CJC H2 CHEM P2 SOL Prelim

CANDIDA NAME CLASS CHEM Paper 2 S Candida Additiona READ T Write your Write in d ERASABLE You may u Do not use Answer a You are r and clear The numb the end of CATH JC2 PR Higher ATE MISTRY Structured ates answe al Material THESE INS r name and c dark blue o r E PENS ARE use a soft pe e staples, pa all questions reminded of presentation ber of mark s f each questi Answ HOLIC JU RELIMINAR r 2 2T Y Questions er on the Q ls: Data Bo STRUCTIO class in the s r black pen NOT ALLOW encil for any d aper clips, hig s. the need f o n in your answ s is given in on or part of wers UNIOR C RY EXAMIN s uestion Pa ooklet ONS FIRST spaces provid in the spa WED] diagrams, gr ghlighters, gl or good Eng wers. brackets [ f the question COLLEGE NATIONS aper T ded above. aces provid e raphs or roug lue or correc lish ] at n. Pa Pa Pa T E ed, on the Q gh working. ction fluid. aper 1 aper 2 aper 3 Total Monday 1 Question Pa For Q 1 Q 2 Q 3 Q 4 Q 5 Q 1 Q 2 Q 3 Q 4 Q 5 [T 9 1 Septemb aper. [PILOT Examiner’s urn over 9647/02 ber 2014 2 hours FRIXION s Use 12 15 15 15 15 20 20 20 20 20 1 40 72 80 92

2 Planning (P) 1 FA1 is a solution of 1.00 mol dm-3 sodium hydroxide, NaOH. FA2 is a solution of 1.00 mol dm -3 acid which is either a monobasic with the formula HA, or dibasic with the formula H2A. You are required to determine the basicity of an acid (either monobasic or dibasic), based on the information given below and the planning of the procedure you have to undertake. Three separate experiments were carried out. Varying volumes of FA1 were measured and then added to different volumes of FA2 and the rise in temperatures, as a result of the mixing were recorded. The relevant measurements and results are tabulated below. Experiment 1 Experiment 2 Experiment 3 Volume of FA1 / cm3 25 25 50 Volume of FA2 / cm3 25 50 25 Temperature rise, ∆T / oC 6.9 4.6 9.1 (a) (i) Write a balanced equation for the reaction between FA1 and FA2 if the acid is monobasic. Do not omit spectator ions. HA(aq) + NaOH(aq) → NaA(aq) + H2O(l) (ii) Write a balanced equation for the reaction between FA1 and FA2 if the acid is dibasic. Do not omit spectator ions. H2A(aq) + 2NaOH(aq) → Na2A(aq) + 2H2O(l) [2] (b) You are provided with the following apparatus and materials: 1 thermometer 1 styrofoam cup Two 50 cm 3 measuring cylinders Sufficient FA1 solution Sufficient FA2 solution Plan a simple procedure by describing the steps you would carry out for Experiment 1 given above so as to make the relevant measurements and recordings. You are to use only the apparatus and materials listed above. 1 Using a measuring cylinder, pour 25 cm 3 of FA1 into a styrofoam cup. 2 Read and record the initial temperature with a thermometer. 3 Using another measuring cylinder, measure 25 cm3 of FA2 and carefully add FA2 to FA1, contained in the styrofoam cup. 4 Stir/swirl gently with the thermometer and read and record the highest temperature reached. [4]

3 [Turn over 9647/02/CJC JC2 Preliminary Exam 2014 (c) Use the results of the relevant experiment s given in the table above, explain clearly how the basicity of the acid is determined. Hence, state the basicity of the acid. Need to compare experiments 2 & 3 as the total volume of the mixture is the same at 75 cm3. HA(aq) + NaOH(aq) → NaA(aq) + H2O(l) H2A(aq) + 2NaOH(aq) → Na2A(aq) + 2H2O(l) Exp 2: 0.025 mol 0.025 mol 0.025 mol 0.0125 mol 0.025 mol 0.025 mol Exp 3: 0.025 mol 0.025 mol 0.025 mol 0.025 mol 0.050 mol 0. 050 mol Since the rise in temperature is doubled in Exp 3 compared to that of Exp 2, FA2 is a dibasic acid, H2A. If the acid is monobasic, then the rise in temperature in Exp 2 and 3 should be the same. The acid is dibasic. [2] (d) (i) Hence, calculate the amount of heat evolved in Experiment 1. In your calculations, you are to assume that the specific heat capacity of the reaction mixture is 4.18 J g-1 K-1 and the density of the reaction mixture is 1 g cm-3. Heat evolved = m c ∆T = (25+25) x 4.18 x 6.9 J = 1.44 x 10 3 J (ii) Using your answers to (d)(i) and the relevant equation in (a), calculate the enthalpy change of neutralisation, ∆Hneu. H2A(aq) + 2NaOH(aq) → Na2A(aq) + 2H2O(l) 0.0125 mol 0.025 mol 0.025 mol ∆Hneu = - 1.44 x 103 0.025 J mol-1 = - 57.7 kJ mol-1 (with correct units & negative sign) [2] (e) From your calculated ∆Hneu value, state whether the acid is strong or weak. Strong acid as the value is approximately – 57.3 kJ mol-1. [1] (f) Identify one source of error in your procedure and explain how you would minimise the error. Heat loss to the surrounding. Perform the experiment in a draught free environment or provide a lid to cover the styrofoam cup or provide lagging to the styrofoam cup. Measuring cylinders are not precise enough to measure volumes of FA1 and FA2. Use of burette to measure their volumes more accurately. Measure the initial temperatures of FA1 and FA2 separately before mixing. The initial temperature of the mixture can then be calculated by the taking the mean/average of the initial temperatures. [1]

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5 [Turn over 9647/02/CJC JC2 Preliminary Exam 2014 2 Scuba diving is a form of underwater diving, in which a diver uses a self-contained underwater breathing apparatus (scuba) to breathe underwater. Scuba diving is an exhilarating sport, and thanks in part to the gas laws, it is also a safe activity for trained individuals who are in good health. A typical dive might be 12 to 19 m, but dives to 27 m are not uncommon. As seawater has a slightly higher density than fresh water, the pressu re exerted by a column of 10 m of seawater is equivalent to a pressure of 1 atm. Pressure increases with increasing depth, so at a depth of 20 m, the pressure of the water will be 2 atm, and so on. The total pressure exerted on the diver is the sum of the atmospheric pressure and the pressure due to the column of water. (a) Consider a diver rising to the surface of the water from a depth of 6 m without breathing. (i) Calculate the total pressure exerted on a diver at 6 m. (ii) Show that, when the diver rises to the surface of the water, the volume of air trapped in the diver’s lungs would be increased by 1.6 times. The most important rule in scuba diving is to breathe continuously, especially when ascending, and to avoid holding breath underwater. This is to avoid barotrauma where physical damage to body tissues is caused by a difference in pressure between a gas space inside, or in contact with the body, and the surrounding waters. The sudden expansion of air can fatally rupture the membranes of the lungs. This is different in free diving, which relies on the diver’s ability to hold his or her breath until resurfacing, rather than