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 ligands for both isomers [1] only awarded if shape is correct correct 3D shape, charge, ligands bonded correctly [1] Note: Naming of isomers not required (d) Copper based nanopesticides like Cu(OH)2 and CuO, have been used to protect crops from bacteria induced diseases. The minimum effective concentration for Cu2+ ion as a pesticide is 6.35 × 10−5 g dm−3. (i) Write the expression for the solubility product of Cu(OH)2, giving its units. [1] Ksp = [Cu2+][OH−]2, mol3 dm−9 [1] (ii) A sample of solid Cu(OH) 2 is added to water. Given that the value of the solubility product of Cu(OH)2 is 2.20 × 10−20, calculate the solubility of Cu(OH)2 in mol dm−3. [1] Let the solubility of Cu(OH)2 be x mol dm−3. Ksp = [Cu2+] [OH−]2 2.20 × 10−20 = x(2x)2 = 4x3 x = 1.77 × 10−7 mol dm−3 [1] (iii) Hence, deduce if the sample in part (d)(ii) is suitable for use as a pesticide. [2] Concentration of Cu2+ ion in g dm−3 = 1.77 × 10−7 × 63.5 = 1.12 × 10−5 g dm−3 [1]

Since the  [Cu2+] is lesser than minimum effective concentration of Cu2+ ion (6.35 × 10−5 g dm−3), the sample is  not suitable for use as a pesticide. 2 = 1 mark (e) Gilman reagent is an organometallic reagent containing two R groups (alkyl or aryl), copper, and lithium. The general formula of Gilman reagents can be expressed as R2CuLi. The Gilman reagent, lithium dimethylcopper, can react with an acyl chloride via nucleophilic reaction to form a ketone. The steps of the reaction are shown in Fig. 1.1. Fig. 1.1 (i) Draw four curly arrows on Fig. 1.1 to complete the mechanism. Include relevant lone pairs and partial charges. [2]  4 arrows,  lone pair,  partial charge 3 = 1 mark (ii) Lithium dimethylcopper can also react with alkyl halides. A product of this reaction is shown. Suggest the structure of the alkyl halide that reacted with lithium dimethylcopper to give the product above. [1]

[1] (f) Transition elements like copper and iron are commonly used as catalysts. The reaction between S 2O82− ions and I− ions is very slow. If a small amount of aqueous iron(II) ions is added to the mixture, the rate of reaction increases. Write two equations to illustrate the catalytic role of Fe 2+ in the S 2O82− / I− reaction. [2] Step 1: 2Fe2+(aq) + S2O82–(aq) → 2SO42–(aq) + 2Fe3+(aq) [1] Step 2: 2Fe3+(aq) + 2I–(aq) → I2(aq) + 2Fe2+(aq) [1] [Total: 19]

2 Chlorine-containing organic compounds are widely used in both industrial and laboratory settings due to their diverse chemical properties and applications. One such compound is 1 -chloro-1-phenylethane, an aromatic halogenoalkane, which exhibits reactivity typical of benzylic halides. A typical reaction is the reaction between 1-chloro-1-phenylethane and hydroxide ions to produce 1-phenylethanol. C6H5CHClCH3 + OH− C6H5CH(OH)CH3 + Cl− The rate of this reaction can be studied by measuring the amount of hydroxide ions that remained in the solution at a given time. The reaction can effectively be stopped if the solution is diluted with an ice-cold solvent. (a) Briefly describe a suitable method for studying the rate of this reaction at a temperature of 40 °C. [3] ✓1 Ensure both reactant solutions are maintained at 40 °C using a thermostatically controlled water bath before mixing. ✓2 Mix known volumes of both reactants and start the stopwatch. ✓3 At known time, take out a sample and add it to ice-cold solvent for quenching Method 1 ✓4 titrate mixture against standard HCl solution. ✓5 repeat steps 3-4 to obtain volume of HCl at known time intervals. ✓6 plot graph of volume of HCl against time. Method 2 ✓4 Use a pH meter to record the pH. ✓5 repeat steps 3-4 to obtain pH readings at known time intervals. ✓6 plot graph of pH against time. 2✓ = 1 mark (b) The reaction was studied by carrying out four experiments at different initial concentrations of the two reagents. Table 2.1 shows the results obtained. Table 2.1 experiment [C6H5CHClCH3] / mol dm−3 [OH−] / mol dm−3 relative rate 1 0.05 0.10 0.5 2 0.10 0.20 1.0 3 0.15 0.10 1.5 4 x 0.15 2.0 (i) Show that the overall order of the reaction is 1. Explain your reasoning. [2]

Comparing experiments 1 & 3, [OH–] kept constant, [C6H5CHClCH3] increases 3 times from 0.05 mol dm -3 to 0.15 mol dm -3, relative rate increases 3 times from 0.5 to 1.5. First order wrt C6H5CHClCH3. [1] Comparing experiments 1 & 2, [C 6H5CHClCH3] doubled from 0.05 mol dm -3 t