RVHS H2 CHEM P3 QP Prelim

River Valley High School 9647/03/PRELIM II/16 [Turn over 2016 Preliminary Examination II RIVER VALLEY HIGH SCHOOL YEAR 6 PRELIMINARY EXAMINATION II CANDIDATE NAME CLASS 6 CENTRE NUMBER S INDEX NUMBER H2 CHEMISTRY 9647/03 Paper 3 Free Response 21 September 2016 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Cover Page Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, class, centre number and index number on all the work you hand in. Write in dark blue or black pen on both sides of paper. 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 any four questions. Begin each question on a fresh sheet of paper. 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. At the end of the examination, fasten all your work securely together, with the cover page on top. This document consists of 10 printed pages and 2 blank pages.

2 River Valley High School 9647/03/PRELIM II/16 2016 Preliminary Examination II Answer any four questions. 1 Many compounds of chlorine are manufactured from brine, NaC l(aq). The electrolysis of brine produces Cl2(g) and NaOH(aq). In some industrial electrolytic cells, these two substances are allowed to react further. The products formed in this second reaction dependp on the operating conditions used. (a) Write balanced equations for the reaction between Cl2(g) and (i) cold aqueous NaOH; [1] (ii) hot aqueous NaOH. [1] (b) Chlorine dioxide, ClO2, is used in the treatment of water. It is formed from ClO3− ions in an acidic solution. ClO3− + 2H+ + e− ⇌ ClO2 + H2O (i) Draw the dot-and-cross diagrams of ClO2 and H2O2. You may assume that there is no dative bond in either compound. [2] (ii) Construct the overall equation for the reaction of ClO3– ions with H2O2 in acidic solution. [1] (iii) What is the role of H2O2 in the reaction? [1] (c) Chlorine can also be used to disinfect water. When chlorine is added to water, it produces hypochlorous acid, HOCl. Hypochlorous acid is a weak acid that dissociates into hypochlorite ions, OCl−, according to the following equation. HOCl(aq) ⇌ H+(aq) + OCl−(aq) Free chlorine refers to the total chlorine content in HOCl and OC l−. The dissociation curve below illustrates the ratio of hypochlorous acid to hypochlorite at different pH values. [Source: www.hach.com] OCl– HOCl

3 River Valley High School 9647/03/PRELIM II/16 [Turn over 2016 Preliminary Examination II (i) Determine the pKa of hypochlorous acid. [1] (ii) Hence, calculate the pH of a 0.0025 mol dm –3 hypochlorous acid solution. [2] To determine whether the free chlorine in a sample of tap water meets the regulatory limit (4 mg Cl per litre), OCl – is quantitatively reduced to Cl − by I− ions, which is in turn oxidised to I2. The I2 is titrated with standard sodium thiosulfate. The following reaction takes place during the titration. I2 + 2S2O32− → 2I− + S4O62− (iii) Construct a balanced equation between OCl–(aq) and acidified KI(aq). [1] (iv) When 2 dm 3 of tap water was tested, 6.0 cm 3 of 0.00455 mol dm−3 sodium thiosulfate was required to discharge the colour of iodine. Calculate the concentration of OCl− in the sample of water. [2] (v) Suggest why the calculated concentration of OC l− has the same value as the concentration of free chlorine. [2] (vi) Determine if the sample of tap water is safe for consumption. [2] (d) Chlorine is also used in organic chemistry to produce the Lewis acid catalyst, FeCl3, for the reaction between methylbenzene and chlorine. (i) Describe the mechanism of the above reaction. [3] (ii) FeCl3 reacts in a similar way with acyl chlorides. Predict the structure of the product of the following reaction. [1] [Total: 20]

4 River Valley High School 9647/03/PRELIM II/16 2016 Preliminary Examination II 2 (a) Nitrous oxide or dinitrogen oxide, N 2O, is commonly known as "laughing gas" due to the euphoric effects of inhaling it. It is used in surgery and dentistry for its anesthetic and analgesic effects. To produce N 2O, ammonium nitrate is decomposed at 170 C. Water is a by-product of this reaction. (i) Write an equation for the production of dinitrogen oxide from ammonium nitrate. [1] (ii) In the manufacturing of N 2O gas, 1 kg of N 2O gas is produced for every 2.1 kg of ammonium nitrate used. Determine the percentage yield of dinitrogen oxide. [2] (b) At 1200 K, in the presence of gold wire, dinitrogen oxide decomposes as shown: 2N2O(g)  2N2(g) + O2(g). To follow the rate of reaction, the change in concentration of a sample of N2O is measure against time. The results are shown below: Time, t / s Concentration of N2O / × 103 mol dm3 0 2.50 1000 2.01 2000 1.62 3000 1.31 4000 1.05 5000 0.85 6000 0.68 7000 0.55 (i) What do you understand by the term half-life of N2O? [1] (ii) Plot the above data on a graph paper. Use the following scale:  2 cm to represent 1000 s on the x-axis; and  2 cm to represent to represent 0. 25 × 103 mol dm3 on the y-axis. [2] (iii) From your graph, deduce the order of the reaction with respect to N2O. [2]

5 River Valley High School 9647/03/PRELIM II/16 [Turn over 2016 Preliminary Examination II (iv) Calculate the rate constant for the reaction and state its units. [2] (v) The gold wire acts as a heterogeneous catalyst in this reaction. Explain the terms in italics and outline the mode of action of the catalyst. [3] (c) Alkenes react with carbenes R2C: to yield cyclopropanes. One way to generate a substituted carbene is by reacting chloroform, CHCl3, with a strong base. The mechanism to generate a carbene is described below. 1. Potassium hydroxide, KOH, removes the proton from CHC l3, leaving behind the electron pair. An anionic intermediate is formed. 2. Cl− is lost and a neutral dichlorocarbene is formed. (i) Based on the description above, draw a mechanism to show the generation of dichlorocarbene. Show relevant lone pairs and use curly arrows to indicate the movement of electron pairs. [3] An example of a reaction between an alkene and a carbene is as shown: (ii) Suggest the structure of the product formed when cyclohexene reacts with chloroform in the presence of KOH. [1] (iii) Suggest the role of carbene in its reaction with an alkene. Explain your answer. [2] (iv) Suggest the type of reaction when carbene reacts with an alkene. [1] [Total: 20]

6 River Valley High School 9647/03/PRELIM II/16 2016 Preliminary Examination II 3 Copper is a rare element, constituting only 6.8 × 10–3 percent of the Earth’s crust by mass. However, it has a wide range of uses such as in alloys, plumbing and in electrical cables. The common oxidation states of copper are +2 and +1. (a) Copper can be obtained by roasting a copper ore, CuFeS 2, to give Cu2S, which is further oxidised to form metallic copper. This impure copper can be purified by electrolysis. (i) Draw a diagram to illustrate the electrolytic cell used in the purification of copper, using CuSO4(aq) as the electrolyte. [2] (ii) If a current of 0.8 A was passed through the cell, calculate the time required for 0.25 g of pure copper to be collected. [2] (b) Anhydrous copper(II) sulfate, CuSO4(s), is a white powder. It dissolves in water to form a pale blue solution. (i) State the species responsible for the pale blue colour of the solution. [1] (ii) Explain why the so