MJC H1 CHEM P2 ANS

No part of this paper should be repro duced for commercial use without the prior written permission of Meridian Junior College. Name : ______________________ Class: ______ Reg Number: _____ Meridian Junior College 2009 JC2 Preliminary Examination Answers H1 Chemistry 8872 14 September 2009 2 hours Paper 2 Candidates to answer Section A on the Question Paper and Section B on separate writing paper. Additional Materials: Data Booklet Writing paper INSTRUCTION TO CANDIDATES Write your name, class and regi s ter number in the spaces pr ovided at the top of this page. Section A Answer all questions. Examiner’s Use Paper 1 MCQ / 30 Q1 / 7 Q2 / 8 Q3 Section B Answer any tw o questions. Paper 2 Section A At the end of the examination, fasten your answers for Section B behind Section A. INFORMATION FOR CANDIDATES / 10 Q4 / 15 Q5 / 20 Q6 / 20 Q7 / 20 Total / 110 Paper 2 Section B 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 English and clear presentation in your answers. This question paper consists of Page 1 to 19. ©chemistry department@meridian jc

Section A 1(a) Magnesium bromide, MgBr 2, is a white solid that r eadily absorbs moisture from the atmosphere. It is often used as a mild sedative. (i) Using magnesium bromide as an exam ple, explain what is meant by the term lattice energy. Lattice energy is the energy released when one mole of MgBr 2 is formed from gaseous Mg2+ and Br- ions at standard conditions. (ii) Use the energy level diagram, t ogether with the giv en data, determine the lattice energy of magnesium bromide. ©chemistry department@meridian jc Enthalpy change of hydration of magnesium ion, ∆Hhyd(Mg2+) -1921 kJ mol-1 Enthalpy change of hydration of bromide ion, ∆Hhyd(Br-) -336 kJ mol-1 Enthalpy change of solution of magnesium bromide, Hsoln( MgBr2) -186 kJ mol-1 ∆Hlatt ө (MgBr2) = + (-1921) + 2(-336) – (-186) ∆Hlatt ө (MgBr2) = - 2407 kJ mol-1 ∆ ∆Hlatt MgBr2 (s) ∆Hsoln (MgBr2) Mg2+(aq) + 2Br-(aq) Hhyd( 2+gM ) + 2 x ∆Hhyd( -Br ) Mg2+(g) + 2Br-(g) 1

(iii) Explain how you would expect the numerical magnitude of the lattice energy of MgBr2 to compare with that of sodium fluoride, NaF. Charge of cation q+ : Na+ < Mg2+ Ionic radius of cation: Mg2+ < Na+ & Ionic radius of anion: F- < Br- Hlatt : MgBr2> NaF [4] (b) (i) With the aid of an equation, define the first ionisation energy of iodine. ©chemistry department@meridian jc 2 I (g) → I + (g) + e (ii) State and explain how you would expec t the first ionisation energy of iodine to compare with that of fluorine. [3] 1st I.E of iodine < 1st I.E of fluorine Down the group, significant incr ease in screening effect outweighs increase in nuclear charge, henc e effective nuclear charge of I < F . The size of the iodine atom is larger hence its valence electrons are further away from the nucleus. Ther e is thus weaker electrostatic attraction between nucleus and the valence electrons of I. Less amount of energy required to remove the valence electron of I. [Total : 7]

2 Early Periodic Tables, such as that devised by Mendeleev, listed the then known elements in order of their relative atomic mass. (a) When Mendeleev created the table, ther e were uncertainties regarding the relative atomic mass of tellurium. It is now known that there are eight isotopes of tellurium. Complete the followi ng table. Give your answers to four significant figures. isotope percentage abundance isotopic mass x percentage abundance tellurium-120 0.09 11 tellurium-122 2.46 300 tellurium-123 0.87 107 tellurium-124 4.61 572 tellurium-125 6.99 874 tellurium-126 18.71 2357 tellurium-128 31.79 4069 tellurium-130 34.48 4482 Hence, calculate the relative atomic mass of tellurium to 1 decimal place. Relative atomic mass of Te = 11 + 300 + 107 + 572 + 874 + 2357 + 4069.12 + 4482.40 100 = 127.7 [3] ©chemistry department@meridian jc 3

(b) Tellur ium (IV) chloride, TeCl4, is a pale yellow solid at room temperature with a melting point and boiling point of 224°C and 380°C respectively. In the molten state, TeC l4 dissociates into TeCl 3 + and Te 2Cl10 2-. Gaseous tellurium (IV) chloride, TeCl4, has a structure similar to sulphur tetrafluoride, SF4. Explain in terms of structure and bonding, the relative electrical conductivity of TeCl4 in the molten and gaseous state. Electrical conductivity of gaseous TeC l 4 < molten TeC l4. This is because gaseous TeCl 4 has a simple molecular structure and thus lacks free mobile ions or delocalised electrons to conduct electricity. Molten TeC l4 can conduct electricity because of the presence of free mobile ions. [2] (c) Tellurium resembles silicon in m any aspects. For example, both are metalloids and are used as semiconductors. Tellurium ( IV) oxide, TeO 2, reacts with concentrated strong bases in the same way as silicon ( IV) oxide, SiO2. (i) Write a balanced equation for the reaction between TeO2 and NaOH. TeO 2 + 2NaOH → Na2TeO3 + H2O (ii) Predict the pH of the solution formed when gaseous tellurium tetrachloride, TeCl4, is bubbled into water. Suggest an equation for this reaction. pH of solution : Between 1.5 - 2.5 Equation: TeCl4 + 2H2O → TeO2 + 4HCl [3] [Total : 8] ©chemistry department@meridian jc 4

3 The following shows a series of reactions that compound Z can undergo. B C H Cl C O O-Na+Cl A ©chemistry department@meridian jc (a) Draw the structures of compounds A to F in the boxes provided below. Cl C H Cl C CH3 O + Step I Step II KMnO4/H+ Heat Cl2, Fe Heat NaOH(aq) heat Ethanolic NaCN, heat Ethanolic NaOH, heat NaOH(aq), heat Cl C H Cl C CH3 OH H K2Cr2O7/H+ F Heat D G H C E Compound Z C H Cl C O O-Na+Cl Cl Cl COOH A B Cl Cl C H Cl C O O - Na + or Cl C H OH C CH3 OH H Cl C H CN C CH3 OH H C D E 5

©chemistry department@meridian jc [6] (b) When treated with NaOH in ethanol, compound Z produces a mixture of isomers, G and H. Draw the full structural formulae of G and H. C C C H H H O H H Cl C C H Cl O C H H H H Cl C H COO- C CH3 OH H F G H [2] (c) State the reagents and conditions required in Steps I and II. Step I Reagents: I2 in NaOH (aq) Conditions: ……… heat …… Step II Reagents: … Steam, concentrated H3PO4…… Conditions: ….300°C, 65 atm … [2] [Total: 10] 6

4 Sulphur compounds occur naturally in petrol. The presence of such compounds gives rise to the emission of toxic oxides of sulphur from car exhausts. One such pollutant produced fr om car emissions is the gas sulphur dioxide, SO2. Sulphur dioxide reacts with water and oxygen in the atmosphere to produce ‘acid rain’, a dilute solution of sulphuric acid, resulting in further damage to urban architecture. One process used to remove sulphur compounds from petroleum and diesel is called “hydroprocessing”. This process uses catalysts consisting of metals deposited as thin layers on porous aluminium supports. Hydroprocessing converts the sulphur compounds in the fuel to the gas hydrogen sulphide, H2S. Some of the hydrogen sulphide produced is oxidised to sulphur