MJC_2008 H2 Chem Prelim Paper 3 (Question Paper)

Class Reg Number Candidate Name ___________________________________ Meridian Junior College JC2 Prelim Examination 2008 H2 Chemistry 9746 10 September 2008 2 hours Paper 3 Free Response Additional Materials Data Booklet Writing Paper Graph Paper INSTRUCTION TO CANDIDATES Write your name, class and regi ster number in the spaces pr ovided at the top of this page. Answer any 4 out of 5 questions. Fasten the writing papers behind the given Cover Page for Questions 1 & 2 and Cover Page for Questions 3, 4 & 5 respectively. Hand in Questions 1 & 2 and 3, 4 & 5 separately . You are advised to spend about 30 min per question. INFORMATION FOR CANDIDATES The number of marks is given in brackets [ ] at the end of each question or part question. You are reminded of the need for good Englis h and clear presentation in your answers. This question paper consists of 14 pages. 1

Answer any 4 out of 5 questions. Begin each question on a fresh page of writing paper. 1(a) Hydrazine, N 2H4, has an ammonia-like odor, and is derived from the same industrial chemistry processes that ma nufacture ammonia. In some cases, hydrazine behaves like ammonia in chemic al reactions. For example, hydrazine is able to form a product when it reacts with boron trifloride, BF 3, in a molar ratio of 1:1. (i) Draw a ‘dot-and-cross diagram’ to s how the electronic structure of a molecule of N2H4. Use the electron repulsion t heory to pedict the shape of this molecule and state its bond angle. [2] (ii) Explain why h ydrazine form a product with BF3 when they react in a molar ratio of 1:1. [1] (iii) Draw a diagram to show the likely shape of the product formed. [1] (b) Hydrazine is used as rocket fuel and to prepare gas precursors used in air bags. Approximately 260 thousand tonnes of hydrazine are manufactured annually. Liquid hydrazine undergoes combustion according to the following equation: N 2H4(l) + O2(g) → N2(g) + 2H2O(l) A chemist conducted an experiment to det ermine the standard enthalpy change of combustion of hydrazine. In the ex periment, 0.210g of hydrazine was burnt as fuel to heat up a beaker containing 200 cm 3 of water. The te mperature of water rose by 4 oC. You may assume the process has 80 % efficiency. (i) Explain what is meant by standard enthalpy change of combustion of hydrazine. [1] (ii) Calculate the standard enthalpy change of combustion of hydrazine. [2] (iii) Given the following data: Enthalpy change of formation of steam = - 242 kJ mol-1 Enthalpy change of vapourisation of water = + 44 kJ mol-1 and using the value y ou have calculated in b(ii), draw an appropriate energy cycle to determine the standard enthalpy of formation of hydrazine. [3] 2

(c) The standard enthalpy change of formation