2025 JC2 Prelims H2 Chem Paper 3 QP and Ans_TJC

H2 P3 Review 1 Schweizer's reagent is used in purifying cellulose. This dark blue compound has the formula, [Cu(NH3)4(H2O)2](OH)2 and contains the tetraamminediaquacopper(II) cation, [Cu(NH3)4(H2O)2]2+. (a) Explain the origin of the dark blue colour of the tetraamminediaquacopper(II) cation. [3] In the gas-phase Cu2+ ion, the five 3d orbitals are degenerate. In the cation, due to the  presence of ligands, the five 3d orbitals are split into two energy levels, with energy gap, ∆E, due to the repulsion between the Cu2+ ion and the ligands. For  Cu2+ with partially filled d-subshell, when a d-electron from lower energy group is  promoted to the higher energy group (d-d transition),  radiation corresponding to ∆E, orange light, is absorbed.  Light of wavelengths not absorbed (blue light) will be seen as the colour of the complex. 5 = 3 marks, 3-4 = 2 marks, 2 = 1 mark (b) When a solution of [Cu(NH 3)4(H2O)2]2+ was gently heated, NH 3 gas was released. A precipitate of Cu(OH)2 and NH4+ ions were also obtained as products. The Cu(OH)2 formed was purified and separated into two samples. One of the samples was added to concentrated hydrochloric acid, forming complex ion X. The other sample of Cu(OH)2 was added to dilute sulfuric acid, forming blue complex ion Y. [Cu(NH3)4(H2O)2]2+, X and Y are of different colours. When [Cu(NH3)4(H2O)2]2+ was strongly heated, a black solid Z is formed. (i) Write an equation for the reaction when a solution of [Cu(NH 3)4(H2O)2]2+ was gently heated. [1] [Cu(NH3)4(H2O)2]2+ → Cu(OH)2 + 2NH4+ + 2NH3 [1] (ii) Write an equation for the reaction of Cu(OH) 2 with concentrated hydrochloric acid, forming X. [1] Cu(OH)2 + 4HCl → [CuCl4]2– + 2H2O + 2H+ [1] OR Cu(OH)2 + 4HCl → H2CuCl4 + 2H2O OR Cu(OH)2 + 4HCl → [CuCl4]2– + 2H3O+ (iii) Suggest the identities of Y and Z. [2] Y: [Cu(H2O)6]2+ [1] Z: CuO [1] (iv) Explain why [Cu(NH3)4(H2O)2]2+ and X are of different colours. [1] The  ligands datively bonded to Cu(II) are different , resulting in  different energy gaps between the d orbitals in the complexes. This will affect the

wavelength of visible light absorbed, and thus colour of transition metal complexes. 2 = 1 mark, with clear explanation (c) Tetraamminediaquacopper(II) cation, [Cu(NH3)4(H2O)2]2+, has two possible stereoisomers. Draw the two stereoisomers. [2] correct relative position of H2O ligan