Chemistry 0620/53 May/June 2025: question paper and mark scheme

Question

You are going to investigate how the rate of reaction between dilute hydrochloric acid and aqueous sodium thiosulfate changes as the concentration of the aqueous sodium thiosulfate decreases. During the reaction, the solution slowly becomes cloudy. As the solution becomes cloudy, it becomes more difficult to see through the solution. Read all of the instructions carefully before starting the experiments. Instructions You are going to do five experiments. Experiment 1 • Use the 50 cm3 measuring cylinder to pour 50 cm3 of aqueous sodium thiosulfate into a 100 cm3 beaker. • Use the 10 cm3 measuring cylinder to pour 5 cm3 of dilute hydrochloric acid into the beaker containing the aqueous sodium thiosulfate. • Immediately start the stop-watch and stir the contents of the beaker. • Stand the beaker on the printed insert and look down from above the beaker as shown in Fig. 1.1. eye 100 cm3 beaker aqueous sodium thiosulfate and dilute hydrochloric acid mixture printed insert printed text Fig. 1.1 • When the text on the printed insert is not visible, stop the stop-watch and record the time to the nearest whole number of seconds in Table 1.1. • Empty the contents of the beaker into the stop-bath and rinse the beaker with distilled water. Experiment 2 • Repeat Experiment 1 using 40 cm3 of aqueous sodium thiosulfate instead of 50 cm3. Add 10 cm3 of distilled water to the beaker, using the 50 cm3 measuring cylinder, before adding the dilute hydrochloric acid. Experiment 3 • Repeat Experiment 2 using 30 cm3 of aqueous sodium thiosulfate and 20 cm3 of distilled water. Experiment 4 • Repeat Experiment 2 using 25 cm3 of aqueous sodium thiosulfate and 25 cm3 of distilled water. Experiment 5 • Repeat Experiment 2 using 20 cm3 of aqueous sodium thiosulfate and 30 cm3 of distilled water. (a) Complete Table 1.1. Table 1.1 volume of volume of volume of dilute time taken for aqueous sodium experiment distilled water hydrochloric text to not be thiosulfate / cm3 acid / cm3 visible / s / cm3 1 50 0 5 2 3 4 5 [4] (b) Write a suitable scale on the y-axis and plot your results from Experiments 1 to 5 on Fig. 1.2. Draw a curve of best fit. time taken for text to not be 20 25 30 35 40 45 50 volume of aqueous sodium thiosulfate / cm3 Fig. 1.2 [4] (c) The relative rate of reaction in each experiment is calculated using the equation shown. 1 relative rate of reaction = time taken (i) Calculate the relative rate of reaction in Experiment 1. Do not give units for your answer. relative rate of reaction = …… [1] (ii) State in which Experiment, 1, 2, 3, 4 or 5, the relative rate of reaction was greatest. …… [1] (d) In each experiment the total volume of aqueous sodium thiosulfate and distilled water is constant. (i) Calculate the volume of distilled water needed when the volume of aqueous sodium thiosulfate is 37 cm3. volume = …… [2] (ii) From your graph in Fig. 1.2, deduce the time for the text to not be visible when the volume of aqueous sodium thiosulfate is 37 cm3. Show clearly on Fig. 1.2 how you worked out your answer. time = …… s [2] (iii) Explain why the total volume of aqueous sodium thiosulfate and distilled water is kept constant. …… [1] (e) (i) State why measuring the volume of the dilute hydrochloric acid with a burette instead of a measuring cylinder is an improvement. …… [1] (ii) Explain why it is not possible to use a volumetric pipette to measure the volume of aqueous sodium thiosulfate in each experiment. …… [1] (f) Explain why it is important not to change the size of the beaker to a larger beaker during the investigation. …… [2] [Total: 19]

Question

You are provided with two solids: solid L and solid M. Do the following tests on solid L and solid M. Record all of your observations at each stage. Tests on solid L Add about 3 cm depth of distilled water to the boiling tube containing solid L. Replace the stopper in the boiling tube and shake the boiling tube to dissolve solid L and form solution L. Divide solution L into four approximately equal portions in three boiling tubes and one test-tube. (a) To the first portion of solution L in a boiling tube, add about 2 cm depth of aqueous sodium carbonate. Record your observations. …… [2] (b) To the second portion of solution L in a boiling tube, add aqueous sodium hydroxide dropwise and then in excess. Record your observations. dropwise …… in excess …… [2] (c) To the third portion of solution L in a boiling tube, add aqueous ammonia dropwise and then in excess. Record your observations. dropwise …… in excess …… [2] (d) To the fourth portion of solution L in the test-tube, add about 1 cm depth of dilute nitric acid followed by a few drops of aqueous silver nitrate. Record your observations. …… [1] (e) Identify solid L. …… [2] Tests on solid M (f) Carry out a flame test on solid M. Record your observations. …… [1] (g) Place the remaining solid M in a boiling tube. Add about 5 cm depth of distilled water to the boiling tube. Put a stopper in the boiling tube and shake the boiling tube to dissolve solid M and form solution M. Divide solution M into two approximately equal portions in two boiling tubes. (i) To the first portion of solution M, add about 1 cm depth of aqueous sodium hydroxide and a piece of aluminium foil. Warm the mixture and test any gas given off. Record your observations. …… [2] (ii) To the second portion of solution M, add a few drops of dilute sulfuric acid. Record your observations. …… [1] (h) Identify solid M. …… [2] [Total: 15]

Question

Lead(II) chloride, PbCl 2, is an insoluble salt. Solid lead(II) chloride can be made by reacting aqueous sodium chloride with aqueous lead(II) nitrate. 2NaCl (aq) + Pb(NO3)2(aq) 2NaNO3(aq) + PbCl 2(s) Describe how to make a pure, dry sample of solid lead(II) chloride. You are provided with solid sodium chloride, solid lead(II) nitrate, distilled water and common laboratory apparatus. …… [6] Notes for use in qualitative analysis Tests for anions anion test test result carbonate, CO32– add dilute acid, then test for carbon effervescence, carbon dioxide dioxide gas produced chloride, Cl – acidify with dilute nitric acid, then white ppt. [in solution] add aqueous silver nitrate bromide, Br – acidify with dilute nitric acid, then cream ppt. [in solution] add aqueous silver nitrate iodide, I– acidify with dilute nitric acid, then yellow ppt. [in solution] add aqueous silver nitrate nitrate, NO3– add aqueous sodium hydroxide, ammonia produced [in solution] then aluminium foil; warm carefully sulfate, SO42– acidify with dilute nitric acid, then white ppt. [in solution] add aqueous barium nitrate sulfite, SO32– add a small volume of acidified the acidified aqueous potassium aqueous potassium manganate(VII) manganate(VII) changes colour from purple to colourless Tests for aqueous cations cation effect of aqueous sodium hydroxide effect of aqueous ammonia aluminium, Al 3+ white ppt., soluble in excess, giving white ppt., insoluble in excess a colourless solution ammonium, NH4+ ammonia produced on warming – calcium, Ca2+ white ppt., insoluble in excess no ppt. or very slight white ppt. chromium(III), Cr3+ green ppt., soluble in excess green ppt., insoluble in excess copper(II), Cu2+ light blue ppt., insoluble in excess light blue ppt., soluble in excess, giving a dark blue solution iron(II), Fe2+ green ppt., insoluble in excess, green ppt., insoluble in excess, ppt. turns brown near surface on ppt. turns brown near surface on standing standing iron(III), Fe3+ red-brown ppt., insoluble in excess red-brown ppt., insoluble in excess zinc, Zn2+ white ppt., soluble in excess, giving white ppt., soluble in excess, giving a colourless solution a colourless solution Tests for gases gas test and test result ammonia, NH3 turns damp red litmus paper blue carbon dioxide, CO2 turns limewater milky chlorine, Cl 2 bleaches damp litmus paper hydrogen, H2 ‘pops’ with a lighted splint oxygen, O2 relights a glowing splint sulfur dioxide, SO2 turns acidified aqueous potassium manganate(VII) from purple to colourless Flame tests for metal ions metal ion flame colour lithium, Li+ red sodium, Na+ yellow potassium, K+ lilac calcium, Ca2+ orange-red barium, Ba2+ light green copper(II), Cu2+ blue-green