Chemistry 0620/51 May/June 2025: question paper and mark scheme

Question

You are going to investigate how the rate of reaction of magnesium ribbon with dilute acid changes as the concentration of the acid is changed. You will use five solutions of the same acid, A, B, C, D and E. Each solution has a different concentration. The acid is in excess in all experiments. Read all of the instructions carefully before starting the experiments. Instructions You are going to do five experiments. (a) Experiment 1 • Use the 50 cm3 measuring cylinder to pour 30 cm3 of acid A into the 100 cm3 conical flask. • Add a coil of magnesium ribbon to the acid in the conical flask and immediately start a stop‑watch. • Continually swirl the mixture in the conical flask until the magnesium ribbon disappears completely. Immediately stop the stop‑watch and record the time in seconds to the nearest second in Table 1.1. • Empty and rinse the conical flask with distilled water. Experiment 2 • Repeat Experiment 1 using 30 cm3 of acid B instead of acid A. Experiment 3 • Repeat Experiment 1 using 30 cm3 of acid C instead of acid A. Experiment 4 • Repeat Experiment 1 using 30 cm3 of acid D instead of acid A. Experiment 5 • Repeat Experiment 1 using 30 cm3 of acid E instead of acid A. Complete Table 1.1. Table 1.1 concentration of acid time for magnesium experiment acid in mol / dm3 to disappear in s 1 A 2.0 2 B 1.5 3 C 1.0 4 D 0.8 5 E 0.5 [4] (b) Write a suitable scale on the y‑axis and plot your results from Experiments 1 to 5 on Fig. 1.1. Draw a line of best fit. time for magnesium to disappear in s 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 concentration of acid in mol / dm3 Fig. 1.1 [4] (c) From your graph in Fig. 1.1, deduce the time for the magnesium to disappear when the concentration of the acid is 1.3 mol / dm3. Show clearly on Fig. 1.1 how you worked out your answer. time for magnesium to disappear = …… s [2] (d) The mean rate of reaction is calculated using the equation shown. length of magnesium ribbon in cm mean rate of reaction = time for magnesium to disappear in s The length of each coil of magnesium ribbon used in all five experiments was 5 cm. (i) Calculate the mean rate of reaction in Experiment 1. Give units for the rate you have calculated. mean rate of reaction = …… units …… [2] (ii) Deduce in which Experiment, 1, 2, 3, 4 or 5, the mean rate of reaction was the slowest. …… [1] (e) Explain why repeating each experiment is an improvement. …… [1] (f) (i) You are going to carry out an experiment to find the temperature change when the magnesium reacts with the acid. • Use the measuring cylinder to pour 30 cm3 of acid A into the 100 cm3 conical flask. • Measure the initial temperature of the acid in the conical flask. Record the initial temperature in Table 1.2. • Add a coil of magnesium ribbon to the acid in the conical flask. • Continually swirl the conical flask until the magnesium ribbon disappears completely. • Measure the final temperature of the acid in the conical flask. Record the final temperature in Table 1.2 and complete Table 1.2. Table 1.2 initial temperature final temperature temperature of acid / °C of acid / °C change / °C [2] (ii) Explain why controlling the temperature of the acid so that it remains constant is an improvement. …… [1] (iii) Explain why using a polystyrene cup instead of the 100 cm3 conical flask does not control the temperature of the acid. …… [1] (iv) Describe how the temperature of the acid can be controlled and kept constant. …… [1] [Total: 19]

Question

You are provided with two solids: solid F and solid G. Do the following tests on solid F and solid G. Record all of your observations at each stage. Tests on solid F (a) (i) To the boiling tube containing solid F, add about 15 cm3 of dilute hydrochloric acid. Test any gas produced. Keep the product for use in (b). Record your observations. …… [3] (ii) Identify the gas made in (a)(i). …… [1] (b) Filter the product from (a)(i) to obtain solution H as the filtrate. Carry out the tests on solid G while the mixture is filtering. Stop filtering after you have completed the tests on solid G. Divide solution H into three approximately equal portions in three test‑tubes. (i) To the first portion of solution H, add about 1 cm depth of dilute nitric acid followed by a few drops of aqueous barium nitrate. Record your observations. …… [1] (ii) To the second portion of solution H, add aqueous sodium hydroxide dropwise and then in excess. Record your observations. dropwise …… in excess …… [2] (iii) To the third portion of solution H, add about 1 cm depth of aqueous sodium carbonate. Record your observations. …… [1] (c) Identify solid F. …… [2] Tests on solid G (d) Carry out a flame test on solid G. Record your observations. …… [1] (e) Put the remaining solid G in a boiling tube. Add about 5 cm depth of water to the boiling tube. Put a stopper in the boiling tube and shake the boiling tube to dissolve solid G and form solution G. Divide solution G into two approximately equal portions in two test‑tubes. (i) To the first portion of solution G, add about 1 cm depth of dilute nitric acid followed by a few drops of aqueous silver nitrate. Record your observations. …… [1] (ii) To the second portion of solution G, add all of the sample of aqueous chlorine. Record your observations. …… [1] (f) Identify solid G. …… [2] [Total: 15]

Question

Solid sodium hydrogencarbonate, NaHCO3, reacts with dilute hydrochloric acid and with dilute ethanoic acid. NaHCO3(s) + HCl (aq) NaCl (aq) + CO2(g) + H2O(l) NaHCO3(s) + CH3COOH(aq) CH3COONa(aq) + CO2(g) + H2O(l) These two reactions are endothermic. Plan an investigation to find which of the two reactions is the most endothermic. Your plan must include how you can use the results to find which reaction is the most endothermic. You are provided with solid sodium hydrogencarbonate, dilute hydrochloric acid, dilute ethanoic acid 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