RI Prelim P2

RAFFLES INSTITUTION (JUNIOR COLLEGE) PRELIMINARY EXAMINATION 2009 Higher 2 CANDIDATE NAME CLASS INDEX NUMBER CHEMISTRY 9746/02 Paper 2 Structured Questions 23 September 2009 1 hour 30 minutes Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, class and index number in the spaces provided on the top of this page. Write in dark blue or black pen in the spaces provided. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions. A Data Booklet is provided. Do not write anything on it. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use This document consists of 14 printed pages. Question 1 2 3 4 5 Total Marks /10 /14 /11 /11 /14 /60

2 © RAFFLES INSTITUTION (JUNIOR COLLEGE) 2009 9746 / 02 / 09 [Turn Over x y z Answer all questions in the spaces provided. 1 Fluorine is the most reactive of all elements and even combines with all noble gases except helium and neon. Compounds containing the element f luorine are commonly called fluorides. (a) (i) Give the electronic configuration of fluorine, F. (ii) On the axes below, sketch the shape(s) of the sing ly–occupied orbital(s) in an atom of F at ground state. [2] (b) With reference to the Data Booklet , briefly explain the following statements. (i) The first ionisation energy of oxygen is lower than that of fluorine. (ii) The second ionisation energy of oxygen is higher than that of fluorine. [3] For Examiner’s Use

3 © RAFFLES INSTITUTION (JUNIOR COLLEGE) 2009 9746 / 02 / 09 [Turn Over (c) It is well known that fluoride, F – , helps prevent tooth decay. Hence, it is commonly added to toothpastes, mouth rinses and water suppli es via water–soluble compounds such as sodium fluoride. However, in high concentrations, F – is lethal. Historically, most cases of fluoride poisoning have been caused by the accidental ingest ion of insecticides containing F – but currently, most cases have been due to the swallowing of toothpaste. A suggested method of treatment in the case of acc idental ingestion is to give the person some milk of magnesia , which is an aqueous suspension of magnesium hydroxide, Mg(OH) 2(s). (i) Given that stomach juices are highly acidic, writ e a balanced equation, with state symbols, for the reaction that occurs when some milk of magnesia enters the stomach. (ii) MgF 2(s) is a sparingly soluble salt. The numerical valu e of its solubility product is 5.16 × 10 –11 at 25 oC. Based on your answer to (c)(i) , suggest how a person who has accidentally swallowed some toothpaste may be successfully treat ed by drinking some milk of magnesia . Give a balanced equation, with state symbols, t o support your answer. For Examiner’s Use

4 © RAFFLES INSTITUTION (JUNIOR COLLEGE) 2009 9746 / 02 / 09 [Turn Over (iii) Using the data below, determine the minimum volum e of the milk of magnesia to be taken, so that the first trace of precipitate fo rms, if a person has swallowed an amount of toothpaste that has an equivalent of 1.0 mg of F – . average volume of liquid in stomach 1 dm 3 amount of Mg(OH) 2 in 1 dm 3 of Brand X milk of magnesia 1.40 mol Assume that the volume of milk of magnesia swallowed is negligible compared to the volume of liquid in stomach. [5] [Total: 10] 2 Air is composed of mainly nitrogen and oxygen, with trace quantities of other gases such as argon, carbon dioxide and even ammonia. (a) What do you understand by the term ideal gas ? [1] For Examiner’s Use

5 © RAFFLES INSTITUTION (JUNIOR COLLEGE) 2009 9746 / 02 / 09 [Turn Over pV ideal gas p 0 (b) (i) A sketch of pV against p for 1 mole of an ideal gas at 100 oC is given below. On the same axes, show how 1 mole each of carbon dioxide and ammonia will behave at 100 o C. Label your graphs clearly. (ii) Briefly explain your answer to (b)(i) . [2] In nature, the gases nitrogen and oxygen can react to form nitrogen monoxide, NO. However, the reaction requires elevated temperatures. (c) (i) Suggest how NO can be formed in nature. (ii) Briefly explain why elevated temperatures are needed for the formation of NO. [2] For Examiner’s Use

6 © RAFFLES INSTITUTION (JUNIOR COLLEGE) 2009 9746 / 02 / 09 [Turn Over The formation of NO has been studied as an equilibrium system. N 2(g) + O 2(g) 2NO(g) (d) Write an expression for the equilibrium constant, K c, for the reaction. [1] An experiment is carried out using the set–up belo w. (e) To begin the reaction, the temperature is raised t o 2000 oC without opening the valve. Given that the numerical value of K c is 6.2 x 10 –14 at 2000 oC, calculate the concentration of NO present when the system reaches equilibrium. [2] F or Examiner’s Use

7 © RAFFLES INSTITUTION (JUNIOR COLLEGE) 2009 9746 / 02 / 09 [Turn Over (f) When the system in (e) has reached equilibrium, the valve is opened at co nstant temperature. Predict, and explain your answers briefly, how the position of equilibrium will change if gas A is (i) argon, (ii) nitrogen. [3] (g) With reference to (f)(ii) , calculate the final pressure of NO when equilibrium is re–established. [3] [Total: 14] For Examiner’s Use

8 © RAFFLES INSTITUTION (JUNIOR COLLEGE) 2009 9746 / 02 / 09 [Turn Over 3 The flowchart below shows reactions involving some calcium compounds. heat CaCO 3(s) gas A + solid B solid D solution C solution E (a) Coal-fired power stations emit flue gases, which co ntain the oxides of nitrogen, carbon and sulfur. To reduce sulfur dioxide emissio n, such power station is fitted with a flue gas desulfurisation (FGD) plant which u ses a slurry of compound D to extract sulfur dioxide gas from the flue gases. (i) Suggest the identity of compound D. Hence write an equation for its reaction with sulfur dioxide, given that one of the products produced is a sulfite. (ii) Calculate the mass of compound D required to extract the sulfur dioxide produced when 1 × 10 9 kg of coal containing 2.5 % of sulfur by mass are burnt. [4] (b) With the aid of equation(s), suggest why solution E has a pH of 7 whereas a solution of beryllium chloride has a pH of approximately 3. [3] H 2O(l) HCl (aq) CO2 (g) For Examiner’s Use

9 © RAFFLES INSTITUTION (JUNIOR COLLEGE) 2009 9746 / 02 / 09 [Turn Over (c) The graph below shows the change in mass when 1.00 g of calc