CJC H2 CHEM P3 Prelim

CATHOLIC JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS Higher 2 CANDIDATE NAME CLASS 2T CHEMISTRY 9647/03 Paper 3 Free Response THURSDAY 25 AUGUST 2011 2 Hours Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. Write in dark blue or black pen on both sides of the paper. [PILOT FRIXION ERASABLE PENS ARE NOT ALLOWED] You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. A Data Booklet is provided You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part of the question. At the end of the examination, fasten all work securely together. This document consists of 11 printed pages and 1 blank page. [Turn over] For Examiner’s Use Paper 3 Q 1 20 Q 2 20 Q 3 20 Q 4 20 Q 5 20 Total 80

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3 9647/03/CJC JC2 Prelim Exam 2011 Answer any four questions. 1 Nanochemistry is an important field of science that explores the use of extremely small particles that have diameters about thousand time s thinner than the width of a human hair. Transition metals and its compounds are often the subject of interest in nanochemistry, as many useful properties are improved at this ultra small scale. (a) Nano-sized iron( III) hydroxide, Fe(OH) 3 exists as a precipitate and can be used as a catalyst to remove aqueous arsenate( V) ions, AsO 4 3- from contaminated water. The Fe(OH)3 with the adsorbed arsenate( V) ions are filtered off, and undergoes an ion- exchange with the arsenate( V) ions removed and Fe(OH) 3 precipitate recovered unchanged. (i) State the electronic configuration of iron in Fe(OH) 3. (ii) State the type of catalysis and briefly explain why transition metal compounds are suitable for such catalysis. (iii) Suggest why the use of nano-sized particles will improve this catalytic property. [4] (b) Nano-sized iron(II) hydroxide exists as a complex solid with the formula Fe(H 2O)4(OH)2 and has no net dipole moment. (i) State the colour and coordination number of the complex iron( II) hydroxide. (ii) Suggest the structure of the complex solid, given that it is a non-polar species. [4] (c) Nano-sized iron(II) hydroxide, Fe(OH)2 is often unstable, due to oxidation by water even though oxygen is not present. The pH of the solution remains largely unchanged, a reddish-brown precipitate and effervescence are observed. (i) Identify the reddish-brown precipitate and suggest an equation for this reaction. (ii) The reaction is not expected to occur under standard conditions and can be considered reversible. Suggest a possible reason why it does in fact occur for an opened bottle of Fe(OH)2 suspended in water. [3] (d) Nano-sized iron( II) hydroxide, Fe(OH) 2 is used for the purpose of removal of toxic selenite(IV) ions, SeO3 2- from water, by reducing selenite(IV) ions to selenium, Se. (i) Construct a half-equation for the reduction of SeO 3 2- to Se in an alkaline solution. (ii) In water that is heavily contaminated with CN - ions, iron( II) hydroxide appears to dissolve. State the type of reaction that is occurring and construct a balanced chemical equation for it. (iii) By selecting E o values from the Data Booklet, and given that the redox potential of the reaction in (i) is – 0.37 V, explain and justify if SeO 3 2- can be removed from water contaminated with CN- ions. [5]

4 9647/03/CJC JC2 Prelim Exam 2011 (e) Electrochemistry is also widely used in the synthesis of nano-sized materials. For instance, anodising of aluminium can be conducted to produce extremely small aluminium oxide. Describe with the aid of a labelled diagram, the set-up used during anodising of aluminium, and the reactions occurring at each electrode. [4] [Total: 20] 2 (a) Vitamin C, C6H8O6, is also known as ascorbic acid. It is a monobasic acid and has a pKa of 4.10. When a 500 mg tablet of Vitamin C is swallowed, it dissolves in the stomach before being absorbed into the bloodstream. The stomach may be assumed to contain 1.0 dm3 of 0.10 mol dm-3 hydrochloric acid (i) Estimate the percentage of Vitamin C that is ionised in the stomach under these conditions. State the assumptions made during the calculation. (ii) The structure of Vitamin C is State which hydrogen atom in the molecule is likely to be acidic, and justify your choice. (iii) Blood has a pH of 7.35 and is saturated with carbon dioxide of concentration 3.2 x 10 -3 mol dm-3. The equilibrium reaction, H2O + CO2 ⇌ H+ + HCO3 - ; Ka = 4.5 x 10-7 mol dm-3 acts as a buffer. (I) Explain clearly how this buffering action minimizes the change in pH on the addition of either acid or alkali. (II) Calculate the concentration of HCO3 - ions in blood. (III) Explain qualitatively how the extent of ionisation of Vitamin C changes as it moves from the stomach to the bloodstream. [8]

5 9647/03/CJC JC2 Prelim Exam 2011 (b) The acid strength of the hydrogen halides in aqueous solution may be related to the enthalpy change for the process, HX(aq) → H+(aq) + X-(aq) where X is a halogen. (i) By constructing a suitable energy cycle and using the data in the following table, calculate the enthalpy change of this process for hydrofluoric acid. Reaction ∆Ho / kJ mol-1 X = F X = C l HX(aq) → HX(g) +48 +18 HX(g) → H(g) + X(g) +568 +432 H(g) → H+(g) + e- +1312 +1312 X(g) + e- → X-(g) -328 -349 H+(g) → H+(aq) -1091 -1091 X-(g) → X-(aq) -515 -381 (ii) Hence calculate the enthalpy change of this process for hydrochloric acid. (iii) Comment on your answers in relation to the strength of hydrofluoric acid (Ka = 6.7 x 10-4 mol dm-3) and hydrochloric acid. [4]

6 9647/03/CJC JC2 Prelim Exam 2011 (c) Each Group II element forms an iodate( V) with the formula M( IO3)2. The iodates( V) have many uses, for example, calcium iodate is an oxidant added to lotions and ointments as an antiseptic and deodorant. (i) Draw the structural formula of the iodate(V) ion, indicating the likely bond angle. (ii) Draw a dot-and-cross diagram corresponding to this structure. (iii) The iodates(V) readily decompose on heating giving oxygen gas as one of the two products formed. (I) Write a likely equation for the decomposition of M(IO3)2. (II) Predict and explain how the thermal decomposition of these iodates( V) varies down the group. The decomposition of the iodates( V) is similar to the decomposition of the nitrates(V). (iv) The percentage purity of a sample of barium iodate( V), Ba( IO3)2 can be determined by the following method. 0.105 g of Ba( IO3)2 (Mr = 487) is added to excess aqueous potassium iodide, K I in acidic medium. The iodine liberated is then titrated against aqueous sodium thiosulfate pentahydrate, Na2S2O3.5H2O (Mr = 248.2) of concentration, 24.8 g dm-3. 25.00 cm 3 of the thiosulfate is used for the titration. By using the reactions given below, and the information described above, calculate the percentage purity of the sample of barium iodate(V). [In the presence of acid and the iodide ions, the following reaction takes place quantitatively: IO3 - + 5 I- + 6 H + → 3 I2 + 3 H 2O. The reaction between iodine and thiosulfate is given by the equation: I2 + 2 S2O3 2- → 2 I- + 2 S4O6 2-]. [8] [Total: 20]

7 9647/03/CJC JC2 Prelim Exam 2011 3 Ethanal occurs commonly in coffee and fruits. In humans, ethanal is formed by oxidation of ethanol consumed from alcoholic drin