HCI H1 Chem Prelim P2

NAME: _______________________________________ CT: ___________ HWA CHONG INSTITUTION C2 PRELIMINARY EXAMINATIONS H1 CHEMISTRY 8872 PAPER 2 This question booklet consists of 12 printed pages. 10 September 2008 2 hr INSTRUCTIONS TO CANDIDATES 1. This paper consists of 12 printed pages. You should also have a Data Booklet and a set of writing paper. 2. Answer all questions in Section A (Structured Questions) in the spaces provided in this Answer Booklet. 3. Answer any two questions in Section B (Free Response Questions) on the writing paper provided. Begin each question on a FRESH sheet of writing paper. 4. At the end of the examination, hand in this answer booklet and your answer scripts to Section B SEPARATELY. INFORMATION FOR CANDIDATES The number of marks is given in brackets [ ] at the end of each question or part question. You may use a calculator. You are reminded of the need for good English and clear presentation in your answers. FOR EXAMINERS’ USE ONLY TOTAL Multiple Choice Section A (Structured) Section B (Free Response) Q1 / 8 Q5 / 20 Q2 / 7 Q6 / 20 Q3 / 13 Q7 / 20 Q4 / 12 / 30 Subtotal / 40 Subtotal / 40 110

Section A Answer all the questions in this section in the spaces provided. 1(a) Molecules of NH3, H2O and HF contain polar covalent bonds. (i) State what is meant by each of the following terms: covalent bond: _______________________________________________________ ___________________________________________________________________ polar bond: _________________________________________________________ ___________________________________________________________________ (ii) Arrange NH 3, H 2O and HF in order of increasing polarity of the bond in the molecules. Explain your answer. ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ [4] (b) The boiling points of the hydrides of some Period 2 elements are shown in the table below. Compound CH4 NH3 H2O HF Boiling point / oC -164 -33 100 19.5 With the aid of a simple diagram, explain why the boiling point of either H2O or HF is much higher than that of methane. _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ [2] 2008 HCI C2 H1 Chemistry Preliminary Examination / Paper 2 2

(c) When a NH 3 molecule reacts with H +, an NH 4 + ion is formed. Draw the structure of the NH4 + ion and state the bond angle. Bond angle = _____________ [2] [Total: 8] 2(a) Define the term first ionisation energy. _______________________________________________________________________ _____________________________________________________________________ [1] (b) On the grid below, sketch the trend in first ionisation energy of the elements across Period 3 of the Periodic Table. Na Mg Al Si P S Ar Cl first ionisation energy [2] 2008 HCI C2 H1 Chemistry Preliminary Examination / Paper 2 3

(c) Elements P, Q and R belong to the same group of the Periodic Table. The first four ionisation energies of P, Q and R are shown below. Element P Q R First ionisation energy / kJ mol−1 496 403 419 Second ionisation energy / kJ mol−1 4562 2633 3052 Third ionisation energy / kJ mol−1 6910 3860 4420 Fourth ionisation energy / kJ mol−1 9543 5080 5877 (i) Deduce which group of the Periodic Table P, Q and R belong to. ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ (ii) Arrange P, Q and R according to increasing atomic number. Explain your answer. ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ [4] [Total: 7] 2008 HCI C2 H1 Chemistry Preliminary Examination / Paper 2 4

3 Chlorine is a Group VII element in the Periodic Table. It was first produced in 1774 by German-Swedish chemist Carl Wilhelm Scheele by heating hydrochloric acid with manganese (IV) oxide. The products were chlorine, manganese (II) chloride and water. This method is not suitable for industrial production of chlorine. Instead, chlorine is now commonly recovered from sea water using electrolysis. Chlorine gas is toxic. A concentration of 1000 ppm (parts per million by volume) is likely to be fatal after a few deep breaths. Chlorine was thus used as an offensive weapon in World War I in Flanders. It was first deployed in the morning of 22 April 1915 when the German army released the gas from hundreds of cylinders. The gas was carried by the wind to the British troops, causing chaos as many men died in agony and countless more were disabled by it. The threat was eventually countered by issuing gas masks. An initial form of the gas mask, termed the “hypo helmet”, was a hood that was dipped in sodium thiosulphate. Chlorine was converted to harmless chloride ions by thiosulphate ions, S 2O3 2−, which were converted to sulphate (VI) ions, SO4 2−. (a) Give the chemical formula of the oxidizing agent used by Scheele. _____________________________________________________________________ [1] (b) Suggest a reason why Scheele's method is not suitable for industrial production of chlorine. _______________________________________________________________________ _____________________________________________________________________ [1] (c) The molar volume of chlorine gas may be taken to be 24 dm 3 under room conditions. (i) Estimate the density of chlorine gas in g cm −3 under room conditions. (ii) Hence suggest why many more British soldiers died from chlorine poisoning in trenches rather than on hill tops. ___________________________________________________________________ ___________________________________________________________________ [2] 2008 HCI C2 H1 Chemistry Preliminary Examination / Paper 2 5

(d) The reaction occurring in the "hypo helmet" is a redox reaction. (i) Write the half-equation for the oxidation reaction. ___________________________________________________________________ (ii) Write the half-equation for the reduction reaction. ___________________________________________________________________ (iii) Hence construct the overall equation for the redox reaction in the "hypo helmet". ___________________________________________________________________ [3] (e) Assuming that each "hypo helmet" effectively absorbed all the chlorine gas in 320 dm 3 of air containing 500 ppm of chlorine before being inactivated, and each helmet carried 30 cm 3 of sodium thiosulphate, calculate the concentration in mol dm −3 of sodium thiosulphate used. [2] (f) Describe the reactions, if any, of chlorine with sodium and with silicon. Include relevant equations in your answer. _______________________________________________________________________ ___________________________