NJC 2008 H2 CM Prelim P3

NATIONAL JUNIOR COLLEGE 2008 PRELIMINARY EXAMINATIONS C H E M I S T R Y 9 7 4 6 / 0 3 Higher 2 2 1 A u g u s t 2 0 0 8 Paper 3 Free Response 2 hours READ THESE INSTRUCTIONS FIRST Write your Name, Registration Number and Chemistry group on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use correction fluid. Answer each question on a fresh sheet of paper. 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 question. This question paper consists of 11 printed pages (including this page)

Answer any four questions. 1 (a) (i) What do you understand by the term enthalpy change of atomisation of magnesium? (ii) Use the following data together with relevant data from the data booklet, construct an energy level diagram to calculate the enthalpy change of atomisation of magnesium. ΔH / kJ mol −1 Lattice energy of magnesium chloride −2500 1st electron affinity for chlorine −349 Standard enthalpy change of formation of magnesium chloride −652 (iii) Suggest, with reason, how the magnitude of the enthalpy change of atomization of magnesium might compare with that of chromium. [7] (b) (i) Explain why a solution of chromium(III) chloride is green while aqueous magnesium chloride is colourless. [4] (ii) When aqueous sodium carbonate is added to aqueous chromium(III) chloride, effervescence was observed. Explain the observation with equations where necessary. [2] (c) Grignard reagent, RMgCl, is useful in organic synthesis as it enables the combination of two organic molecules into a bigger one by forming a carbon- carbon bond. It is prepared by the reaction of magnesium with a halogenoalkane. One of the most important uses of the Grignard reagent is its reaction with carbonyl compounds to form alcohols as shown below. RM g C l +C O R' H C R' R H OH + Mg(OH)ClH2O (i) Given the carbon-magnesium bond in Grignard reagent is highly polar, state the type of reaction between R –MgCl and the aldehyde. [1]

(ii) R-MgCl reacts with propanal to give compound A. Compound A gives a yellow precipitate with I2 in aqueous NaOH. When compound A reacts with excess concentrated H 2SO4 at 170 oC, compounds B and C are formed. Compound B upon reaction with hot acidified potassium manganate(VII), produces compound D and effervescence is observed . Compound C exists as a pair of geometric isomers. Deduce the structure of compounds A - D. [6] [ T o t a l : 2 0 ]

2 Adrenaline is a naturally occurring hormone and stimulant released by the nervous system to prepare the body for immediate physical action during the time of danger or stress. After the discovery of the action of adrenaline, a series of stimulant drugs which are structurally and therapeutically similar to adrenaline were developed by pharmaceutical chemists. One such stimulant is amphetamine as shown below. HO HO CH2 OH CH2 HN CH3 CH2 C CH3 HN H 2 Adrenaline Amphetamine (a) Even though adrenaline and amphetamine are structurally similar, both of them have different solubility in water. State which compound is more soluble in water, giving your reasons. [2] (b) Amphetamine is prepared commercially from compound X via a series of reactions as shown. C H 2 CH CH CHC H 3 CH 2 Amphetamine (i) What types of reaction are reactions I, II and III? (ii) Suggest reagents and conditions for reaction III. (iii) Suggest a chemical test to distinguish between compound X and amphetamine. [6] Starting Compound C O C 3 HO N 2 3 H2 C NH C H2 C CH3 H NH2 NH 3 I II Compound X I II

(c) Amphetamine is able to undergo further reactions to form other compounds. Draw the structural formulae of the organic compounds formed when amphetamine is reacted with (i) excess CH 3Cl (ii) acid anhydride (an acid derivative), H3C C O C CH3 O O . [4] (d) Compound E, an isomer of amphetamine, forms a white precipitate when aqueous bromine is added to it. Suggest a structure for Compound E. [2] (e) In order to determine the base strength of amphetamine, 50 cm 3 of 1.0 mol dm –3 of hydrochloric acid was added to 55 cm 3 of 1.0 mol dm –3 of amphetamine in a polystyrene cup. The changes in temperature were recorded and plotted in Figure 1. 28 29 30 31 32 33 34 35 36 37 38 02468 Time/ minutes Temperature/ oC 10 Figure 1 (i) State one precaution you should observe when carrying out the experiment. [1] (ii) By means of appropriate calculations, deduce the base strength of amphetamine. (You may assume the density of the solution is 1.00g cm –3 and its specific heat capacity is 4.18 J g–1 K–1.) [5] [Total: 20]

3 Benzene diazonium salts, such as C 6H5N2 +Cl−, are important intermediates in the synthesis of dyes. (a) An experiment was conducted to study the reaction of 0.00346 mol of benzenediazonium chloride, F, with water at 25o C and 101 kPa. C6H5N2 +Cl − (aq) + H2O(l) → C6H5OH(aq) + N2(g) + HCl(aq) benzenediazonium chloride (F) phenol The progress of the reaction was followed by measuring the volume of nitrogen gas, Vt, produced at intervals of time after the start of the reaction. The mixture was left until no further reaction appeared to be occurring and the total volume of gas produced, V ∞ , was measured. The table below shows some of the experimental results. Time / s 0 30 60 90 150 210 Vt / cm3 0 15.7 27.7 37.7 52.7 63.0 (i) What is the volume of gas, V ∞ , collected when the reaction is complete? (ii) By using a graphical method, determine the order of reaction and hence its rate constant. [6] (b) What would be the qualitative effect on the rate constant of changing: (i) the concentration of benzenediazonium chloride (ii) the temperature at which the measurements were made? [2] (c) When the rate of this reaction was measured, at 20o C, with a concentration of F (benzenediazonium chloride) of 0.06 mol dm−3, in the presence of nitric acid, the results at a series of values for pH were as shown below: What can be deduced from these results? [3] pH Rate x10−5 / appropriate units 2.15 44.6 2.45 22.6 2.57 16.9

(d) Sunset Yellow, a dye, made from a benzenediazonium salt is used as a food colourant in sweets, orange squash and jams. It may be synthesized by the following reactions: Sunset Yellow II NaOH suitable conditions I HO 3S NH 2 Na O3S Na O3S HO N N Na O