NYJC H1 Chem Prelim 08 P2

NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAM Higher 1 CANDIDATE NAME CLASS TUTOR’S NAME CHEMISTRY 8872/02 Paper 2 10 Sep 2008 Candidates answer Section A on the Question Paper. 2 hours Candidates answer Section A on the Question Paper Additional Materials: Answer Paper 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. For Examiner’s Use A1 A2 A3 A4 B1 B2 B3 Section A Answer all 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. Total This document consists of 15 printed pages. [Turn over

2 For examiner’s use only Section A Answer all questions in this section in the spaces provided. 1 The rate of hydrolysis of a chloroalkane, RC l with aqueous sodium hydroxide depends on the concentration of the r eactants at a fixed temperature T1. The values for the initial rates of hydrolysis at various initial concentrations have been determined. These are shown in the table below. Expt Initial conc. of NaOH / mol dm-3 Initial conc. of RCl / mol dm-3 Initial rate / mol dm-3 s-1 1 0.0200 0.0150 4.0 x 10-4 2 0.0200 0.0225 6.0 x 10-4 3 0.0300 0.0225 9.0 x 10-4 4 0.0600 0.0300 x (a) Deduce the orders of reaction with respect to each of the reactants. Hence, write the rate equation for the hydrolysis of the chloroalkane. [3] (b) Based on your answer above, calculate the magnitude of x. [1]

3 For examiner’s use only (c) Describe and explain, with an appropriate diagram, how t he rate of this reaction is affected when the experiment is repeated at a higher temperature, T2. [3] [Total: 7 marks] 2 Nitrogen and phosphorous ar e in the same group in the Periodic Table. The information in the following table is provided for compounds of nitrogen and phosphorous. Compound M r Boiling point / K NH3 17.0 240 PH3 34.0 185 (a) Briefly explain the difference in boiling points of NH 3 and PH3. [2] ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... (b) Name and sketch the type of hybrid orbital used by nitrogen in forming the N – H bond in NH 3. [2]

4 For examiner’s nly use o ……………………………………………………………………………………………... (c) (i) Identify the ion formed by NH3 when it dissolves in HCl(aq). [1] (ii) The bond angle in NH 3 differs from the ion it formed in part (c)(i). By considering the change in the number and type of electron pairs around the central atom, explain why. [1] [Total: 6 marks] 3 Nitric acid, HNO 3, known to early chemists as aqua fortis (meaning "strong water"), is a major commercial acid. It is available as the concentrated acid which is a 70% aqueous solution. Although colour less when pure, the acid turns yellow when exposed to light as a result of decomposition. 4HNO3(aq) ⎯⎯→ 4NO2(g) + 2H2O(l) + O2(g) ----------------- (1) Pure nitric acid can be prepared by dist illation from a mixture of potassium nitrate and concentrated sulphuric acid as shown KNO3 + H2SO4 ⎯⎯→ KHSO4 + HNO3 ----------------- (2) This method was at one time used in t he commercial preparation of nitric acid, but it has now been replaced by the Oswa ld process, discovered in 1902 by the German chemist Wilhelm Ostw ald, who received the Nobe l Prize in chemistry in 1909 for his work on catalysts. In the Ostwald process, ammonia gas is burned in oxygen over a platinum catalyst. The produc t is nitric oxide, NO, which reacts with oxygen to give nitrogen diox ide. When dissolved in water, NO 2 produces nitric acid. The equations are 4 N H 3(g) + 5O2(g) ⎯⎯→ 4NO(g) + 6H2O(g) ; ΔH1 = − 906 kJ mol-1 2NO(g) + O 2(g) ⎯⎯→ 2NO2(g) ; ΔH2 = −114 kJ mol-1 3 N O 2(g) + H2O(l) ⎯⎯→ 2HNO3(aq) + NO(g) ; ΔH3 = −137 kJ mol-1 The largest single use of nitric acid is in the production of ammonium nitrate, NH4NO3, which is largely used as an agricultural fertiliser. It is also used to manufacture chemicals for nylon and to make explosives like nitroglycerine, C3H5(NO3)3. In making the explosive nitroglycerine, glycerol, C3H5(OH)3, which is obtained from fats is used as the starting material. T he manufacturing process is shown in two stages as shown below:

5 Stage 1 For examiner’s use only CH2OCOR CH2OCOR CH2OCOR CH2OH CHOH CH2OH + 3 RCO2 − Na+ fats glycerol Stage 2 CH2OH CH2ONO2 Nitroglycerine is hazardous to handle in t he pure state. It is very sensitive to shock, exploding by the following reaction: 4C 3H5(ONO2)3(l) ⎯⎯→ 6N2(g) + 10H2O(g) + 12CO2(g) + O2(g) ; ΔH4 = − 5.720 × 103 kJ mol-1 The reaction is highly exothermic and produces large volumes of gas. Alfred Nobel (1833 -1896) discovered that when nitroglycerine was mixed with clay and similar materials, it was much less sh ock-sensitive. The mixture is called dynamite. One stick of dynamite weighs 500 g and contains 80% of nitroglycerine by mass. (a) Explain why nitric acid in a bottl e often turns yellow after some time. [1] ……………………………………………………………………………………………... (b) State the type of reaction involved in the making of pure nitric acid as exemplified in reaction (2). Explain your answer. [2] ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... (c) Draw the dot and cross diagram for the NO molecule and hence suggest a reason for the reactivity of NO. [2] CHOH CH2OH + 3HNO3 CHONO2 CH2ONO2 + H2O

6 (d) (i) Using relevant data from the Data Booklet and the given reaction, calculate the average bond energy of the bond found in the NO molecule. For examiner’s use only 4 N H 3(g) + 5O2(g) ⎯⎯→ 4NO(g) + 6H2O(g) ; ΔH1 = − 906 kJ mol-1 [2] (ii) Suggest a reason for the discrepan cy between your answer in part (d)(i) and the experimentally determined value of of 629 kJ mol -1. [1] ……………………………………………………………………………………………... ……………………………………………………………………………………………... (e) Using appropriate enthalpy change values given for the Ostwald Process and the following energy cycle, calculate the enthalpy change for reaction (1). [2] 2HNO 3(aq) + NO2(g) 3NO2(g) + H2O(l) + ½O2(g) 2HNO3(aq) + NO(g) + ½O2(g)

7 (f) State the reagents and conditions used for the formation of glycerol from fats. [1] For examiner’s use only ……………………………………………………………………………………………………... (g) The molecule of glycerine has the following structure: C H CC H H HH ONOON N O O O O O O x y (i) State the C–O–N angle ( x) and the O–N–O bond angle ( y) as indicated in the diagram above. [2] (ii) Dynamite is still widely used in many countries for clearing of obstacles and for mining resources. Its potential danger however cannot be underestimated. Calculate the average change in tem perature of the surrounding 20 m 3 of air around one stick of exploding dynamite. [Volumetric heat capac ity of air = 1.297 kJ m-3 K-1] [2] [Total: 15 marks]

8 4 Umpolung in organic chemistry is the chemical modification of a functional group with the aim of the reversal of polarity of that group. This allows the functional group to undergo reactions that will otherwise not be possible. For examiner’s us