MI Prelim P3 QP

Class Adm No Candidate Name: This question paper consists of 11 printed pages and 1 blank page. 2018 Preliminary Exams Pre-University 3 H2 CHEMISTRY 9729/03 Paper 3 Free Response 18th Sept 2018 2 hours Candidates answer on separate paper. Additional materials: Answer Paper 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. 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. You are reminded of the need for good English and clear presentation in your answers. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. Question Section A Section B Total 1 2 3 4 5 Marks 23 20 17 20 20 80

2 Section A Answer all questions from this section. 1 Oxalic acid is a weak diprotic Br Ønsted acid with p Ka1 and p Ka2 values of 1.25 and 4.14 respectively. 25 cm 3 of 0.50 mol dm –3 oxalic acid was titrated against a solution of 1 mol dm –3 of sodium hydroxide. Only 1 to 2 drops of phenolphthalein indicator was used in this titration. It was found that the first colour change occurred at 19.40 cm3. A sketch (not drawn to scale) of the pH titration curve is shown below. (a) Suggest why the value of pKa2 is larger than pKa1. [2] (b) (i) Calculate the value of pH at point W. [1] (ii) Given that the pH at point X is 1.25, calculate the volume of NaOH added at point X. [2] (c) (i) Define the term buffer. [1] (ii) Write an equation to illustrate how the reaction mixture found at point Y can maintain the pH of a solution when a small amount of OH–(aq) is added. [1] (d) (i) Describe how you would recognise the end point of the titration described. [1] (ii) Explain why only 1 or 2 drops of phenolphthalein indicator was used in the titration. [1] pKa1 ⇌ volume of NaOH added / cm3 pKa2 ⇌ pH Y X W

3 [Turn over The oxalate ion, C 2O42−, is known to form complexes with transition metal ions such as platinum(II). The structure of the oxalate ion is as follows. (e) (i) It is observed that a solution of aqueous platinum( II) ions turns universal indicator orange. Given that platinum( II) is a transition metal ion known to form square planar complexes, explain the observation seen with the aid of a chemical equation. [3] Oxaliplatin is a platinum( II) based anti-cancer drug used to treat colorectal cancer. It contains one C2O42− ligand. O O O O Pt N N H2 H2 oxaliplatin When a concentrated Cl– solution is added to oxaliplatin, ligand exchange occurs and the Cl– ligands take the place of C2O42− in a two-step reaction. O O O O Pt N N H2 H2 Cl Pt N N H2 H2 O O O O +C l- Cl Pt N N H2 H2 O O O O Cl Pt N N H2 H2 Cl +C l- +C 2O4 2- Step I Step II X Y (ii) By consideration of entropy changes, predict with explanation if Step I is likely to be feasible if diluted Cl– solution was used instead. Assume that C l– and C2O42− have the same ligand strength. State the missing numerical data necessary to confirm your prediction. [2] (iii) Determine if any the three complexes, oxaliplatin, X and/or Y can exhibit cis-trans isomerism. Explain your answer. [2] −O OO O−

4 (iv) Hydrated crystals of complex Y have the structural formula [Pt(C6H14N2)Cl2].2H2O. When excess AgNO3 solution was added to an aqueous solution containing 0.0094 mol of an isomer of [Pt(C6H14N2)Cl2].2H2O, 1.35 g of a white precipitate was obtained. Determine the structural formula of this isomer. [3] (f) (i) Determine the number of protons, neutrons and electrons in 52Cr3+. [1] (ii) State the full electronic configuration of 52Cr3+. [1] (iii) Draw and label the d orbitals in the valence shell of 52Cr3+. You are required to show the orientation of the orbitals with respect to the x, y and z axes. [2] [Total: 23]

5 [Turn over 2 (a) State the relationship between the Faraday constant, F, the charge on the electron, e, and the Avogadro number, L. [1] (b) Copper metal containing impurities such as zinc, iron, and silver can be purified via electrolysis. (i) Draw a well-labelled diagram of a suitable set-up for purifying impure copper and state the reaction occurring at the cathode. [3] A fixed current was passed through the cell for a duration of time to determine the experimental values of F and L. The following are the results obtained. current passed through the cell = 0.500 A duration of time = 30.0 min change in mass of cathode = 0.299 g (ii) Use the data above and relevant information from the Data Booklet to calculate a value of L. [3] (c) Rechargeable lithium ion batteries have been developed in Japan. The standard electromotive force of a rechargeable lithium ion battery is 3.70 V. During discharge, Li + ions flow from the anode to the cathode through the electrolyte. During the discharging process, the ion-electron equation at the cathode is CoO2 + Li+ + e- → LiCoO2, and the ion-electron equation at the anode is LiC6 → C6 + Li+ + e- where CoO2 and graphite, C 6, are layered solid electrodes which can have Li + ions and Li atoms embedded between their layers respectively. A schematic of the rechargeable lithium ion battery during discharge is shown below.

6 (i) By considering the overall equation of the battery during discharge, calculate a value for the standard Gibbs free energy of the reaction in kJ mol-1. [1] (ii) A fully charged battery cell starts with 10.0 g of CoO2 and 10.0 g of LiC6. Calculate the mass of electrode A when this cell is fully discharged. [3] (iii) The fully discharged cell is plugged into a power source to recharge it. During the charging process, the direction of electron-flow is reversed. Determine the mass of electrode B when the cell is fully charged. [1] (iv) Suggest two reasons why water cannot be used as the electrolyte solvent in rechargeable lithium ion batteries. [2] (v) Rechargeable lithium ion batteries tend to perform poorly when used in cold countries. Suggest a reason for this. [1] (d) Lithium oxide is a white solid that is used to lower the melting point of ceramic glazes. (i) Write an equation, with state symbols, to represent the lattice energy of lithium oxide. [1] (ii) Use the following data, together with appropriate data from the Data Booklet , to calculate a value for the lattice energy of lithium oxide. 1 st electron affinity of oxygen = -141.1 kJ mol-1 2nd electron affinity of oxygen = +798 kJ mol-1 enthalpy change of atomisation of lithium = +159.4 kJ mol-1 enthalpy change of formation of lithium oxide = -597.9 kJ mol-1 [4] [Total: 20]