MI 2008 PU2 Promotional H1Chem P2 Qn

Class Adm No Candidate Name: _______________________ Promotional Examinations 2008 Pre-university 2 CHEMISTRY HIGHER 1 8872/2 Monday 8th Sept 2008 2 hours Additional materials: Answer paper Data Booklet 5 pieces of foolscap paper Cover page READ THESE INSTRUCTIONS FIRST Do not open this booklet until you are told to do so. Write your name, index number 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. Section A Answer all questions. Section B Answer any two questions on the writing papers. At the end of the examination, hand in your answers to Section A and Section B separately. Attach the cover page to your answer scripts for section B. The number of marks is given in brackets[ ] at the end of each question or part question. This question paper consists of 17 printed pages.

Section A: Structured Questions (40m) Answer ALL questions in the spaces provided. 1. Acidified KMnO 4, which oxidizes iron(II) compounds, is used to estimate the amount of iron present in substances such as flour. 1.00g of flour was shaken vigorously with dilute H2SO4 and the volume of the solution was made up to 100cm3 in a volumetric flask. A 10.0cm3 portion of this solution was titrated against 5.00x10 -6 moldm-3 of KMnO4 solution. 11.00cm3 of the KMnO 4 solution was required to completely oxidized the iron(II) salt to iron(III) salt. (i) Write a balanced equation to show the above reaction. [2] ……………………………………………………………………………………………… ………………………………………………………………………………………………. (ii) Calculate the concentration of Fe2+ in the flour solution. [2] (iii) Hence, determine with calculation, if this flour meets the requirement of containing 16.5mg of iron per 100g of flour by law. [2] [Total marks: 6m] [Turn over 2

2. Sulphur and its isotopes are mainly used in medical applications. Sulphur also known as brimstone has been used for thousands of years to cure skin ailments. Ancient cultures relied on sulphur to treat a variety of illnesses as well as to make dyes and other useful products. (a) The relative isotopic mass and percentage abundances of the isotopes of a sample of sulphur are shown in the table. relative isotopic mass % abundance 32S 94.93 34S 5.07 Calculate the relative atomic mass for sulphur in the sample. [1] (b) Oxides of sulphur contribute towards air pollution. Draw a dot and cross diagram to show the bonding in a molecule of sulphur trioxide, SO 3. Hence state the shape and bond angle of SO3 [ 3 ] (c) The melting points of silicon dioxide and sulphur trioxide are shown as follows: compound melting point / oC SiO2 2830 SO3 17 Briefly relate these melting points to the structure and bonding in each of these oxides. [3] ……………………………………………………………………………………………… ………………………………………………………………………………………………. ……………………………………………………………………………………………… ………………………………………………………………………………………………. [Turn over 3

(d) The lattice energy of sodium oxide is – 2000 kJ mol -1. Explain how you would expect the numerical magnitude of the lattice energy of sodium sulphide, Na2S, to compare with that of sodium oxide, Na2O. Hence, predict the relative melting points of Na2O and Na2S . [ 2 ] ……………………………………………………………………………………………… ………………………………………………………………………………………………. ……………………………………………………………………………………………… ………………………………………………………………………………………………. ……………………………………………………………………………………………… ………………………………………………………………………………………………. ……………………………………………………………………………………………… ………………………………………………………………………………………………. [Total marks: 9m] [Turn over 4

3. (a) A dilute aqueous solution of hydrogen peroxide, used to bleach hair, decomposes as shown by the equation. 2H2O2(aq) Æ 2H2O(g) + O2(g) The decomposition of hydrogen peroxide is known to be a first order reaction. (i) Explain the meaning of the phrase in Italics. [1] ……………………………………………………………………………………………… ………………………………………………………………………………………………. (ii) Hence, write a rate equation to show the decomposition of hydrogen peroxide. [1] ……………………………………………………………………………………………… ………………………………………………………………………………………………. (b) Graphical method can also be used to determine the order of reaction for the decomposition of hydrogen peroxide. Hence, sketch (i) a graph of concentration of hydrogen peroxide against time (ii) a graph of rate of reaction against concentration of hydrogen peroxide to show that the decomposition of hydrogen peroxide is a first order reaction. [3] (i) (ii) [Turn over 5

(c) (i) Calculate the standard enthalpy change of reaction for the decomposition of hydrogen peroxide. [ Given : ΔHf o (H2O2) = -187.83 kJ mol-1, ΔHf o(H2O) = -285.83kJ mol-1 ] [2] (ii) Hence, sketch a clearly labelled energy profile diagram to illustrate the decomposition of H 2O2. [ 1 ] [Total marks: 8m] [Turn over 6

4. Trifluroethyl vithyl ether was the first fluorinated volatile compound to be used in anaesthetia. A series of fluorinated compounds was investigated by Robbins(1946) who observed that the anaesthetic potency of a fluorinated compound was greatly increased with the insertion of a second compound. This work has been extended and Fluoethane was discovered in 1956. Fluothane, CF 3CHBrCl, is a volatile liquid commonly used as an anaesthetic in hospital. In anaesthetic systems, sodalime (which contains mainly NaOH) is used to remove the CO 2 breathed out by patients. In hospital, Fluoethane is administered before an operation is conducted. Fluoethane wll be converted in vapour when inside the human body and absorbed by the human body. For every 50ml of fluothane, the vapour concentration absorbed by human body when injected is 2.4%. For a typical person to be unconscious for 8h, 0.05g of Fluothane has to be absorbed. . To confirm that Fluothane does not undergo hydrolysis, a mixture of Fluothane and sodalime( which contains mainly NaOH) was stirred for an hour. It was then acidified with nitric acid and tested with aqueous silver nitrate. (a) Draw a structure of Fluothane. [1] (b) In an operation, the patient needs to be unconscious for 8h. (i) Calculate the number of moles of Fluothane that must be absorbed by the body. [1] [Turn over 7

(iii) Hence or otherwise, calculate the volume of Fluothane that needs to be injected into the human body. Assume that the density of Fluothane is 0.8 gcm-3 [2] (c) Assuming that the bromide ions are produced in the hydrolysis of Fluothane, describe what would be observed when the product is acidified and tested with aqueous silver nitrate. Support your answer with and equation. [2] ……………………………………………………………………………………………… ………………………………………………………………………………………………. ……………………………………………………………………………………………… ………………………………………………………………………………………………. (d) The attempted hydrolysis did not break the C-Br bond in Fluothane. Hence it may be assumed that the other two types of bond, namely the C-Cl and C-F bond in Fluothane also remain intact in the hydrolysis. Give a reason for this assumption [2] ……………………………………………………………………………………………… ………………………………………………………………………………………………. ……………………………………………………………………………………………… ………………………………………………………………………………………………. (e) Another alkyhalide with the molecular formula CH 3CH2Cl undergone a reac