MI Prelim H2 Chem P3 Suggested answers_for exchange

Class Adm No Candidate Name: 2025 Preliminary Examination Pre-University 3 H2 CHEMISTRY 9729/03 Paper 3 Free Response 15 Sep 2025 2 hours Candidates answer on separate paper. Additional materials: Data Booklet Suggested answers 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] They have high melting and boiling points. They have higher densities. (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] Blue solution: CuSO4 - Pink solid: Cu [1] Cu2O + H2SO4 → Cu + CuSO4 + H2O [1] (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] Presence of ligands in the solution cause the degenerate 3d orbitals to split into two different energy levels. Different ligands split the d-orbitals to different extents. (b) A sample of copper(II) sulfate solution was added to an excess o f 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] Comparing mole ratio, 2 mol S2O32– : 1 mol I2 2 mol Cu2+ : 1 mol I2 Hence, 1 mol Cu2+ : 1 mol S2O32– Amount of Cu2+ in 25 cm3 = (20/1000) x 0.10 = 2.00 x10-3 mol ; Amount of Cu2+ in the original solution = (250/25) x 2.00 x10-3 = 2.00 x 10-2 mol ;

3 [Turn over (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 1-butanol to butanoic acid. (i) State the oxidation number of the underlined carbon in 1-butanol, CH3CH2CH2CH2OH.