QP for exchange

Class Adm No Candidate Name: This question paper consists of 22 printed pages. 2025 Preliminary Examination Pre-University 3 H2 CHEMISTRY 9729/03 Paper 3 Free Response 15 Sep 2025 2 hours Candidates answer on the Question Paper Additional materials: Data Booklet READ THESE INSTRUCTIONS FIRST Do not turn over this question paper until you are told to do so Write your name, class and admission 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 page 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. The number of marks is given in brackets [ ] at the end of each question or part question. Question A B Total 1 2 3 4 / 5 Marks 21 16 23 20 80 H

2 Section A Answer all the questions in this section. 1 Copper and iron are transition metals that form a wide array of complexes due to their ability to exhibit multiple oxidation states. Copper and iron complexes showcase diverse chemical and biological roles, ranging from catalysis to electrochemical processes. (a) (i) State two physical properties of transition metals which differ from that of main group elements. [2] (ii) When copper(I) oxide solid, Cu2O was added to sulfuric acid, H2SO4(aq), and warmed, a blue solution and a pink solid were observed. Identify the blue solution and the pink solid and write a balanced chemical equation for the reaction. [2] (iii) Ethylenediaminetetraacetic acid (EDTA) is a compound used in chelation therapy. Suggest why [Cu(EDTA)]2− and [Cu(NH3)4(H2O)2]2+ complexes have different colours. [1] ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………...

3 [Turn over (b) A sample of copper(II) sulfate solution was added to an excess of aqueous potassium iodide to make a 250 cm3 solution. 2CuSO4 + 4KI ⟶ 2CuI + I2 + 2K2SO4 The amount of iodine produced can be found by titrating a sample of this solution with sodium thiosulfate, Na2S2O3, solution. 25.0 cm3 of the iodine-containing solution required 20.00 cm3 of 0.10 mol dm−3 sodium thiosulfate solution for complete reaction. I2 + 2S2O32– ⟶ S4O62– + 2I– Calculate the amount of copper(II) sulfate present in the original sample. [2] ………………………………………………………………………………………………………………............ ………………………………………………………………………………………………………………............ ………………………………………………………………………………………………………………............ ………………………………………………………………………………………………………………............ ………………………………………………………………………………………………………………............ ………………………………………………………………………………………………………………............ ………………………………………………………………………………………………………………............ ………………………………………………………………………………………………………………............ ………………………………………………………………………………………………………………............ ………………………………………………………………………………………………………………............ ………………………………………………………………………………………………………………............ ………………………………………………………………………………………………………………............ ………………………………………………………………………………………………………………............ ………………………………………………………………………………………………………………............

4 (c) Chromium is also a transition metal. Potassium dichromate(VI), K2Cr2O7, is commonly used as an oxidising agent in organic chemistry reactions. For example, a solution of K2Cr2O7 can be used to oxidise butan-1-ol to butanoic acid. (i) State the oxidation number of the underlined carbon in butan-1-ol, CH3CH2CH2CH2OH. [1] (ii) By writing the oxidation and reduction half equations, construct an ionic equation for the reaction between potassium dichromate(VI) and butan-1-ol in an acidic solution. [3] A student proposed a reaction mechanism for the oxidation of butan-1-ol and suggested that steps 1 and 2 of the mechanism are as follows: Step 1: Protonation of butan-1-ol to form a better leaving group. Step 2: Breaking of the C−O bond to form a carbocation. (iii) Draw the structure of the protonated intermediate in step 1. [1] (iv) It was found that the proposed mechanism was not feasible as the carbocation formed in step 2 was unstable. Suggest why the carbocation formed was unstable. [1] ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………... ………………………………………………………………………………………………………………...

5 [Turn over (d) A team of scientists is designing a power source for field sensors. The aim is to develop a sustainable electrochemical cell using abundant and recyclable materials. After consideration, the team narrows their choice of materials to zinc, copper, iron, and silver electrodes. (i) Propose a combination of two materials that will give rise to the most efficient electrochemical cell with the highest voltage. Explain your answer and justify by means of a calculation. [3] (ii) Draw a fully labelled diagram of the electrochemical cell proposed in (d)(i), indicating clearly the direction of electron flow. [3] (iii) State the observations observed over time as the electrochemical cell is allowed to run. [1] (iv) Suggest one potential challenge the team of scientists might face should they proceed with large scale productions of the electrochemical cells proposed in (d)(i). [1] ………………………………………………………………………………………………………………... …………………………………………………………………………