NJC Structured Paper Prelim

Registration Number ST group Name NATIONAL JUNIOR COLLEGE H1 Chemistry Preliminary Examination CHEMISTRY PAPER 2 21 August 2008 TIME: 2 hours INSTRUCTIONS TO CANDIDATES Do not open this booklet until you are told to do so. Write your name, class and registration number in the spaces provided at the top of this page. Do not open this booklet until you are told to do so. Write your name, class and registration number in the spaces provided at the top of this page. Section A: Answer ALL questions in the spaces provided on the question paper. The number of marks is given in brackets [ ] at the end of each question or part question. Section B: Answer any two questions on the writing paper provided. Begin each question on a fresh sheet of paper. Staple your answers at the back of this booklet. Circle the questions answered on the cover page of this booklet. The number of marks is given in brackets [ ] at the end of each question or part question. INFORMATION FOR CANDIDATES A Data Booklet is provided. You may use a calculator. For Examiner’s Use Section A 1 / 9 2 / 9 3 / 12 4 / 10 Section B 5 / 20 6 / 20 7 / 20 Total P2 This question paper consists of 10 printed pages (including this page).

Section A: Structured Questions (40 Marks) 1. Use of Data Booklet is relevant to this question. Gallium was first discovered by Lecoq de Boisbaudran in 1875 when he was studying zinc blende from the Pyrennees. In present day, it is now widely distributed in minute quantities and is used in thermometers for temperatures up to 100 0C, where it is an alloy with indium and tin in liquid state. It is also used in the semi- conductor industry in the manufacture of diodes and transistors. (a) (i) Describe the relative charges and masses of protons, neutrons and electrons. Explain fully how these particles are arranged in a Gallium atom. [3] (ii) State the number of protons, electrons and neutrons in an atom of 69Ga. [1] (b) Gallium exists as two isotopes, 69Ga and 71Ga in the proportion of 3:2. (i) Define the term isotope. [1] (ii) Calculate the relative atomic mass of gallium. [2] (iii) A gaseous mixture of 69Ga3+ and 71Ga3+ is passed through an electric field which is at right angles to their direction of travel. State and explain which one of the two ions will be deflected to a larger angle. [2] [Total: 9] 2

2. Nitrogen monoxide can be produced by the oxidation of ammonia according to the equation below. 4NH3 (g) + 5O2 (g) → 4NO (g) + 6H2O (g) (a) (i) The NO bond energy in nitrogen monoxide is 607 kJ mol –1. Explain what is meant by the term bond energy with reference to the NO molecule. [1] (ii) Calculate the enthalpy change for the reaction above using (a)(i) and other bond energy values from the Data Booklet. [3] (b) The standard enthalpy change of formation of the respective compounds are given below. Compound kJ mol –1ΔHf Ө / NH3 (g) – 46 NO (g) +90 H2O (l) –285.8 (i) Suggest a value for ΔHf Ө of O2(g) and explain your answer briefly. [2] (ii) Calculate the enthalpy change of the reaction above. [2] (iii) Compare the answers obtained in (a)(ii) and (b)(ii) and suggest a reason for the difference. [1] [Total: 9] 3

3. (a) Draw the structures of the organic product(s) formed when Compound A below reacts with each of the following reagents. CH H C CH3 C CH3 H Compound A Reagents and Conditions Organic Product(s) formed (i) HBr(g) (ii) KMnO4 ; dilute H2SO4 ; reflux (iii) Cold dilute alkaline KMnO4 (iv) Cl2(g) / AlCl3 ; dark [8] 4

(b) State the observation and write a balanced equation when Compound A reacts with the reagents and conditions in (a)(iii). [2] (c) State the type of isomerism exhibited by Compound A and hence draw the structures of the two isomers formed. [2] [Total: 12] 5

4. (a) Chlorine was discovered in 1774 by Swedish Chemist C.W. Scheele. He added manganese dioxide, MnO2 to hydrochloric acid and found that a gas was evolved. In this reaction, hydrochlor ic acid converts manganese dioxide to manganese(II) chloride, MnCl 2. The other products were water and the unknown gas. Scheele initially thought that the gas was oxygen. It was only until 1810 that Sir Humphery Davy identified the gas as an element and named it Chlorine, which means greenish-yellow in Greek, due to its appearance. [For this question, 1 mole of chlorine gas occupies a volume of 24 dm 3 at room temperature and pressure.] (i) Suggest a balanced equation for the reaction that Scheele performed. [1] (ii) Use your equation to calculate the volume of chlorine produced at room temperature and pressure when 5 g of manganese dioxide reacts with 25.0cm 3 of 6.0 mol dm−3 of hydrochloric acid. [2] (iii) Which species in the above reaction acts as the reducing agent? [1] (b) Chlorine gas is toxic. It affects the eyes at concentrations of only 3 ppm and it irritates the throat at 15 ppm. At 50 ppm, it can cause danger to human health even for a short time. Due to its toxic nature, chlorine was thus used as an offensive weapon in World War I in Flanders. It was first deployed in 1915 when the German army released the gas from hundreds of cylinders. The threat of causing many men dying in agony was eventually counter ed by issuing gas masks, termed the “hypo helmet”, which was a hood that was dipped in aqueous sodium thiosulphate, Na 2S2O3. (i) Using the above information, calculate the number of moles of chlorine in 24 dm3 of air that would be dangerous to human health. [2] 6

(ii) When chlorine gas was absorbed by sodium thiosulphate found in the “hypo helment”, chlorine was reduced to chloride while thiosulphate ions, S2O3 2− was oxidized to sulphate ions, SO4 2−. Write a balanced equation for the reaction between chlorine and thiosulphate ions in an acidic medium. [1] (iii) Assuming that each treated “hypo helmet” effectively absorbed 500 cm 3 of chlorine gas during a battle at room temperature and pressure, and the production of each “hypo helmet” required 700 cm 3 of aqueous sodium thioshulphate, calculate the concentration in mol dm −3 of sodium thiosulphate required. [3] [Total: 10] 7

8 Section B: Free−Response Questions (40 Marks) 5. (a) (i) Sketch a graph of ionic radius against proton number for the Period 3 elements from sodium to chlorine. Explain the shape of your graph. [3] (ii) When heated in oxygen, magnesium, aluminium and sulphur form oxides. (I) State the formulae of the oxides formed. (II) By means of equations, indicate whether or not each of these oxides react with sodium hydroxide and hydrochloric acid. [5] (iii) Arrange the melting point of the oxi des formed by magnesium, aluminium and sulphur in increasing order. Explain your answer. [3] (b) The kinetics of the reaction below can be followed via iodometric titrations. IO3 −(aq) + 6H+(aq) + 5I−(aq) → 3I2(aq) + 3H2O(l) The results are shown in the table below: Volume / cm3 Expt. IO3 −(aq) I −(aq) H+(aq) H 2O Total Initial Rate / mol dm−3 min−1 1 100 500 600 800 2000 1.0 2 50 250 600 100 1000 4.0 3 100 250 600 50 1000 8.0 4 50 125 600 225 1000 2.0 All solutions used are of concentration 1 mol dm−3. (i) Use the above da