2024 ACJC H2 Chemistry Prelim P4 (QP)

2 © ACJC 2024 9729/04/Prelim/2024 [Turn over Answer all the questions in the spaces provided. 1 To determine the concentration of NaOH and Na2CO3 in FA 1 Aqueous NaOH can be used to remove CO 2 from exhaled air in spacecrafts. NaOH reacts with CO2 according to the following equation: 2NaOH(aq) + CO2(g) → Na2CO3(aq) + H2O(l) A sample of aqueous NaOH was used to absorb CO2 in the air for some time. The resulting mixture of NaOH and Na2CO3 formed was labelled as FA 1. You are to determine the concentration of NaOH and Na 2CO3 in FA 1 by titration with an aqueous solution of HCl, FA 2. You are provided with the following. FA 1, is a mixture of NaOH and Na2CO3 FA 2, 2.50 mol dm−3 of hydrochloric acid, HCl methyl orange indicator thymol blue indicator distilled water (a) Titration of FA 1 against FA 2 using thymol blue indicator. 1. Fill a burette with FA 2. 2. Using a pipette, transfer 25.0 cm3 of FA 1 into a 250 cm3 conical flask. 3. Add a few drops of thymol blue indicator into the conical flask. 4. Run FA 2 from the burette into the flask. The end point is reached when the solution changes from blue to green. 5. Record your titration results in Table 1.1 6. Repeat points 2 to 5 until consistent titre values are obtained. Table 1.1 1 2 3 final burette reading / cm3 13.80 13.80 initial burette reading / cm3 0.00 0.00 volume of FA 2 added / cm3 [3]

3 © ACJC 2024 9729/04/Prelim/2024 [Turn over From your titrations, obtain a suitable volume of FA 2 to be used in your calculations. Show clearly how you obtained this volume. volume of FA 2 = ……………………….. [1] (b) Titration of FA 1 against FA 2 using methyl orange indicator. Repeat points 1 to 6 in (a) but add methyl orange at point 3 in place of thymol blue. Using this indicator, the end point is reached when the solution changes from yellow to orange. Record your titration results in Table 1.2. Table 1.2 1 2 3 final burette reading / cm3 18.80 19.00 initial burette reading / cm3 0.00 0.20 volume of FA 2 added / cm3 [3] From your titrations, obtain a suitable volume of FA 2 to be used in your calculations. Show clearly how you obtained this volume. volume of FA 2 = ……………………….. [1] (c) Calculation When thymol blue is used as the indicator in part (a), the following reactions occur. Reaction 1: NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) Reaction 2: Na2CO3(aq) + HCl(aq) → NaCl(aq) + NaHCO3(aq) When methyl orange is used as the indicator in part (b), the following reactions occur. Reaction 1: NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) Reaction 2: Na2CO3(aq) + HCl(aq) → NaCl(aq) + NaHCO3(aq) Reaction 3: NaHCO3(aq) + HCl (aq) → NaCl(aq) + H2O(l) + CO2(g) The NaHCO3 in Reaction 3 is the product from reaction of Na 2CO3 with H Cl in Reaction 2.

4 © ACJC 2024 9729/04/Prelim/2024 [Turn over (i) Calculate the volume of hydrochloric acid that reacted with NaHCO3 in 25.0 cm3 of FA 1 in Reaction 3. volume of hydrochloric acid = ……………………..[1] (ii) By considering Reactions 2 and 3, determine the volume of hydrochloric acid that reacted with sodium carbonate in 25.0 cm3 of FA 1 to form NaHCO3. volume of hydrochloric acid = ……………………….. [1] (iii) Calculate the volume of hydrochloric acid that reacted with the sodium hydroxide present in 25.0 cm3 of FA 1. volume of hydrochloric acid = ……………………….. [1] (iv) Using your answer in (c)(ii) and the equation for Reaction 2, calculate the amount of sodium carbonate present in 25.0 cm3 of FA 1. amount of sodium carbonate = ……………………….. [1] (v) Calculate the concentration of sodium carbonate, Na2CO3 in FA 1. concentration of sodium carbonate = ……………………….. [1] (vi) From your answer to (c)(iii) and the equation for Reaction 1 , calculate the amount of sodium hydroxide present in 25.0 cm3 of FA 1. amount of sodium hydroxide = ……………………….. [1]

5 © ACJC 2024 9729/04/Prelim/2024 [Turn over (vii) Calculate the concentration of sodium hydroxide, NaOH in FA 1. concentration of sodium hydroxide = ……………………….. [1] (d) Give one reason why after 25.0 cm3 of FA 1 is pipetted into a conical flask, the stock solution FA 1 should be immediately covered with a sealant film. State the effect of not covering FA 1 with a sealant film on the calculated concentration of NaOH. ………..……………………………………………………………….……………….………. ………..……………………………………………………………….……………….………. ………………………………………………………………………………….....…………[2] [Total: 17]

6 © ACJC 2024 9729/04/Prelim/2024 [Turn over 2 Determination of the identity of the acid and the enthalpy change of neutralisation, Hneut You are provided with the following solutions: FA 3 is a solution of 1.60 mol dm-3 sodium hydroxide, NaOH FA 4 is a solution of 2.00 mol dm-3 of either hydrochloric acid, HCl or sulfuric acid, H2SO4. In this question, you are to perform a series of 6 experiments where different volumes of FA 3 and FA 4 are mixed to give a total volume of 50 cm 3. The temperature change, T, of the reaction mixture for each experiment will be determined and a graph of T against the volume of FA 3 will be plotted. You will then use the data from your graph to determine the identity of the acid and the enthalpy change of neutralization for the reaction between aqueous sodium hydroxide and the acid. (a) Procedure Experiment 1 1) Place one polystyrene cup inside a second polystyrene cup. Place these into a 250 cm3 beaker to prevent them from tipping over. 2) Use a measuring cylinder to transfer 10.0 cm3 of FA 3 into the polystyrene cup. 3) Measure the temperature of FA 3 in the polystyrene cup and record the initial temperature of FA 3, T1, in Table 2.1 on the next page. 4) Use another measuring cylinder to transfer 40.0 cm 3 of FA 4 into the same polystyrene cup. 5) Stir the mixture in the polystyrene cup with the thermometer. Measure and record the highest temperature, T2 in Table 2.1 on the next page. 6) Rinse the polystyrene cup and thermometer with distilled water and dry them with paper towels. Experiment 2 to 4 7) Repeat steps 1 to 6 using 20.0 cm3, 30.0 cm3 and 40.0 cm 3 of FA 3, each time using an appropriate volume of FA 4, such that the total volume of the reaction mixture is 50.0 cm3. 8) Record all measurements of volumes of FA 3 and FA 4, temperature (T1 and T2) and temperature change, T, in Table 2.1 on the next page.

7 © ACJC 2024 9729/04/Prelim/2024 [Turn over (b) Results Table 2.1 experiment 1 2 3 4 5 6 volume of FA 3 / cm3 10.0 20.0 30.0 40.0 25.0 35.0 volume of FA 4 / cm3 40.0 initial temperature, T1 / oC 31.0 31.0 31.0 31.0 31.0 31.0 final temperature, T2 / oC 35.6 40.0 42.0 36.6 42.4 39.6 temperature change, T / oC [3] (c) (i) Plot a graph of T (y–axis) against volume of FA 3 used (x–axis) in Fig 2.1 using the four experimental results that you have obtained. The scales for both axes must be chosen to provide an origin.

8 © ACJC 2024 9729/04/Prelim/2024 [Turn over Fig. 2.1 By considering your plotted points, select two other volumes of FA 3 that will allow you to draw two best fit lines to identify the volume of FA 3 needed to produce the maximum temperature change, Tmax. In each experiment, ensure