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 of hydrazine gas is +235 kJ mol-1. (i) U sing appropriate data from the Data Booklet , draw an energy level diagram to calculate the average bond energy of N-H bond in hydrazine. [3] (ii) Suggest a reason for the difference in the N-H bond energy value obtained from (c)(i) with the value given in the Data Booklet. [1] (d)(i) When sodium thiosulphate is react ed separately with bromine and iodine, different products are formed. Using t he following data, and data from the Data Booklet, describe and explain the difference. S 4O6 2- + 2e ƒ 2S2O3 2- E θ = +0.09V 4SO2 + 4H+ + 6e ƒ S4O6 2- + 2H2O E θ = +0.51V SO4 2- + 4H+ + 2e ƒ SO2 + 2H2O E θ = +0.17V [3] (ii) Describe and explain why the trend of boiling points of hydrogen halides differs from the trend of thermal stability of hydrogen halides down Group VII. [3] [Total : 20] 3

2 An example of a gas-phase reaction is the decomposition of nitryl chloride NO2Cl. At 700K, NO2Cl decomposes according to the following equation. 2 NO2Cl (g) → 2 NO2 (g) + Cl2 (g) The initial pressure of NO 2Cl is 0.0524 atm. The rate of the reaction is followed by measuring the partial pressure of Cl 2 as it changes with time, t. In such as an experiment, the partial pressure of Cl2 increased as follows. Time, t /s 0 300 900 1500 2000 3000 Partial pressure of Cl2 / x 102 atm 0 0.48 1.18 1.68 1.94 2.30 (a) (i) When the reaction is complete, the partial pressure of Cl 2 is 0.0262 atm. Plot a graph to show how the partial pressure of Cl 2 changes during the first 3000s of the reaction and determine the order of reaction with respect to NO2Cl. Hence, write a rate equation for the decomposition of NO2Cl. [4] (ii) Calculate the rate constant for the above reaction, stating its units. [1] (iii) At a certain temperatur e, the decomposition of NO2Cl follows a two-step mechanism as shown: Step 1 : NO 2Cl → NO2 + Cl Step 2 : NO 2Cl + Cl → NO2 + Cl2 The enthalpy change for the over all process is -15 kJ mol -1. For the first step, the activation energy for the forward reaction is 35 kJ mol -1 and that of a reverse reaction is 25 kJ mol -1. The activation energy for the reverse reaction of the second step is 35 kJ mol -1. Draw a labelled energy profile diagram based on the above given data. [2] (iv) The rate of decomposition of NO 2Cl is found to increase when temperature is raised. With the aid of a sketch of the Maxwell-Boltzmann distribution curve, explain how a small increase in temperature can lead to a large increase in the rate of decomposition. [3] 4

(b) C4H8 can exist in 4 non-cyclic isomers forms, W, X, Y and Z. W has the structure as shown: CC H H H CH2CH3 W reacts with chlorine under two different conditions. Reaction I: In the absence of light, W reacts with Cl 2 dissolved in CC l4 to give an equimolar mixture of two isomers A and B. Reaction II: W reacts with Cl 2 gas under ultraviolet light to give H 2C=CH(CH2CH2Cl). However, the presence of trace amount of nitrogen m onoxide may decrease the rate of reaction. (i) Draw the isomeric structures of X, Y and Z and identify the types of isomerism present. [3] (ii) Draw the displayed formulae of isomers A and B in reaction I. [2] (iii) Explain why the following step rarely occurs in reaction II. H 2C=CH(CH2CH3) + Cl • → H2C=CH(CH2CH2Cl) + H • [1] (iv) Suggest why the presence of nitr ogen monoxide will slow down reaction II. [1] (c) Chlorine forms many compound s with different oxidation states. With the aid of balanced equations, explain how different chlorine-containing products are obtained when chlorine reacts with sodium hydroxide under different conditions. [3] [Total : 20] 5

3(a) Alloys are mixtures of two or more metals. Transition elements are usually alloyed with one another to enhance their pr operties. For instance, titanium can be alloyed with iron and copper to form a strong, light-weight and corrosion resistant material for aerospace and automobile. (i) Explain what is meant by the term transition element. [1] (ii) When 50g of the titanium-iron alloy is completely oxidized, 35.3g of FeO and a violet oxide of titanium, compound A, is obtained. The mass of oxygen measured in compound A is found to be 11.3g. Determine the molecular formula of this oxide. Hence calculate the oxidation number of titanium. [2] (iii) On strong heating, compound A undergoes disproportionation to form two oxides. One of the oxide is a yellow solid and the other oxide is a white solid, compound B. Write a balanced equation fo r the disproportionation r e a c t i o n . [1] (iv) State the full electronic conf iguration of Ti in compound B. Hence, explain why