MI Prelim P3 Ans

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] Removal of H + for p Ka2 is from a negatively charged species while that for p Ka1 is from a neutral species, hence dissociation for p Ka2 is more difficult / (COOH)COO – is less acidic than (COOH)2 and the value of pKa1 is larger. or The first conjugate base of oxalic acid is stabilised by an intramolecular ion-dipole interaction with the OH group of (COOH)COO –, hence dispersing the negative charge to a greater degree and increasing the acid strength. Thus the first p Ka value is smaller than the second pKa as the second conjugate base formed does not have this extra stability. (b) (i) Calculate the value of pH at point W. [1] [H+] = ඥ(10ିଵ.ଶହ)(0.50) = 0.1677 mol dm-3 pH = 0.776 pKa1 ⇌ volume of NaOH added / cm3 pKa2 ⇌ pH Y X W

3 [Turn over (ii) Given that the pH at point X is 1.25, calculate the volume of NaOH added at point X. [2] Amount of oxalic acid = 0.50 x ଶହ ଵ଴଴଴ = 0.0125 mol pH = pKa1 + log ቀ ሾsaltሿ ሾacidሿቁ Let ݔbe the amount of salt (COOH)COO–, which is also the amount of NaOH added. 1.25 = 1.25 + V ܸ/)ݔ൰ ௫ ଴.଴ଵଶହି௫ = 1 ݔ0.00625 mol vol. of NaOH added = ଴.଴଴଺ଶହ ଵ = 6.25 cm3 or pH = pKa1, X is at MBC, which occurs at HEP(1). Amount of oxalic acid reacted = 0.0125/2 = 0.00625 mol = Amount of NaOH added vol. of NaOH added = ଴.଴଴଺ଶହ ଵ = 6.25 cm3 or Since pH = pKa1, X is at MBC, which occurs at HEP(1). Since EP(1) vol. of NaOH = ቀ ଴.଴ଵଶହ ଵ ቁ = 0.0125 dm3 = 12.5 cm3 vol. of NaOH added at HEP(1) = 12.5/2 = 6.25 cm3 (c) (i) Define the term buffer. [1] A buffer is a solution which is able to resist pH changes when small quantities of acid or base are added. (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] (COOH)COO- + OH-  (COO-)COO– + H2O (d) (i) Describe how you would recognise the end point of the titration described. [1] Colourless solution turns pale pink. (ii) Explain why only 1 or 2 drops of phenolphthalein indicator was used in the titration. [1] Indicators are weak acids/bases. Addition of more indicator could cause a significant change in the pH of the solution, affecting the accuracy of the titration.

4 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] In water, Pt2+ exists as the tetraaqua complex [Pt(H2O)4]2+ [Pt(H2O)4]2+ → [Pt(H2O)3(OH)]+ + H+ Transition metal ions such as Pt have a high charge density and can polarise the H 2O ligands to release H+, making the solution acidic. Oxaliplatin is a platinum( II) based anti-cancer drug used to treat colorectal cancer. It contains one C2O42− ligand. 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 OO O−

5 [Turn over (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] Not feasible, as entropy change of the ligand exchange reaction is negative / reaction is entropically disfavoured due to a decrease in the number of particles. Enthalpy change of the ligand exchange reaction. accept ∆G; BE(Pt-Cl) and BE(Pt-O) (iii) Determine if any the three complexes, oxaliplatin, X and/or Y can exhibit cis-trans isomerism. Explain your answer. [2] None of the three complexes can exhibit cis-trans isomerism. The bidentate ligand cannot be forced into a trans conformation. OWTTE (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] Amount of AgCl = 1.35 / (107.9+35.5) = 0.009414 mol = Amount of free Cl– ∴ 1 mol of isomer contains 1 mol of free Cl–. [Pt(C6H14N2)(Cl)(H2O)]+Cl–.H2O (f) (i) Determine the number of protons, neutrons and electrons in 52Cr3+. [1] 24 protons, 28 neutrons, 21 electrons (ii) State the full electronic configuration of 52Cr3+. [1] 1s22s22p63s23p63d3

6 (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]

7 [Turn over 2 (a) State the relationship between the Faraday constant, F, the charge on the electron, e, and the Avogadro number, L. [1] F = Le (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] Cathode reaction: Cu2+(aq) + 2e → Cu(s) 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. cur