2019 DHS Prelim H2 Chem P4 ANS

© DHS 2019 [Turn over Name: Index Number: Class: DUNMAN HIGH SCHOOL Preliminary Examinations 2019 Year 6 H2 CHEMISTRY 9729/04 Paper 4 Practical INSTRUCTIONS TO CANDIDATES 1 Write your name, index number and class on this cover page. 2 Give details of the practical shift and laboratory where appropriate, in the boxes provided. 3 Write in dark blue or black pen. 4 You may use an HB pencil for any diagrams or graphs. 5 Do not use staples, paper clips, glue or correction fluid. 6 Protective eye goggles and gloves must be worn at ALL TIMES. Answer ALL questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. The number of marks is given in brackets, [ ], at the end of each question or part question. Shift Laboratory For Examiner’s Use Question No. Marks 1 17 2 10 3 14 4 14 Total 55

2 © DHS 2019 9729/04 Answer all questions in the spaces provided. 1 Determination of acid concentration and enthalpy change of neutralisation using calorimetry When an acid is added into an alkali, an exothermic reaction takes place and the temperature of the mixture increases. Maximum heat is given out when stoichiometric amounts of H + and OH– are added together. FA 1 is an aqueous solution prepared by mixing equal volumes of y mol dm3 hydrochloric acid, HCl, and y mol dm3 sulfuric acid, H2SO4 FA 2 is 2.00 mol dm3 sodium hydroxide, NaOH In this question, you are to follow the neutralisation of known volumes of FA 2 , NaOH , by measuring the highest temperature obtained as different volumes of FA 1 are added. By measuring the maximum temperature rise for different mixtures of the two reagents, you are to determine the following.  the value of y, concentration of the acids present in FA 1  the enthalpy change of neutralisation, Hneut, for the reaction H+(aq) + OH(aq)  H2O(l) (a) Method 1 Fill the burette with FA 2. 2 Support the Styrofoam cup in a 250 cm3 glass beaker. 3 Run 20.00 cm3 FA 2 from the burette into the Styrofoam cup. Stir and measure the temperature of this FA 2. 4 Measure 20 cm3 FA 1 in a measuring cylinder. 5 Tip the FA 1 in the measuring cylinder into the FA 2 cup, stir and record the maximum temperature obtained in the reaction. 6 Rinse and carefully dry the Styrofoam cup. 7 Repeat steps 1 to 6 three more times, each time using 20.00 cm 3 of FA 2. Use 30.0 cm3, 40.0 cm3 and 50.0 cm3 of FA 1 respectively in these different experiments. Carry out two further experiments. Choose volumes of FA 1 which will allow you to investigate more precisely the volume of FA 1 that produces the highest temperature rise when added to 20.00 cm3 of FA 2. Record your results in an appropriate format in the next page. Record all measurements of volume, temperature and temperature change, T.

3 © DHS 2019 9729/04 (i) Experimental results Expt No. Vol of FA 1/ cm3 Vol of FA 2/ cm3 Temperature of FA 2/ C OR initial temperature/ C Maximum Temperature / C T / C 1 20.0 20.00 2 30.0 20.00 3 40.0 20.00 4 50.0 20.00 5 35.0 20.00 6 25.0 20.00 [5] (ii) Plot T (yaxis) against volume of FA 1 (xaxis) on the grid below. Draw a line of best fit through the points where the temperature rise is increasing and another line through the points where the temperature rise is decreasing. [3]

4 © DHS 2019 9729/04 (iii) Read from the graph the maximum temperature change, Tmax, and the volume, Vneut, of FA 1 needed to obtain this value. Record these values in the spaces provided below. maximum temperature change, Tmax = ……10.0………….C volume of FA 1 used at Tmax = Vneut = ……34.0…………... cm3 [1] (b) Use your results from (a)(iii) to calculate: (i) the concentration, in mol dm3, of the hydrogen ions in FA 1. Moles of OH used = 2.0 x ( 20.00 1000 ) = 0.0400 mol H+(aq) + OH(aq)  H2O(l) Total moles of H+ in FA 1 required for neutralisation = 0.0400 mol Total concentration of H+ in FA 1 = 0.0400 0.0340 = 1.1764 = 1.18 mol dm3 concentration of hydrogen ions in FA 1 = ………1.18 mol dm3 …………… [1] (ii) Hence, the value of y. Since FA 1 is an aqueous solution prepared by mixing equal volumes of y mol dm3 hydrochloric acid, HCl, and y mol dm3 sulfuric acid, H2SO4; mol ratio of H+ from H2SO4 : H+ from HCl 2 : 1 concentration of HCl in FA 1 = 1 3 x (1.1764) = 0.392 mol dm3 Since equal volumes of both acids were mixed to obtain FA 1 Concentration of HCl(aq) before dilution = y = 2 x 0.392 = 0.784 mol dm3 value of y is …0.784 ………………….. [1] (iii) the heat change for the reaction and hence the enthalpy change of neutralisation, Hneut, for the reaction H+(aq) + OH(aq)  H2O(l) [Assume that 4.18 J of energy is needed to raise the temperature of 1 g of the solution by 1 K] Heat released = mcTmax = (20.00 + 34.0)(4.18)(10.0) = 2257.2 J Hneut =  2257.2 0.0400 = 56430 J mol1 = 56.4 kJ mol1 [4]

5 © DHS 2019 9729/04 (c) A student suggested using a burette rather than a measuring cylinder to measure the volume of FA 1 to improve the precision in volume measurement. However, he was advised not to do so by his teacher. Suggest a disadvantage of using a burette and explain how the calculated Hneut would be affected if volume of FA 1 had been added from a burette. Disadvantage Takes longer to add FA 1 into FA 2. Impact on Hneut Since addition of FA 1 from a burette is a slow process, heat is lost during the slow process. Heat loss reduces Tmax and would result in a lower value for the heat change and so a lower numerical / less exothermic value for Hneut. [2] [Total:17] 2 Determination of calcium ion concentration using complexometric titration The calcium content of milk and tap water and the amount of calcium carbonate in various solid samples can be determined using complexometric titrations. This method uses ethylenediaminetetraacetic acid, EDTA, which forms a complex with calcium ions in a 1:1 ratio. A blue dye called Patton –Reeder indicator is used to identify the end –point. The dye forms a pink complex with calcium ions. As this complex is less stable than the EDTA complex, the dye is displaced by EDTA as the titration proceeds . The solution turns blue at the end–point due to the uncomplexed dye. FA 3 is a brand of milk. In 2(a), you perform titrations to determine the calcium content of milk. You are also provided with FA 2, 2.00 mol dm3 sodium hydroxide, NaOH FA 4, 0.0170 mol dm–3 EDTA Patton–Reeder indicator As EDTA is harmful to the environment, FA 4 should be disposed in the waste bottle. You should also wear gloves throughout the experiment. (a) (i) 1. Fill the burette with FA 4. 2. Use a pipette to transfer 10.0 cm3 of FA 3 into a 250 cm3 conical flask. 3. Using appropriate measuring cylinders, add 36.0 cm 3 of deionised water then 8.0 cm 3 of FA 2 into the conical flask. Swirl and allow the solution to stand for about 2 minutes. 4. Add half a spatula of Patton–Reeder indicator into the conical flask and swirl the mixture to dissolve the indicator. The mixture is pink at this point. 5. Run FA 4 from the burette into the conical flask. The end–point is reached when the mixture loses