Question 120 marks
Question
You are going to investigate the rate at which hydrogen gas is made when magnesium reacts with two different solutions of dilute hydrochloric acid, C and D, with different concentrations. The dilute hydrochloric acid is in excess in both experiments. Read all of the instructions carefully before starting the experiments. Instructions You are going to do two experiments using the apparatus shown. rubber delivery tube clamp bung inverted 100 cm3 measuring cylinder conical flask trough dilute hydrochloric acid water (a) Experiment 1 ● Use a measuring cylinder to pour 50 cm3 of dilute hydrochloric acid C into a conical flask. ● Use a thermometer to measure the initial temperature of the dilute hydrochloric acid. Record the initial temperature in the space provided. ● Set the apparatus up as shown in the diagram, ensuring the inverted measuring cylinder is full of water. ● Remove the bung from the conical flask, leaving the delivery tube in the measuring cylinder. ● Add a coiled 5 cm length of magnesium ribbon to the conical flask, immediately replace the bung and start the timer. ● Measure the volume of gas collected in the inverted measuring cylinder every 20 seconds for 160 seconds. Record the volume of gas collected in the table. ● Use the thermometer to measure the final temperature of the dilute hydrochloric acid in the conical flask. Record the final temperature in the space provided. initial temperature …… °C final temperature …… °C time / s 20 40 60 80 100 120 140 160 volume of gas collected / cm3 [2] (b) Experiment 2 ● Empty and rinse the conical flask with distilled water. ● Repeat Experiment 1 using 50 cm3 of dilute hydrochloric acid D instead of dilute hydrochloric acid C. initial temperature …… °C final temperature …… °C time / s 20 40 60 80 100 120 140 160 volume of gas collected / cm3 [3] (c) Complete a suitable scale on the y-axis and plot your results from Experiments 1 and 2 on the grid. Draw two smooth line graphs. The lines must pass through (0,0). Clearly label your lines. volume of gas collected / cm3 0 0 20 40 60 80 100 120 140 160 time / s [5] (d) From your graph, deduce the volume of gas that was collected after 50 seconds in Experiment 2. Show clearly on the grid how you worked out your answer. volume of gas = …… [3] (e) Explain what can be deduced about the concentrations of dilute hydrochloric acid C and dilute hydrochloric acid D. …… [2] (f) (i) State what happens to the temperature of the dilute hydrochloric acid during Experiment 1. . …… [1] (ii) State what effect this temperature change has on the total volume of gas made when the reaction has finished. . …… [1] (iii) Describe a change that can be made to the apparatus or reagents to reduce the temperature change of the acid in Experiment 1. . …… [1] (g) Suggest why it is important to replace the bung in the conical flask immediately after adding the magnesium ribbon. …… [1] (h) State the advantage of measuring the volume of gas collected every 10 seconds rather than every 20 seconds. …… [1] [Total: 20]
Mark scheme
1(a) initial and final temperatures recorded and all eight volumes of hydrogen recorded 1 final temperature is greater than initial temperature 1 1(b) final temperature is greater than initial temperature and final temperature is comparable to supervisor 1 volume of hydrogen increases as time increases in BOTH experiments 1 volume of hydrogen at 40 seconds in Experiment 2 is lower than volume of hydrogen at 40 seconds in Experiment 1. 1 1(c) y-axis scale in linear and points extend over halfway up scale 1 all points for Experiment 1 and 2 plotted correctly 2 two best fit curves 1 lines both extended to (0,0) and are labelled 1 1(d) correct working shown on graph 1 correct reading from their working shown on graph. 1 cm3 1 1(e) solution C is more concentrated than solution D 1 as reaction is faster 1 1(f)(i) increases 1 1(f)(ii) none / stays the same 1 1(f)(iii) use a water bath / use a larger volume of acid 1 1(g) minimise gas loss 1 1(h) (more points / data so) better / smoother graph 1