Chemistry · Oct / Nov 2020 · Paper 62 · Question 2
0620
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The question as text
A student investigated the rate of a reaction between sodium metabisulfite and potassium iodate. In the reaction, starch was used as an indicator. At first the reacting mixture remained colourless but then suddenly changed to a blue-black colour. Five experiments were done. In each experiment the total volume of liquid was 45 cm3. Experiment 1 ● Using a 10 cm3 measuring cylinder, 5 cm3 of aqueous sodium metabisulfite was poured into a beaker. ● Using another 10 cm3 measuring cylinder, 5 cm3 of aqueous starch was poured into the beaker. ● Using a 25 cm3 measuring cylinder, 15 cm3 of distilled water was poured into the beaker. ● Using another 25 cm3 measuring cylinder, 20 cm3 of aqueous potassium iodate was poured into the beaker. At the same time a stop-clock was started. ● The mixture in the beaker was stirred until a sudden colour change was seen. ● The stop-clock was immediately stopped and the time recorded. ● The beaker was rinsed with water. Experiment 2 ● Experiment 1 was repeated using 17 cm3 of distilled water and 18 cm3 of aqueous potassium iodate. Experiment 3 ● Experiment 1 was repeated using 21 cm3 of distilled water and 14 cm3 of aqueous potassium iodate. Experiment 4 ● Experiment 1 was repeated using 23 cm3 of distilled water and 12 cm3 of aqueous potassium iodate. Experiment 5 ● Experiment 1 was repeated using 25 cm3 of distilled water and 10 cm3 of aqueous potassium iodate.
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2(a)[5]
all volumes of sodium metabisulfite completed as 5
all volumes of water (15, 17, 21, 23, 25) correct.
all times recorded correctly (38, 42, 53, 61, 72)
all five times in seconds only
2(b)[3]
all 5 points plotted correctly
suitable best fit curve drawn. Line must go through / within half a square of correctly plotted points
2(c)(i)[2]
correct working shown on graph
time correct for their working
2(c)(ii)[1]
19
2(d)[1]
line is below plotted line and does not meet / touch plotted line.
2(e)(i)[1]
0.02
2(e)(ii)[1]
1
2(f)[1]
to keep total volume constant / so concentration of sodium metabisulphite does not change
2(g)[2]
change: use a pipette / burette (in place of a measuring cylinder)
explanation: more accurate / precise (than a measuring cylinder)
2(h)[1]
repeat and compare the results
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