C
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Reaction of sodium
thiosulphate and
hydrochloric acid
GCSE Investigation
C
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The reaction…
• Sodium thiosulphate, Na2S2O3 reacts
with HCl to give a reaction mixture that
gets more cloudy as yellow sulphur is
formed.
• Na2S2O3(aq) + 2HCl(aq)  2NaCl(aq) + S(s) +
SO2(g) + H2O(l)
C
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What will we measure?
• The time for a cross beneath the
reaction flask to disappear as the
reaction gets cloudy.
• We will do the reaction at several
temperatures and try to work out the
rate of reaction.
C
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A
The Investigation
Answer Q’s
• The investigation will involve looking at how
concentration can affect the rate of this reaction.
• We can control concentration by diluting solutions of
known strenghts
• We want to investigate 5 or 6 concentrations – what
would be a suitable range for investigation of
reactions in solution?
• How many times would you attempt to repeat the
reaction to improve reliability of results?
A
Answer Q’s
Errors
• There are several errors in the experiment.
• The devices for measuring volume are at best
accurate to 0.2 mL.
• What would the result be of having too much or too
little of each reagent?
• If the total volume of the reaction is 50mL what is the
percentage error from measuring volume?
• Error % = error/range x 100
• Especially at lower temperature the end of the
reaction is difficult to judge.
• Why is it inappropriate to use reaction times to
fractions of 1/100 seconds?
D
Do this
Background
• Using your background knowledge write a short
account of how and why you would expect
concentration to affect react rate.
• Then use the reference materials to make a
prediction of how rate will change with concentration
– for higher (Level 8) you need to make a prediction
that has numbers in it!. In your write up quote the
source of the information (including web addresses if
the internet is used).
The Reaction
Na2S2O3 + 2HCl  2NaCl + S + SO2 +H2O
When hydrochloric acid is added to
the sodium thiosulphate a reaction
occurs which produces a cloudy
suspension of solid sulphur.
The time it takes for the cross to disappear
can be used to investigate rate of reaction.
The Plan (1)
• Describe what you will be investigating
and what you will be measuring.
• Give a list of the apparatus you plan to
use and a method which could be
followed by a student who has not done
the experiment. Your method should
include quantities of chemicals.
Plan (2)
• Describe how you will make the experiment
more reliable and what things you will have to
look out for that could go wrong.
• Include some details on safety of the
chemicals and the lab.
• Give background research into things that will
affect reaction rate and try to make a
quantitative prediction – how much faster will
the experiment go when you change
concentration?
• Do a trial experiment that can be used to help
decide how exactly to do the reaction.
Changing Concentration
• It is important that the total volume of the
reaction stays the same.
• If you add more acid you need to take away
the same amount of water. For example…
Amount of
thiosulphate (cm3)
Amount of acid (cm3)
Amount of water (cm3)
40
40
40
40
10
8
6
4
0
2
4
6
Analysis of results (1)
• Describe what the basic pattern of
results shows – what is the effect of the
changes you make?
• Describe the pattern in more detail
using the numbers you have obtained in
your results.
• Produce a graph of the results with a
line of best fit. Make sure the axes are
labelled with the correct units.
Analysis of results (2)
• Using your results calculate the reaction
rate for each concentration using…
• Rate = 1000/time to react (units = 1000/s)
• Produce another graph with Rate on the
y-axis and concentration on the x-axis.
• Does the rate double if you double the
concentration? If it does not then how
much faster does it get? How does the
increase in rate relate to collision
theory?
Evaluation
• Did the experiment work effectively to answer
the aim of the investigation?
• Did any of the results you obtained not fit the
expected pattern? If they did say which ones
and think of why this result might be off.
• What could you do to improve the experiment
or make it more accurate?
• Describe in detail an experiment that you
could do to test more ideas about collision
theory.
I’ve checked results from the experiments and while some were not too bad most were
pants!! So I have put together the best results so we can use them as a class
1000/av
g
1
2
3
4
5
6
7
8
9
avg
10
34
36
36
37
34
35
37
33
35
35.1
28
8
37
38
38
39
38
38
39
38
39
38.2
26
6
43
41
44
44
45
44
45
42
44
43.6
23
4
50
49
52
51
50
48
50
50
51
50.2
20
2
61
63
66
62
65
66
62
65
65
63.8
16
Choose 3 sets of results from sets 1-9 – do not use the same set as the person sitting next to you.
Then work out values for your average and then 1000/average.
Remember to put the correct units into the table for volume and time…
Rewrite your own table using the results you have Making sure you have the correct units in the table….
thiosulphate
acid
water
40
10
0
40
8
2
40
6
4
40
4
6
40
2
8
1
2
3
average
1000/aver
age