TJC H2 CHEM P3 Qn Prelim

1 2016 TJC H2 Chemistry Preliminary Exam [Turn Over CHEMISTRY 9647/03 Paper 3 Free Response 13th September 2016 2 hours Candidates answer on separate paper. Additional materials: Answer paper Graph Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, Civics Group, Centre number and Index number in the spaces provided on the cover page and on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a HB pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, glue or correction fluid. Answer any four questions. 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. This document consists of 15 printed pages.

2 2016 TJC H2 Chemistry Preliminary Exam [Turn Over Answer any four questions 1 Ethanedioic acid is a substance found in many plant foods. Cabbage is among the plant foods with high ethanedioic acid content. However, its anion, C2O42- can bind to iron to form iron(II) ethanedioate, which renders much of the iron in cabbage unusable by the body. (a) 50.0 cm3 sample of iron(II) ethanediaote, FeC2O4 was extracted from 300 g of cabbage, diluted in water and the solution made up to 250 cm 3. A 25.0 cm3 portion of this solution was acidified and required 26.90 cm 3 of 0.0100 mol dm -3 potassium manganate(VII) for oxidation of iron(II) to iron(III) and ethanedioate ions to carbon dioxide. (i) State the change in oxidation number for manganese and carbon in the reaction. [1] (ii) Write down all the relevant ion-electron half equations and hence the overall redox equation for the reaction between potassium manganate(VII) and iron(II) ethanedioate. [2] (iii) Calculate the concentration, in mol dm -3, of iron(II) ethanedioate in the original sample. [2] (iv) Calculate the number of moles of iron in each gram of cabbage. [1] (b) Ethanedioic acid dissociates in water according to the following equation. HOOC-COOH + H2O HOOC-COO- + H3O+ The table below compares the Ka values of three organic acids. Formula Ka Ethanoic acid CH3COOH 1.74 x 10-5 Ethanedioic acid HO2CCO2H 6.46 x 10-2 Oxoethanoic acid (CHO)COOH 4.79 x 10-4 With reference to the K a values, comment on the order of acidity of the three organic acids. [2]

3 2016 TJC H2 Chemistry Preliminary Exam [Turn Over (c) Compound A, with molecular formula C5H8O, decolourises aqueous bromine and reacts with PC l5 giving off white fumes. Upon refluxing A with acidified potassium manganate(VII), a symmetrical product B, C 5H6O5, is formed. B does not give a red precipitate with Fehling’s solution but a n orange precipit ate is observed with 2,4-dinitrophenylhydrazine. 1 mol e of B also reacts with 1 mol e of Na 2CO3 with effervescence observed. B reacts with NaBH 4 to form C. Deduce the structures of compounds A, B and C, explaining the chemistry of the reactions involved. [8] (d) (i) Describe what you see when separate samples of sodium and sulfur are burned in excess air. Write equations for the reactions that occur. [2] (ii) The products resulting from the reactions in (d)(i) both react with water. Write equations for these two reactions and describe the effect of the resulting solutions on Universal Indicator solution. [2] [Total: 20]

4 2016 TJC H2 Chemistry Preliminary Exam [Turn Over 2 Thallium is a Group III element discovered in 1861, and its toxicity was quickly noted. Thallium isotopes and compounds have useful applications, sometimes despite their toxic nature. (a) Thallium forms compounds similar to aluminium, and thallium(III) chloride and thallium(III) fluoride have very different structures and bonding. (i) Draw and state the shape of thallium(III) chloride, TlCl3. [1] (ii) In gaseous phase, thallium(III) chloride is able to form a compound with a Mr of 621. Draw a dot-and-cross diagram of this compound, and state the Cl-Tl-Cl bond angle. [2] (iii) Explain, in terms of structure and bonding, why thallium(III) fluoride has a high melting point of 550 oC. [2] (iv) Thallium is also able to form thallium(I) chloride, which is crystalline in nature. In the crystal lattice of sodium and potassium chlorides, the co -ordination number of each ion is 6. In the crystal lattice of thallium(I) chloride, TlCl, the co-ordination number has a different value. Suggest an explanation for the co -ordination number in the TlCl lattice being different from those in NaCl and KCl. [1]

5 2016 TJC H2 Chemistry Preliminary Exam [Turn Over (b) Thallium(III) chloride, TlCl3, reacts with hot water to produce thallium(III) oxide precipitate and hydrochloric acid. (i) Write a balanced equation for the above reaction. [1] (ii) In an experiment, a 15 g impure sample of thallium(III) chloride (M r = 310.5) was mixed with hot water, and filtered. Water was then added to the filtrate to make a standard solution in a 250 cm 3 volumetric flask. A 25.0 cm 3 aliquot was titrated against 0.5 mol dm -3 aqueous sodium hydroxide, requi ring a titre of 22.00 cm 3. Determine the percentage purity of the sample. You may assume that the impurities are insoluble in water and are inert. [2] (c) Thallium(I) sulfate (Mr = 504.1) was often employed as a rodenticide and is highly poisonous. The lethal dosage is 16 ppm of the rat’s mass. Assuming a typical rat has