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.