Copy of ACSBR 6092 Sec 4 Prelims 2024 P3

Anglo-Chinese School (Barker Road) Preliminary Examination 2024 Secondary 4 Express Chemistry 6092/3 2 1 (a) When sodium carbonate is crystallised from solution, it forms crystal s of hydrated sodium carbonate, Na2CO3•xH2O, where x is the amount of water of crystallisation present. You are going to determine the amount of water of crystallisation in Na2CO3•xH2O. In this question, you will perform a titration and the equation for the reaction in the titration is as follows: Na2CO3 + 2HCl → 2NaCl + CO2 + H2O The data from the titration will then be used to calculate: • the molar concentration of Na2CO3 in Q, • the relative molecular mass, Mr, of Na2CO3•xH2O and hence the value of x. Read all the instructions below carefully before starting the experiment in Question 1. Instructions P is 1.50 mol/dm3 aqueous hydrochloric acid. Q contains 200 g/dm3 of Na2CO3•xH2O in deionised water. (i) Fill the burette with P. Use the pipette to transfer 25.0 cm3 of Q into a conical flask. Add a few drops of the methyl orange indicator to the solution in the conical flask. Add P from the burette, swirling the flask constantly. At the end-point, one drop of P will turn methyl orange from yellow to orange. Record your titration results in the table below. Repeat the titration as many times as you consider necessary to achieve consistent results. Results [5]

Anglo-Chinese School (Barker Road) Preliminary Examination 2024 Secondary 4 Express Chemistry 6092/3 3 (ii) From your titration results, obtain an average volume of P to be used in your calculations. Show clearly how you obtained this volume. average volume of P = …………….……… cm3 [1] (iii) P is 1.50 mol/dm3 aqueous hydrochloric acid. Use your answer from (a)(ii) and the equation for the reaction to c alculate the molar concentration of Na2CO3 in Q. molar concentration of Q = …………………… mol/dm3 [2] (iv) Use your answer from (a)(iii) to calculate the Mr of Na2CO3•xH2O to 1 decimal place; and hence the value of x to the nearest whole number. You must show your working. [Mr: Na2CO3, 106; H2O, 18] Mr of Na2CO3•xH2O = …………….……… x = …………….……… [2]

Anglo-Chinese School (Barker Road) Preliminary Examination 2024 Secondary 4 Express Chemistry 6092/3 4 (v) Before the start of the experiment, a student accidentally rinsed the pipette with solution P instead of solution Q, before filling it up with solution Q. Describe and explain the effect this would have on the amount of water of crystallisation. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. …………………………………………………………………………………………... [2]

Anglo-Chinese School (Barker Road) Preliminary Examination 2024 Secondary 4 Express Chemistry 6092/3 5 (b) Gravimetric analysis is a laboratory technique used to determine the amount of a substance by measuring its mass. The amount of water of crystallisation in sodium carbonate crystals can also be determined gravimetrically. The water of crystallisation in sodium carbonate crystals can be removed by heating. Sodium carbonate crystals do not decompose at temperatures achievable by a Bunsen flame. Use this information to outline a step -wise procedure to determine the value of x in sodium carbonate crystals, Na2CO3•xH2O, gravimetrically. In your procedure you should include: • the apparatus you would use, • the measurements you would take (tabulated in a recording table), and • explain how your measurements would help to determine the value of x. You can assume the apparatus and reagents normally found in school laboratory are available. ………………………………………………………………………………………………………….. ………………………………………………………………………………………………………….. ………………………………………………………………………………………………………….. ………………………………………………………………………………………………………….. ………………………………………………………………………………………………………….. ………………………………………………………………………………………………………….. ………………………………………………………………………………………………………….. ………………………………………………………………………………………………………….. ………………………………………………………………………………………………………….. ………………………………………………………………………………………………………….. ………………………………………………………………………………………………………….. ………………………………………………………………………………………………………….. ………………………………………………………………………………………………………….. ………………………………………………………………………………………………………….. ………………………………………………………………………………………………………….. …………………………………………………………………………………………………… [6] [Total: 18]

Anglo-Chinese School (Barker Road) Preliminary Examination 2024 Secondary 4 Express Chemistry 6092/3 6 2 The surrounding temperature changes when sodium carbonate reacts with aqueous hydrochloric acid. In this experiment, you will investigate the enthalpy change of this reaction. Na2CO3 + 2HCl → 2NaCl + CO2 + H2O Read all the instructions below carefully before starting the experiment in Question 2. Instructions 1. Weigh the container labelled X with the sodium carbonate. 2. Using a measuring cylinder, add 100 cm 3 of hydrochloric acid, which is an excess, to the Styrofoam cup and measure the temperature of the acid. Record the initial temperature in the table. 3. Start a stopwatch and measure the temperature of the acid every minute for 2 minutes. Record the temperatures. 4. At 3 minutes, transfer the sodium carbonate from the container to the Styrofoam cup. Stir the contents and continue to measure, and record, the temperature every minute up to 8 minutes. 5. Reweigh the container with any residual solid. (a) (i) Record your results below: mass of container with sodium carbonate = …………….. g mass of container after adding sodium carbonate to the cup = …………….. g time / min 0 1 2 3 4 5 6 7 8 temperature / oC [2]

Anglo-Chinese School (Barker Road) Preliminary Examination 2024 Secondary 4 Express Chemistry 6092/3 7 (ii) Use your results to plot a graph of temperature against time on the grid below. Complete the graph by drawing two straight lines of best fit. One line should be drawn using data from 0 to 2 minutes and the other line should be drawn using data from 4 to 8 minutes. Extend both of these lines to 3 minutes. Draw a vertical line at 3 minutes to connect these lines. Use your lines from the graph to calculate the maximum temperature change in the experiment. maximum temperature change = …………….……… oC [5]

Anglo-Chinese School (Barker Road) Preliminary Examination 2024 Secondary 4 Express Chemistry 6092/3 8 (iii) The amount of heat change in this experiment can be calculated using the expression shown. heat change (in J) = mass of solution (in g) x maximum temperature change (in oC) x 4.2 Use the expression to calculate, in J, the amount of heat change. Assume that 1.0 cm3 of solution has a mass of 1.0 g. amount of heat change = ……………………… J [1] (iv) Calculate the number of moles of sodium carbonate used in the experiment. [Mr: Na2CO3, 106] number of moles of Na2CO3 = ……………………… mol [1] (v) Calculate the enthalpy change of reaction, in kJ/mol, when one mole of sodium carbonate reacts with hydrochloric acid. Include a sign to indicate if this is an exothermic or endothermic reaction. enthalpy change of reaction = ……………………… kJ/mol [2]

Anglo-Chinese School (Barker Road) Preliminary Examination 2024 Secondary 4 Express Chemistry 6092/3 9 (vi) State one source of error in the results obtained and explain the effect this would have on the value of the enthalpy change of reaction calc