SRJC 2008 JC2 Prelims H1 Paper 2 Solutions

1 SERANGOON JUNIOR COLLEGE General Certificate of Education Advanced Level Higher 1 CHEMISTRY 8872/02 Preliminary Examination Paper 2 SOLUTIONS 1 Iron tablets containing iron ( II) sulphate can be used to supplement the daily iron intake in diet. The Health Sciences Authority had r egulated that all commercially available iron tablets must contain 15 to 18% by mass of iron. (a) Acidified potassium manganate (VII), KMnO 4, which oxidises iron (II) sulphate, is used to estimate the amount of iron (II) ions, Fe 2+, present in Brand A iron tablet. 2.00 g of the iron tablet was dissolved in dilute acid and the volume of the solution was made up to 100 cm3 in a volumetric flask. A 10.0 cm 3 portion of this solution was titrated against 2.50 x 10 -4 mol dm -3 of potassium manganate (VII) solution. 13.50 cm 3 of the manganate (VII) solution was required to completely oxidise the Fe2+ present. (i) Write the overall equation of the reaction between MnO4 - and Fe2+. Oxidation: Fe2+ Fe3+ + e Reduction: MnO4 - + 8H+ + 5e Mn2+ + 4H2O 5Fe 2+(aq) + MnO4 -(aq) + 8H+(aq) Æ 5Fe3+(aq)+ Mn2+(aq) + 4H2O (l) (ii) Calculate the concentration of Fe2+ in the 100 cm3 solution. amount of MnO4 - in 13.50 cm3 = 410 x 5 . 2 x1000 5 . 13 − = 3.370 x 10-6 mol from (3) mole ratio: 5Fe2+ ≡ MnO4 - amount of Fe2+ in 10 cm3 = 3.37 x 10-6 x 5 = 1.6875 x 10-5 mol amount of Fe 2+ in 100 cm3 = 1.6875 x 10-5 x 10 = 1.6875 x 10-4 mol concentration of Fe 2+ in 100 cm3 solution = 1.6875 x 10-4 / 0.1 = 1.69 x 10 -3 mol dm-3 (iii) Hence, state with reasoning if the Brand A iron tablet meet s the regulation stated by the Health Sciences Authority. Mass of Fe 2+ present in 2g of tablet A = 1.6875 x 10-4 x 55.8 = 9.4163 x 10 -3 g Percentage of Fe present in tablet A = (9.4163 x 10 -3 / 2) x 100 % = 0.471 % No , the iron tablet does no meet the requirement of HSA as its iron content is not within the range 15 to 18% by mass.

2 (b) Write the electronic configurations of Fe and Fe2+ ion. Fe atom: 1s22s22p63s23p63d64s2 F e 2+ ion: 1s22s22p63s23p63d6 (c) On the diagram below, show and la bel clearly how the beams of the following particles are deflected when subjected to an electric field. (i) e l e c t r o n (ii) proton (iii) Fe atom You should relate clearly the magnitude and the direction of deflection of each beam to the other. (Any pencil markings will NOT be graded.) Fe atom + - Electron / e Proton / p [Total: 8 m]

3 CH CH 2 2 Styrene is an aromatic hydr ocarbon that is a precursor to polystyrene, an important synthetic material that is used in the maki ng of many substances such as plastic, rubber, insulation and fiberglass. The diagram below shows an energy cycle involving the compound styrene. CH2CH3 8 CO2 5 H2O+ + H2 (g) styrene 1HΔ 2HΔ ) (2 21 2 g O+) (2 21 2 g O+ (a) (i) Use the following data to calculate the standard enthalpy change of hydrogenation of styrene. o CHΔ hydrogen = –286 kJ mol -1 o CHΔ styrene = –4393 kJ mol -1 o CHΔ ethylbenzene = –4562 kJ mol -1 1HΔ = o CHΔ (styrene) + o CHΔ (H2) – o CHΔ (ethylbenzene) = (–4393) + (–286) – (–4562) = –117 kJ mol-1 (ii) Calculate the amount of heat evol ved when 1.000 g of styrene was burnt completely to heat up 200 cm 3 of water by 45.0 °C. Assume that the specific heat capacities of all solutions are 4.18 J g -1 K-1, and that all solutions have a density of 1.0 g cm-3. Heat evolved, Q = m c ΔT = (200) (4.18) (45) = 37600 J (accept 37620) (iii) Using the answer in (ii) , calculate the enthalpy change of combustion of styrene. (l)(g) (l) (l) ) (2 21 2 g O+ ) (2 21 2 g O+2HΔ 1HΔ 43 ΔHΔH + 43 ΔHΔH + o CHΔ = – 1041 37620 = –3910 kJ mol-1 (accept -3912)

4 (iv) Comment on the difference in values of standard enthalpy change of combustion of styrene in (i) and (iii) Heat has been lost to the surroundings/ poor insulation. CH CH 2 CHClCH3 + HCl (g) styrene (b) Give the following information, o fHΔ styrene = +104 kJ mol -1 o fHΔ hydrogen chloride = –92 kJ mol -1 o fHΔ (2-chloroethyl)benzene = –58 kJ mol -1 (i) calculate the enthalpy change for the following reaction. rxnHΔ = o fHΔ (C8H9Cl) o fHΔ −[ (C8H8) + o fHΔ (HCl)] = - 58 - [104 + (-92)] = - 70 kJ mol-1 (ii) The reaction above can be performed when ethylbenzene is treated with a small quantity of chlorine in the presence of ultraviolet light. Both (1-chloroethyl)benzene and (2-chl oroethyl)benzene are produced. Predict the approximate ratio in which they are formed. (1-chloroethyl)benzene : (2-chloroethyl)benzene = 2 : 3

5 (iii) A student has accidentally poured some hot acidified potassium manganate (VII) into a bottle of styrene, forming benzoic acid. In order to salvage the styrene, he decided to adopt the following synthetic route. CO2H CH2OH CH2Cl CH2CN CH2CO2HCH2CH2OHCH CH 2 Suggest the correct reagents and conditions for the synthetic steps 1 to 5. 1 2 3 4 5 For (1) Reagents: LiA lH4, Condition: dry ether, room temperature For (2) Reagents: PCl 5 / SOCl2, PCl3 Condition: room temperature For (3) Reagents: KCN, NaCN Condition: ethanol, reflux/heat For (4) Reagents: H 2SO4 (aqueous) Condition: heat For (5) Reagents: concentrated H 2SO4 Condition: 170 0C (iv) Complete the following table below: Number of carbon atom(s) Compound sp sp 2 sp 3 CH CH 2 0 8 0 CH2CH3 0 6 2 [Total: 12 m]

6 3(a) Draw the dot-n-c ross diagram of BH3NH3 and state the type of bond between boron and nitrogen. N B N B N B HH Cl H H H HC l H H H Cl B N HH H HH H dative bond [0.5] [0.5] (b) By referring to the intermolecular intera ction involved, explain the difference in boiling points between ethane and its amino-borane analogue. • Ethane molecules are non-polar molecule s, which are held together by weak induced dipole-dipole interaction. • NH 3BH3 molecules are polar, which are held together by stronger permanent dipole-dipole interaction. • More energy is needed to overcome the stronger pd-pd interaction, the BH3NH3 has higher boiling point. (c) Write an equation to illustrate the acidic nature of ammonia carboxyborane NBH H H C H H OH (aq) O NBH H H C H H O-(aq) + H+(aq) O (d) Suggest a name of the carbon analogue of ammonia carboxyborane. [1] Propanoic acid (e) Construct a balanced equation for th e reaction between borazine and hydrogen chloride. Calculate the volume of HC l (measured at room temperature and pressure) needed to react completely with 16.08g of borazine. N B N B N B H H H H H H + 3HCl N B N B N B H ClH H H Cl HH Cl H H H Amount of borazine = ) 0 . 1 60 . 14 3 8 . 10 3 (+ × + × OR 08 . 16 × = 0.200 mol 1 mol borazine ≡ 3 mol HCl Amount of HCl needed = 0.600 mol Volume of HCl at rtp = 0.600 × 24.0 = 14.4 dm-3

7 (f) Suggest the reagents for the synthesis of B-trimethyl-N-trichloroborazine. Draw the displayed formula of this product. B(CH3)3 and ClNH3Cl N B N B N B Cl Cl CH3 CH3 CH3 Cl [Total: 10m] 4 (a)(i) Explain, using a relevant equation, how the H 2CO3/HCO3 - buffer responds to prevent acidosis from occurring. When pH drops, there is additional H3O+: Eqn: HCO3 – (aq) + H+ (aq) → H2CO3 (aq) The added