Question 119 marks
Question
You are going to investigate the rate at which hydrogen gas is made when magnesium ribbon reacts with dilute ethanoic acid at two different temperatures. Read all of the instructions carefully before starting the experiments. Instructions You are going to do two experiments using the apparatus shown in Fig. 1.1. rubber delivery tube clamp bung inverted 100 cm3 measuring cylinder conical flask trough dilute ethanoic acid water Fig. 1.1 (a) Experiment 1 ● Use a 50 cm3 measuring cylinder to pour 50 cm3 of dilute ethanoic acid into a conical flask. ● Use a thermometer to measure the initial temperature of the dilute ethanoic acid. Record the initial temperature in Table 1.1. ● Set the apparatus up as shown in Fig. 1.1, ensuring the inverted 100 cm3 measuring cylinder is full of water. ● Remove the bung from the conical flask, ensuring the delivery tube remains in the measuring cylinder. ● Add a coil of magnesium ribbon to the conical flask, immediately put the bung back into the conical flask and start the timer. If the magnesium sticks to the side of the flask, gently shake the flask so that it is washed off the side. ● Measure the volume of gas collected in the inverted measuring cylinder every 15 seconds for 150 seconds. Record the volume of gas collected in Table 1.2. ● Use the thermometer to measure the final temperature of the mixture in the conical flask. Record the final temperature in Table 1.1. ● Rinse out the conical flask with distilled water. Experiment 2 ● Repeat Experiment 1 but warm the acid by about 10 °C after it has been poured into the conical flask and before the initial temperature is measured and recorded in Table 1.1. ● Complete Table 1.1. Table 1.1 initial temperature final temperature average temperature experiment / °C / °C / °C 1 2 Table 1.2 time / s 15 30 45 60 75 90 105 120 135 150 volume of gas collected in Experiment 1 / cm3 volume of gas collected in Experiment 2 / cm3 [5] (b) Complete a suitable scale on the y-axis and plot your results for Experiments 1 and 2 on Fig. 1.2. Draw two lines of best fit. Both lines must start at (0,0). Label your lines. volume of gas collected / cm3 0 0 20 40 60 80 100 120 140 160 time / s Fig. 1.2 [4] (c) Deduce the volume of gas collected between 50 seconds and 70 seconds in Experiment 1. Show clearly on Fig. 1.2 how you worked out your answer. volume of gas collected between 50 seconds and 70 seconds …… cm3 [3] (d) Use Fig. 1.2 to explain which experiment had the faster rate of reaction. experiment with faster rate …… explanation …… [1] (e) During Experiment 1, the temperature of the mixture in the conical flask changed. (i) State how this change in temperature alters the rate of the reaction. . …… [1] (ii) Describe how the apparatus could be altered to minimise the change in temperature. . …… . …… [1] (f) Some gas escapes in the short time between adding the magnesium ribbon to the conical flask and putting the bung back into the conical flask. Explain how the apparatus could be altered so that no gas escapes and the results of the experiment are more accurate. You may draw a diagram to help explain your answer. …… [2] (g) Describe one other way in which the results of Experiments 1 and 2 could be made more accurate. …… [1] (h) Sketch on Fig. 1.2 the graph you would expect when Experiment 2 is repeated using ethanoic acid with a higher concentration. Label your line H. [1] [Total: 19]
Mark scheme
1(a) initial temperature in Experiment 2 is between 5 °C and 30 °C above initial temperature in Experiment 1 1 both mean temperatures calculated correctly 1 all temperatures and temperature changes recorded to a consistent number of decimal places 1 gas volumes increase from left to right across Table 1.2 and volumes larger in Experiment 2 than in Experiment 1 1 final volume of gas in Experiment 1 is comparable to supervisor 1 1(b) sensible y-axis scale which is linear and points extend over halfway up scale 1 all points plotted correctly for Experiment 2 1 best fit lines 1 lines labelled 1 1(c) working on graph to show readings at 50 seconds and 70 seconds 1 correct readings 1 correct calculation of difference between the two volumes 1 1(d) Experiment 2 and line steeper / gas volume greater 1 1(e)(i) speeds up / faster / increases 1 1(e)(ii) stand flask in a water bath (at initial temperature of the acid) 1 1(f) M1 Place one reagent in tube inside boiling tube/flask 1 M2 Tip tube to start reaction OR 1 M1 Use a divided flask M2 Tip flask to start reaction OR M1 suspend the magnesium (above the acid) in the flask M2 Tip the flask 1(g) repeat and find mean 1 OR use a burette in place of measuring cylinder (to measure the volume of acid) 1(h) graph line starts at (0,0) and rises more steeply than Experiment 2 line 1