2025 JC2 Prelims H2 Chem Paper 4 QP and Ans TJC

Review of H2 Paper 4 1 Determination of the percentage by mass of iron in the wire Iron wire contains impurities. In this experiment, you will investigate the percentage by mass of iron in a sample of iron wire. A sample of iron wire is reacted with an excess of sulfuric acid to produce a solution of iron(II) sulfate. You will titrate the solution of iron( II) sulfate with potassium manganate( VII) of known concentration to determine the amount of iron(II) ions present and hence percentage by mass of iron in the wire. You may assume that impurities do not react with potassium manganate(VII). Iron(II) ions react with manganate(VII) ions according to the equation shown. 5Fe2+ + MnO4– + 8H+ → 5Fe3+ + Mn2+ + 4H2O FA 1 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4. FA 2 is a diluted solution of FeSO4 prepared as follows: • 48.9 g of iron wire was reacted with sulfuric acid to make 1.00 dm3 of solution. • 34.00 cm3 of the solution was then made up to 250 cm3 with deionised water. FA 3 is dilute sulfuric acid, H2SO4. (a) (i) Procedure 1. Fill the burette with FA 1. 2. Pipette 25.0 cm3 of FA 2 into a 250 cm3 conical flask. 3. Use a measuring cylinder to add 25 cm3 of FA 3 into the conical flask. 4. Add FA 1 from the burette until the solution in the conical flask turns to a permanent pale pink colour. 5. Record your titration results, to an appropriate level of precision in the space on page 3. 6. Repeat steps 2 to 5 until consistent results are obtained.

Titration results Final Burette Reading / cm3 22.80 22.80 Initial Burette Reading / cm3 0.00 0.00 Volume of FA1 / cm3 22.80 22.80   Correct headers with units [1] All burette readings to 0.05 cm3 [1] Correct calculation of titre volumes i.e. final – initial burette reading and consistent results within 0.10 cm3. [1] [3] (ii) From your titrations, obtain a suitable volume of FA 1 to be used in your calculations. Show clearly how you obtained this volume. Average titre volume = (22.80 + 22.80)/2 = 22.80 cm3 Use (at least two) titre values within 0.20 cm3 to correctly calculate average volume of FA 1. Working must be shown or ticks put next to the two (or more) consistent titres selected. [1] Accuracy [2] 2 marks if difference is  0.20 cm3 1 mark if difference is  0.40 cm3 0 mark if difference is  0.40 cm3 volume of FA 1 = ……………………………………… [3] (b) (i) Calculate the amount of iron(II) ions present in 25.0 cm3 of FA 2. Amount of MnO4– used = (𝟐𝟐.𝟖𝟎)(𝟎.𝟎𝟐𝟎𝟎) (𝟏𝟎𝟎𝟎) = 4.56 × 10−4 mol Amount of iron(II) ions present = 4.56 × 10−4 × 5 = 2.28 × 10−3 mol [1] 𝑎𝑛𝑠 𝑖𝑛 (𝑎)(𝑖𝑖) 1000 × 0.0200 × 5 amount of iron(II) ions = …………………………………… [1]

(ii) Calculate the mass of iron present in 25.0 cm3 of FA 2. [Ar: Fe, 55.8] Mass of iron present = 2.28 × 10−3 × 55.8 = 0.127 g [1] ans in (b)(i) × 55.8 mass of iron = …………………………………… [1] (iii) Calculate the percentage by mass of iron in the sample of iron wire. Mass of iron in wire sample = 𝟎.𝟏𝟐𝟕 𝟐𝟓.𝟎 × 𝟐𝟓𝟎 𝟑𝟒.𝟎𝟎 × 1000 = 37.4 g % by mass of iron in wire sample = 𝟑𝟕.𝟒 𝟒𝟖.𝟗 × 100% = 76.5% [1] Mass of iron in the wire sample = 𝑎𝑛𝑠 𝑖𝑛 (𝑏)(𝑖𝑖) 25.0 × 250 34.00 × 1000 [1] % by mass of iron in the wire sample = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑖𝑟𝑜𝑛 𝑖𝑛 𝑡ℎ𝑒 𝑤𝑖𝑟𝑒 48.9 × 100% percentage by mass of iron = …………………………………… [2] (c) A student suggested that when a piece of iron wire was dissolved in a known amount of excess sulfuric acid, the amount of iron that reacted with the acid could be determined by titrating the mixture containing the remaining acid with a known concentration of sodium hydroxide. Explain whether the student was correct. Student was incorrect as Fe2+ in the mixture can react with NaOH(aq) to form Fe(OH)2. [1]. Hence, volume of NaOH(aq) is larger than expected. [1] [Total: 12]

2 Determination of enthalpy change for the reaction between sodium carbonate with water and carbon dioxide You will determine the enthalpy change, Hr, for the reaction of sodium carbonate, Na2CO3, with water and carbon dioxide to form sodium hydrogencarbonate, NaHCO 3, via reactions 1 and 2. Na2CO3(s) + H2O(l) + CO2(g) ⟶ 2NaHCO3(s) Hr In reaction 1, you will react sodium hydrogencarbonate with dilute hydrochloric acid and find the temperature change. NaHCO3(s) + HCl(aq) ⟶ NaCl(aq) + H2O(l) + CO2(g) H1 In reaction 2, the reaction of sodium carbonate with dilute hydrochloric acid will be considered. Na2CO3(s) + 2HCl(aq) ⟶ 2NaCl(aq) + H2O(l) + CO2(g) H2 You will then use the results to calculate the enthalpy change, Hr. FA 4 is sodium hydrogencarbonate, NaHCO3. FA 5 is 2.0 mol dm–3 hydrochloric acid, HCl. Handle all chemicals with care and follow all safely precautions at all times. (a) Reaction 1: Reaction between sodium hydrogencarbonate and dilute hydrochloric acid Read through the method and prepare two suitable tables in the space provided on page 6 to record: • all weighings to an appropriate level of precision • all values of temperature, to an appropriate level of precision • all values of time, recorded to the nearest minute. It is important that you measure each temperature at the specified time. 1. Weigh the container with FA 4 and record the balance reading in your table on page 6. 2. Place the polystyrene cup in the 250 cm3 beaker. 3. Use the 25 cm 3 measuring cylinder to transfer 25 cm 3 of the acid, FA 5, into the polystyrene cup. The acid is in excess. 4. Place the thermometer in the acid and record the initial temperature in your second table on page 6. Tilt the cup if necessary, so that the bulb of the thermometer is fully covered. This is the temperature at t = 0 minute. Start timing. 5. Record the temperature of the acid at 1 minute and at 2 minutes. 6. At time t = 2½ minutes, carefully tip all the FA 4, in small portions to avoid spray, into the acid and stir to dissolve it. Avoid inhalation of the acid vapour. 7. Record the temperature of the solution from t = 3 to 8 minutes at 1 minute intervals. 8. Reweigh the container with any residual FA 4. Record the balance reading and the mass of FA 4 used.

Mass of weighing bottle and FA4 / g 8.76 Mass of weighing bottle and residue FA4 / g 6.33 Mass of FA4 used / g 2.43 time / min 0.0 1.0 2.0 2.5 3.0 4.0 5.0 6.0 7.0 8.0 temp / ºC 33.5 33.5 33.5 27.0 27.5 28.0 28.0 28.5 28.5 [✓] mass table with correct headers and units with reweighing [✓] headers and units for temperature and time. accept if 2.5 min not indicated in table [✓] mass reading in 2 or 3 dp [✓] precision of temp to 1 dp (ignore dp for time) 2[✓] = [1] [1] 3 mass readings + 9 temperature readings but penalise if have temperature reading at 2.5 min + correct trend for endothermic reaction (b) (i) Plot a graph of temperature on the y-axis against time on x-axis on the grid below. You will use the graph to determine the theoretical temperature change at 2½ minutes. The scale for temperature should extend at least 1 ºC below your lowest recorded temperature.

Draw two straight lines of best fit on your graph, one for the temperature of the acid before adding FA 4 and the other for the warming of the solution once the reaction is complete. Extrapolate the two lines to 2½ minutes. [3] [1] axis label and scale chosen such that plotted points occupy more than half the available graph area in both x and y directions. [1] all points plotted to within half a small square [1] two lines of best fit drawn – one for before adding and the other for warming of solution and both lines extrapolated to 2½ minutes + min temp must be lower than minimum data point (ii) Determine the change in temperature at 2½ minutes. [1] T = 26.8 – 33.5 = – 6.7 ºC (record to half smallest division based on graph) [1] correct reading of both initial and lowest temperatures and T calculated

(iii) Use your answer to (b)(ii) to calculate the heat energy abs