Chemistry 0620/51 October/November 2022: question paper and mark scheme

Question

You are going to investigate the rate of the reaction between sodium metabisulfite and potassium iodate at different temperatures. Read all of the instructions carefully before starting the experiments. Instructions You are going to do five experiments at different temperatures. In each experiment the total volume of liquid is 80 cm3. (a) Experiment 1 ● Use a 100 cm3 measuring cylinder to pour 70 cm3 of aqueous potassium iodate into a 250 cm3 beaker. ● Use the first 10 cm3 measuring cylinder to pour 5 cm3 of aqueous starch into the beaker containing the aqueous potassium iodate. ● Use the second 10 cm3 measuring cylinder to pour 5 cm3 of aqueous sodium metabisulfite into the beaker. At the same time start the timer. ● Stir the mixture in the beaker with a thermometer and continue to stir until a sudden colour change is seen. ● Stop the timer and record the time in seconds to the nearest whole number in the table. ● Measure the temperature of the mixture in the beaker and record the temperature in the table. ● Rinse the beaker with water. Experiment 2 ● Use the 100 cm3 measuring cylinder to pour 70 cm3 of aqueous potassium iodate into a 250 cm3 beaker. ● Use the first 10 cm3 measuring cylinder to pour 5 cm3 of aqueous starch into the beaker containing the aqueous potassium iodate. ● Warm the aqueous potassium iodate and starch mixture over a Bunsen burner until the temperature of the solution is about 35 °C. Carefully remove the beaker from above the Bunsen burner. ● Use the second 10 cm3 measuring cylinder to pour 5 cm3 of aqueous sodium metabisulfite into the beaker. At the same time start the timer. ● Stir the mixture in the beaker with the thermometer and continue to stir until a sudden colour change is seen. ● Stop the timer and record the time in seconds to the nearest whole number in the table. ● Measure the temperature of the mixture in the beaker and record the temperature in the table. ● Rinse the beaker with water. Experiment 3 ● Repeat Experiment 2 but warm the aqueous potassium iodate and starch mixture until the temperature of the solution is about 40 °C. Experiment 4 ● Repeat Experiment 2 but warm the aqueous potassium iodate and starch mixture until the temperature of the solution is about 45 °C. Experiment 5 ● Repeat Experiment 2 but warm the aqueous potassium iodate and starch mixture until the temperature of the solution is about 50 °C. Complete the table. experiment 1 2 3 4 5 temperature at end of experiment / °C time taken to change colour / s [5] (b) Complete a suitable scale on the y-axis and plot your results from Experiments 1 to 5 on the grid. Draw a line of best fit through your points. time taken to change colour / s 0 10 20 30 40 50 60 temperature at end of experiment / C [4] (c) Deduce which experiment had the fastest rate of reaction. …… [1] (d) From your graph, deduce the time taken for the mixture to change colour at a temperature of 60.0 °C. Show clearly on the grid how you worked out your answer. time taken to change colour = …… [3] (e) Experiments are often repeated and the results compared to check that they are reliable. Suggest why this is difficult to do for these experiments. …… [1] (f) Suggest why the aqueous potassium iodate is warmed before the aqueous sodium metabisulfite is added rather than after it has been added. …… [1] (g) A polystyrene cup can be used instead of the beaker in this experiment. (i) Explain the advantage of transferring the warmed potassium iodate to a polystyrene cup rather than leaving it in the beaker. . …… . …… . …… [2] (ii) Suggest why it is not a good idea to put the aqueous potassium iodate in a polystyrene cup before it is warmed. . …… . …… [1] (h) Sketch on the grid the graph obtained when the experiments are repeated using aqueous potassium iodate of a higher concentration. [1] [Total: 19]

Question

You are provided with two substances, solid N and solution O. Do the following tests on the substances, recording all of your observations at each stage. tests on solid N (a) Carefully add the dilute hydrochloric acid for Question 2 to the boiling tube containing solid N. Test any gas produced. Keep the contents of the boiling tube for use in (b). Record your observations. …… [2] (b) Filter the mixture formed in the boiling tube in (a) into a test-tube until you have about 2 cm depth of filtrate in the test-tube. The filtrate collected is solution P. Divide solution P into two approximately equal portions in two test-tubes. (i) To the first portion of solution P, add aqueous sodium hydroxide gradually until it is in excess. Record your observations. . …… . …… [2] (ii) To the second portion of solution P, add aqueous ammonia gradually until it is in excess. Record your observations. . …… . …… [2] (c) Identify solid N. …… [2] tests on solution O (d) Carry out a flame test on solution O. Record your observations. …… [1] Divide the remaining solution O into three approximately equal portions in three test-tubes. (e) Test the pH of the first portion of solution O. pH = …… [1] (f) To the second portion of solution O add about 1 cm depth of dilute nitric acid followed by a few drops of barium nitrate solution. Record your observations. …… [1] (g) To the third portion of solution O add aqueous copper(II) sulfate gradually until it is in excess. Record your observations. …… [2] (h) Identify solution O. …… [2] [Total: 15]

Question

Many fizzy drinks contain phosphoric acid. Phosphoric acid reacts with sodium hydrogencarbonate to make carbon dioxide gas. Value Coke and Kola Koola are two fizzy drinks which contain phosphoric acid as the only acid. Plan an investigation to find which of these two fizzy drinks contains the highest concentration of phosphoric acid. Include in your answer how your results will tell you which drink contains the highest concentration of phosphoric acid. You are provided with samples of both fizzy drinks, solid sodium hydrogencarbonate and common laboratory apparatus. …… [6] Notes for use in qualitative analysis Tests for anions anion test test result carbonate (CO32–) add dilute acid effervescence, carbon dioxide produced chloride (Cl –) acidify with dilute nitric acid, then add white ppt. [in solution] aqueous silver nitrate bromide (Br –) acidify with dilute nitric acid, then add cream ppt. [in solution] aqueous silver nitrate iodide (I–) acidify with dilute nitric acid, then add yellow ppt. [in solution] aqueous silver nitrate nitrate (NO3–) add aqueous sodium hydroxide, then ammonia produced [in solution] aluminium foil; warm carefully sulfate (SO42–) acidify, then add aqueous white ppt. [in solution] barium nitrate sulfite (SO32–) add dilute hydrochloric acid, warm sulfur dioxide produced gently and test for the presence of will turn acidified aqueous sulfur dioxide potassium manganate(VII) 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 a white ppt., insoluble in excess 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 grey-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 iron(III) (Fe3+) red-brown ppt., insoluble in excess red-brown ppt., insoluble in excess zinc (Zn2+) white ppt., soluble in excess, giving a white ppt., soluble in excess, giving a colourless solution colourless solution Tests for gases Flame tests for metal ions gas test and test result metal ion flame colour ammonia (NH3) turns damp red litmus paper blue lithium (Li+) red carbon dioxide (CO2) turns limewater milky sodium (Na+) yellow chlorine (Cl 2) bleaches damp litmus paper potassium (K+) lilac hydrogen (H2) ‘pops’ with a lighted splint copper(II) (Cu2+) blue-green oxygen (O2) relights a glowing splint sulfur dioxide (SO2) turns acidified aqueous potassium manganate(VII) from purple to colourless