VJC H2 CHEM P2 Answers Prelim

 VJC 2014 9647/02/PRELIM/14 [Turn over 2014 Preliminary Examination H2 Chemistry Paper 2 1 Planning (P) Aqueous chlorine, Cl2, displaces iodine, I2, from aqueous potassium iodide. If a solution of chlorine is mixed with an excess of aqueous potassium iodide, iodine is displaced in a 1:1 molar ratio with chlorine. The concentration of chlorine in the original solution can therefore be calculated from the concentration of the displaced iodine. I2(aq) + 2Na2S2O3(aq)  2NaI(aq) + Na2S4O6(aq) (a) Write an equation for the reaction between aqueous ch lorine, and an excess of aqueous potassium iodide. Cl2(aq) + 2KI(aq)  I2(aq) + 2KCl(aq) [1] [Accept ionic equation, state symbols are optional.] (b) You are to plan an experiment to determine as accurately as possible the concentration o f a saturated aqueous solution of chlorine by titration. The approximate solubility of chlorine is 5 g dm–3 at 25 oC. You are provided with the following materials:  saturated aqueous chlorine;  solid sodium thiosulfate Na2S2O3.5H2O;  concentrated aqueous potassium iodide. This will be used in excess.  the apparatus and chemicals normally found in a school or college laboratory. Your plan should include details of:  a calculation of the approximate concentration of saturated aqueous chlorine in mol dm–3 at 25 oC; [Ar: Cl, 35.5]  a detailed description of the method for preparing a solution of aqueous sodium thiosulfate that can be used in the titration. In a titration, it is usual for the titre volume to be approximately equal to that of the volume of solut ion pipetted. Calculate the mass of sodium thiosulfate, Na 2S2O3.5H2O, which will produce a solution suitable for use in this titration. The relevant calculations and reasoning must be shown in full; [Ar: H, 1.0; O, 16.0; Na, 23.0; S, 32.1]  a step–by–step description of how you would carry out sufficient titrations using a suitable indicator to allow an accurate end–point to be obtained;  an outline calculation to show how the results are to be used to determine the accurate concentration of the aqueous chlorine. Assume that the titre volume used is w cm3. Approximate concentration of saturated Cl2(aq) = = 0.0704 mol dm–3 Assume 10.0 cm3 of saturated Cl2(aq) is pipetted in each titration. No. of moles of saturated Cl2(aq) used = 0.0704 x = 7.04 x 10–4 mol = no. of moles of I2 formed

 VJC 2014 9647/02/PRELIM/14 2 No. of moles of Na2S2O3 needed for titration = 2 x 7.04 x 10–4 = 1.41 x 10–3 mol Assume 10.0 cm 3 of Na 2S2O3 is needed for each titration and 100 cm 3 volumetric flask is used to prepare Na2S2O3 solution. Concentration of Na2S2O3 needed = 1.41 x 10–3  = 0.141 mol dm–3 No. of moles of Na2S2O3.5H2O needed = 0.141 x = 0.0141 mol Mass of Na2S2O3.5H2O needed = 0.0141 x [2(23.0) + 2(32.1) + 3(16.0) + 5(18.0)] = 3.50 g [Accept other combinations of pipette and volumetric flask volumes with correct justification.] Procedure [mass and volumes ecf from above justification]: Step 1: Accurately weigh 3.50 g of Na2S2O3.5H2O in a weighing bottle and completely dissolve it in distilled water using a beaker. Pour the solution and the washings into a 100 cm3 volumetric flask and make up to the mark with distilled water. Shake well to obtain a homogenous solution. Label this solution FA1 and pour it into a burette. Step 2: Pipette 10.0 cm3 of saturated aqueous chlorine into a conical flask. Step 3: Using a measuring cylinder, add 10 cm 3 of concentrated K I(aq) into the conical flask. Step 4: Titrate the iodine liberated in t his solution with FA1. When the colour of the solution turns pale yellow, add about 1 cm 3 of starch indicator. The solution will turn blue–black. Continue the titration and the end –point is reached when the colour just disappears. Record the results. Step 5: Repeat steps 2 to 4 as many times as necessary to obtain consistent results to within ±0.10 cm3. Calculations: Assume 10.0 cm3 of Cl2(aq) reacted with exactly w cm3 of FA1 of concentration 0.141 mol dm–3. No. of moles of FA1 used = 0.141 x = 1.41w x 10–4 mol No. of moles of I2 formed = 1.41w x 10–4  2 = 7.05w x 10–5 mol = no. of moles of Cl2(aq) used

 VJC 2014 9647/02/PRELIM/14 [Turn over 3 Concentration of Cl2(aq) used = 7.05w x 10–5  = 7.05w x 10–3 mol dm–3 [10] (c) State one hazar d that must be considered when planning the experiment and describe a precaution that should be taken to keep risks from this hazard to a minimum. Chlorine gas escapes from saturated aqueous chlorine which is poisonous. Perform the titration in a fume cupboard. OR Chlorine gas escapes from saturated aqueous chlorine which causes eye irritation. Perform the titration in a fume cupboard. OR Saturated aqueous chlorine is acidic/oxidising. Wear safety gloves when performing the titration. [1] [Total: 12] 2 (a) On Planet Uranus, it is postulated that the number of subshells associated with each principal quantum number and the respective energy levels of the subshells are similar to that on Earth. Each orbital contains a maximum of two electron s. However, the number of orbitals that make up a subshell may or may not be identical to that on Earth. Figure 1 represents the sketch of the successive ionisation energies of all the electrons of an element T on Planet Uranus. Figure 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 No. of ionisation log IE I I I I I I I I I I I I I I I I I I I Figure 1 No. of electrons removed lg IE 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

 VJC 2014 9647/02/PRELIM/14 4 (i) By interpre ting figure 1, suggest with reasoning, which period of the Periodic Table element T belongs to. Since there are a total of three large increments between successive ionisation energies, this implies that the 3 rd, 12 th and 18 th electrons are removed fro m different principal quantum shells. Hence, T belongs to the fourth period of the Periodic Table. (ii) Using figure 1, deduce the number of 2p orbitals present in element T. The 18th and 19th IE belong to 1s subshell. OR The 16th and 17th IE belong to 2s subshell. Thus, 12th to 15th IE, i.e. four electrons are removed from a 2p subshell. Since each orbital can accommodate a maximum of 2 electrons, there are two 2p orbitals. (iii) State the full electronic configuration of element T. 1s2 2s2 2p4 3s2 3p4 3d3 4s2 [3] (b) The hydrolysis of ester to form acid and alcohol is an equilibrium reaction. Using ethyl ethanoate as an example, the reaction for the hydrolysis is shown below. CH3CO2CH2CH3(l) + H2O(l) ⇌ CH3CO2H(l) + CH3CH2OH(l) Draw two labelled Maxwell–Boltzmann distribution curves to illustrate clearly how the addition of concentrated sulfuric acid and heating of reaction mixture can increase the rate of hydrolysis. (Written explanation is not required.) Addition of concentrated sulfuric acid: Heating of reaction mixture: Correct axes for both Correct graph for addition of concentrated sulfuric acid with illustration/shading Correct graphs for heating of reaction mixture [3] number of molecules with a given energy energy Ea T1 T2