NYJC H1 Chem Prelim 2012 P2 Answers

H1 Chemistry 8872/02 NYJC J2/12 PX [Turn over NANYANG JUNIOR COLLEGE JC 2 Preliminary Examinations Higher 1 CANDIDATE NAME Answers CLASS TUTOR’S NAME CHEMISTRY 8872/02 Paper 2 10 September 2012 2 hours Candidates answer on the Question Paper. Additional Materials: 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. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Section A, Answer all the questions. Section B, Answer two questions on separate answer paper. 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. Section A 1 12 2 4 3 5 4 6 5 13 Total 40 This paper consists of 13 printed pages and 0 blank page

H1 Chemistry 8872/02 NYJC J2/12 PX Section A Answer all the questions. Write your answers on the spaces provided. 1 Nitrides are a large class of compounds with a wide range of properties and applications. They are often used as refractory materials. Two examples are magnesium nitride, Mg3N2 and boron nitride, BN. (a) Draw a dot-and-cross diagram to show the bonding in magnesium nitride. [2] Mg N 2+ 3- 23 (b) Suggest the electrical conductivity of magnesium nitride in liquid state. [2] In liquid state, there is presence of free mobile ions of Mg2+ and N3- to carry charges. Hence Mg3N2 can conduct electricity. (c) Boron nitride is a good lubricant like graphite. Explain, in terms of structure and bonding, this property of boron nitride. [2] Boron nitride has giant covalent/molecular structure consisting of B and N atoms held by strong covalent bonds. In between the layers, they are held by weak dispersion/van der Waals’ forces. (d) Suggest one reason why magnesium gives the nitride, Mg 3N2, in addition to its oxide when burned in air. Construct a balanced equation for the combustio n of magnesium in air. [2] Magnesium burns in air to produce an oxide layer. The unreacted magnesium below the oxide layer could therefore only reacts with nitrogen. 4Mg + 1 2 O2 + N2 → MgO + Mg3N2

3 H1 Chemistry 8872/02 NYJC J2/12 PX [Turn over (e) A 2.00 g sample of the powder obtained from burning magnesium in air was boiled with water. The ammonia that was evolved neutralised 12.0 cm 3 of 0.500 mol dm−3 hydrochloric acid. (i) Construct a balanced equation for the reaction with water. [1] MgO is only slightly soluble in water. Mg3N2 + 6H2O → 3Mg(OH)2 + 2NH3 (ii) Calculate the percentage of magnesium nitride in the 2.00 g sample. [3] Mg3N2 + 6H2O → 3Mg(OH)2 + 2NH3 n(NH3) produced = n(HCl) required = 3312.00.5 x = 6.000 x 10 mol dm1000 −− n(Mg3N2) present in sample = 1 2 n(NH3) = 3.000 x 10−3 mol dm−3 m(Mg3N2) = 3.000 x 10−3 x 100.9 = 0.3027 g % by mass of Mg3N2 in sample = 0.3027/2.00 x 100% = 15.1% [Total:12]

4 H1 Chemistry 8872/02 NYJC J2/12 PX 0 2 The formation of magnesium oxide from its elements may be represented by a Born-Haber cycle as shown below. Mg2+(g) + O2−(g) H3 +702 Mg (g) + O(g) − 3889 + 150 H2 Mg (s) + ½ O2(g) H1 MgO (s) (a) Name the enthalpy changes represented by H1 is the enthalpy change of formation of MgO. H2 is half the bond energy of O2. (b) Using values from the Data Booklet and the energy values given in the above cycle, calculate the value of H1. H1 = +150 + 2186 + 248 + 702 + (−3889) = −603 kJ mol−1 [2] [Total:4] Energy / kJ mol−1

5 H1 Chemistry 8872/02 NYJC J2/12 PX [Turn over 3 Consider the following reaction where colourless bromide ions react with hydrogen peroxide to form a reddish-brown bromine solution. 2Br− + H2O2 + 2H+ Br2 + 2H2O H < 0 (a) Explain what is meant by the term dynamic equilibrium. [2] A reversible reaction is in dynamic equilibrium when • the rates of forward and backward reactions are equal, and • the concentrations of reactants and products remain constant. (b) Predict and explain the effect on the position of equilibrium when (i) a catalyst is added, [1] • A catalyst does not affect the position of equilibrium because • it increases the rate of forward and backward reactions by the same extent. (ii) temperature is increased. [2] • By Le Chatelier’s Principle, the position of equilibrium will shift to the left • as the endothermic reaction is favoured more than the exothermic reaction to remove some of the added heat [Total: 5]

6 H1 Chemistry 8872/02 NYJC J2/12 PX 4 Cinnamaldehyde is used in fragrances for its jasmine-like odour. CH CHCHO cinnamaldehyde (a) Name the two functional groups, other than the phenyl group, that are present in cinnamaldehyde. ....alkene............................................ .....aldehyde/alkanal..................... [2] (b) Cinnamaldehyde can exhibit geometric isomerism. Draw and label the structure of the geometric isomers and explain how it arises. C C H H CHO C C H H CHO Cis trans Restricted rotation around C=C bond [3] (c) CH CH2CHO is a structural isomer of cinnamaldehyde. Describe a simple chemical test you could carry out to distinguish the two isomers. Reagents and conditions Fehlings solution warm Observations Brick red ppt forms for cinnamaldehyde. Product CH CH2CO2 - (d) Cinnamaldehyde reacts with hydrogen gas in the presence of nickel catalyst to give a saturated compound A. (i) State the type of reaction taking place. Reduction OR Addition

7 H1 Chemistry 8872/02 NYJC J2/12 PX [Turn over (ii) Draw the structural formula of compound A. C H C H CH2OH H H [3] (e) Draw the structural formula of each of the organic products formed when cinnamaldehyde is treated with the following reagents: (i) acidified potassium dichromate(VI) when heated under reflux, C C H H COOH (ii) 2,4-dinitrophenylhydrazine. C C H H CH N N H NO2 O2N [2] [Total: 13]

8 H1 Chemistry 8872/02 NYJC J2/12 PX 5 Use of the Data Booklet is relevant to this question. Part of the Periodic Table is shown below. Li Be B C N O F Ne Na Mg Al Si P S Cl Ar (a) From the elements shown in the table, identify the one which exhibits each of the following property. Write your answers in the table. [4] (i) It has the highest first ionisation energy. (ii) It has the largest ionic radius. (iii) It has an electronegativity similar to that of aluminium. (iv) It has a hydride that forms the strongest intermolecular hydrogen bonds. (v) It has a trifluoride with molecules of trigonal shape. (vi) It has a chloride that neither reacts with nor dissolves in water. (vii) It has an ox ide with a giant structure and a chloride which is readily h ydrolysed in water. (viii) It has an oxide that produces a strong acid when treated with water. (i) Ne (ii) P (iii) Be (iv) F (v) B / P / N (vi) C (vii) Al / Si (viii) P / S (b) Write equations for the reactions in (vii) and (viii). [2] AlCl3 + 6H2O → [Al(H2O)6]3+ + 3Cl- [Al(H2O)6]3+ + H2O [Al(H2O)5OH]2+ + H3O+ SiCl4(l) + 4H2O(l) → Si(OH)4(s) + 4HCl(l) Either one P4O10(s) + 6H2O(l) → 4H3PO4(aq) SO2(g) + H2O(l) → H2SO3(aq) SO3(g) + H2O(l) → H2SO4(aq) [Total: 6] Group I II III IV V VI VII 0 Period 2 Period 3 Period 3

9 H1 Chemis