Chemistry · May / June 2021 · Paper 61 · Question 2
0620
As printed in the paper
The question as text
A student investigated the temperature decrease when sodium hydrogencarbonate reacts with dilute hydrochloric acid. The student did six experiments. Experiment 1 ● Using a measuring cylinder, 25 cm3 of dilute hydrochloric acid was poured into a conical flask. ● The initial temperature of the acid was measured using a thermometer. ● 1 g of sodium hydrogencarbonate was added to the conical flask. At the same time a stop‑clock was started. ● The acid and sodium hydrogencarbonate mixture in the conical flask was stirred continuously using the thermometer. ● The temperature of the mixture after 1 minute was measured. ● The conical flask was rinsed with distilled water. Experiment 2 ● Experiment 1 was repeated using 2 g of sodium hydrogencarbonate instead of 1 g. Experiment 3 ● Experiment 1 was repeated using 3 g of sodium hydrogencarbonate instead of 1 g. Experiment 4 ● Experiment 1 was repeated using 5 g of sodium hydrogencarbonate instead of 1 g. Experiment 5 ● Experiment 1 was repeated using 6 g of sodium hydrogencarbonate instead of 1 g. Experiment 6 ● Experiment 1 was repeated using 7 g of sodium hydrogencarbonate instead of 1 g.
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2(a)[4]
all temperatures and temperature changes completed and all temperatures and temperature changes recorded to the same precision
all temperatures recorded correctly
(22.0, 22.0, 22.0, 22.5, 23.0, 23.0) and
(19.5, 17.0, 14.5, 13.5, 14.0, 14.0)
all temperature changes calculated correctly
(2.5, 5.0, 7.5, 9.0, 9.0, 9.0)
2(b)[4]
all points plotted correctly
ruler drawn straight line through first 4 points
(ruler) drawn straight line through last three points
straight lines have been extended so that they meet / cross
2(c)(i)[3]
values read correctly from graph (9.0 °C and 3.6 g)
correct indication on graph
units (°C and g)
2(c)(ii)[1]
(all) acid used up / sodium hydrogen carbonate in excess
2(d)[2]
correct line should be identical to plotted line up to 1.8 g and then becomes horizontal.
temp change of between 4.0 and 5.0 where line becomes horizontal / levels off
mass of between 1.0 and 2.5 where line becomes horizontal / levels off
2(e)[4]
change: use a pipette
explanation: more accurate than a measuring cylinder
change: use a polystyrene / styrofoam cup
explanation: insulator / reduces heat gain
Tests on solid E
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