RVHS 2026 Prelim Paper 4 QP

1 River Valley High School 9476/04/Prelims/26 [Turn over RIVER VALLEY HIGH SCHOOL Preliminary Examination CANDIDATE NAME CLASS CENTRE NUMBER S INDEX NUMBER CHEMISTRY 9476/04 Paper 4 Practical 2 hours 30 minutes You must answer on the question paper. You will need: The materials and apparatus listed in the confidential instructions INSTRUCTIONS • Answer all questions. • Use a black or dark blue pen. You may use an HB pencil for any diagrams or graphs. • Write your name, centre number and index number in the boxes at the top of the page. • Write your answer to each question in the space provided. • Do not use an erasable pen. Do not use correction fluid or tape. • You may use an approved calculator. • Qualitative analysis notes are printed in the question paper. • Write the details of the shift and laboratory in the boxes provided. INFORMATION • The total mark for this paper is 50. • The number of marks for each question or part question is shown in brackets [ ]. This document has 20 pages. Any blank pages are indicated. Shift Laboratory For Examiner’s Use 1 2 3 4 Total

2 River Valley High School 9476/04/Prelims/26 Answer all the questions in this section in the spaces provided. 1 To determine the percentage by mass of copper in an alloy Copper forms compounds containing Cu 2+ and Cu + ions. The compounds containing Cu 2+ ions tend to be relatively stable. The addition of an excess of potassium iodide, KI, to a solution of Cu2+ ions produces iodine, I2, and a stable precipitate of Cu I. For titration to be accurate, it is necessary that all Cu 2+ ions are reduced to Cu+ ions. The I2 turns the solution brown. Equation 1: 2Cu2+ + 4I− → 2CuI + I2 I2 has a relatively low solubility in water. However, the presence of an excess of I− ions in the reaction mixture allows the soluble tri -iodide ion, I3−, to form as shown by equation 2. This ensures that the I2 formed as shown in equation 1 is fully dissolved. Equation 2: I2 + I− → I3− The I3− ions formed may be titrated against a standard solution of Na 2S2O3 as shown in equation 3. Equation 3: I3− + 2S2O32− → S4O62− + 3I− In 1(a), you will perform titrations to determine the percentage by mass of copper present in an alloy. FA 1 is an aqueous solution containing 11.54 g dm−3 of the alloy. You are also provided with FA 2, 0.100 mol dm−3 sodium thiosulfate, Na2S2O3, FA 3, 0.220 mol dm–3 potassium iodide, starch indicator. In this experiment, you will determine the percentage by mass of copper in the alloy used to prepare FA 1. You will titrate FA 1 against FA 2. (a) (i) Titration of FA 1 against FA 2 1. Fill a burette with FA 2. 2. Use a 25.0 cm 3 pipette to transfer 25.0 cm 3 of FA 1 into a 250 cm 3 conical flask. 3. Use a measuring cylinder to add about 40 cm 3 of FA 3 to this flask. A white precipitate forms in a brown solution. 4. Run FA 2 from the burette into this flask. Near end point, when the brown solution becomes pale, add about 1 cm3 of starch indicator. 5. Continue adding FA 2 slowly. The end point is reached when the solution first becomes colourless. The white precipitate remains. 6. Record your titratio n results, to an appropriate level of precision, in the space provided. 7. Repeat steps 2 to 6 until consistent results are obtained.

3 River Valley High School 9476/04/Prelims/26 [Turn over Results I II III IV V (ii) From your titrations, obtain a suitable volume of FA 2, VFA 2, to be used in your calculations. Show clearly how you obtained this volume. VFA 2 = …………………………….. [1] (b) Calculate the mass of copper in the 11.54 g of the alloy used to prepare 1.00 dm 3 of FA 1, using your answer in 1 (a)(ii). Hence, determine the percentage by mass of copper in the alloy. [Ar: Cu. 63.5] Mass of copper in 11.54 g of the alloy = …………………………….. [2] Percentage by mass of copper in the alloy = …………………………….. [1]

4 River Valley High School 9476/04/Prelims/26 (c) A large excess of potassium iodide is used in each titration. Calculate the ratio of the amount of iodide ions added in each titration to the minimum amount of iodide ions required to reduce the copper(II) ions used. Ratio = …………. : …………. [2] (d) Suggest how the titration would be affected if the pota ssium iodide solution was not added in excess. …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………….............................. [1] [Total: 12]

5 River Valley High School 9476/04/Prelims/26 [Turn over 2 To determine order of reaction with respect to thiosulfate ions, S2O32– You are provided with the following reagents. FA 4, containing 37.0 g dm–3 of hydrated sodium thiosulfate, Na2S2O3.5H2O, FA 5, 2.0 mol dm–3 hydrochloric acid, HCl, deionised water. Aqueous thiosulfate ions, S2O32–, decompose in the presence of hydrogen ions, H+. S2O32–(aq) + 2H+(aq) → S(s) + SO2(g) + H2O(l) The rate of reaction can be followed by observing the formation of a fixed amount of sul fur as a fine suspension of solid particles in the solution. The rate of decomposition at a fixed temperature is affected by the • concentration of thiosulfate ions, [S2O32–] • concentration of hydrogen ions, [H+] In a series of experiments where the concentration of a reagent is changed • 1/time can be used as a measure of rate • the volu me of the reagent used can be taken as a measure of its concentration, providing the total volume of the mixture is kept constant in each experiment The rate equation, where m is the rate order with respect to S2O32–, can be simplified to rate = k[S2O32–]m Taking logarithms of the factors in this equation gives lg(rate) = m lg[S2O32–] + lg k By substituting for rate and concentration, as described above, the following is derived lg(1/t) = m lg(volume of FA 4) + constant Hence, by plotting a graph of lg(1/ t) against lg(volume of FA 4), a best-fit straight line graph can be drawn, the gradient of which is the required rate order. The order of reaction with respect to thiosu lfate ions can be obtained by plotting a graph of lg(1/t) against lg(volume of FA 4).

6 River Valley High School 9476/04/Prelims/26 (a) Method Experiment 1 • Fill the burette, to be labelled as FA 4, with FA 4. • Run 50.00 cm3 of FA 4 from the burette into a dry 250 cm3 beaker. • Measure 5 cm3 of FA 5 in a 10 cm3 measuring cylinder. • Tip the acid FA 5 from the measuring cylinder into the beaker and immediately start a stopwatch • Swirl the beaker to mix the reagents and place the beaker over the printing on the insert provided. • Observe the printing by looking down through the solution in the beaker. • Stop the stopwatch when the printing is just no longer visible. • Record the time, t, taken to the nearest second. Note: Small amounts of SO 2 will be produced during the reaction. Minimise inhalation of SO 2. Discard the reaction mixture immediately down the sink after recording the time. Experiment 2 • Empty and rinse the beaker used in experiment 1. • Carefully dry the beaker with a paper towel. • Refill the burette labelled FA 4 with FA 4. • Fill the second burette, to be labelled as water, with deionised water. • Run 15.00 cm3 of FA 4 and 35.00 cm3 of deionised water from the burettes into the 250 cm3 beaker. • Measure 5 cm3 of FA 5 in a 10 cm3 measuring cylinder. • Tip the acid FA 5 into the beaker, start the stopwatch, mix the reagents and place over the printing as in experiment 1. • Stop the stopwatch when the printing is just no longer visible. • Record the time taken, t, to the nearest second. (b) In experiment 1 you will have obtained the time taken for a ‘fast’ reaction and in experiment 2 the time taken for a ‘slow’ reaction. You are to