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The Rate of Reaction Between Sodium Thiosulphate and Hydrochloric Acid

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The Rate of Reaction Between Sodium Thiosulphate and Hydrochloric Acid

AIM: to investigate the effect of changing the concentration of Sodium
Thiosulphate on the rate of reaction between Sodium Thiosulphate and
Hydrochloric acid.

THE REACTION: when Sodium Thiosulphate reacts with hydrochloric acid
sulphur is produced. The sulphur forms in very small particles and
causes the solution to cloud over and turn a yellow colour. This
causes the cross to fade and eventually disappear.

Sodium Thiosulphate + Hydrochloric acid »» Sulphur + Sodium Chloride +
Sulphur Dioxide + Water

NA2S2O3 + 2HCL »» S + 2NaCl + SO2 + H2O

(aq) + (aq) »» (s) + (aq) + (g) + (l)

PREDICTION: As the concentration of Sodium Thiosulphate increases the
length of time for cross to disappear decreases (inverse). This is
because the increase of concentration of Sodium Thiosulphate will
increase the rate of reaction between Hydrochloric acid and sodium
Thiosulphate particles.

SCIENTIFIC REASONS FOR PREDICTION: the results from preliminary
experiments (see preliminary experiments) support the prediction made.
From the results you can see that there is a directly proportional
relationship between the concentration and the rate of reaction. If
you increase the concentration then the rate of reaction will also

PARTICLE THEORY: the reaction will only exist if two things are in

1. If the particles are close enough to each other

2. When they do collide they have the energy

By heating or stirring the solution it affects the energy levels of
the particles. These variables remain fixed throughout the experiment.

Changing the concentration of sodium Thiosulphate:

Low concentration of sodium Thiosulphate:




2 products/30seconds

O = sodium Thiosulphate

X = Hydrochloric acid

XO = collision

Increased concentration of sodium Thiosulphate




4 products/30seconds

High concentration of sodium Thiosulphate




6 products/30seconds

There is more sodium Thiosulphate than HCL and still means that only 6
collisions can take place. The HCL restricts the number of collisions.
Without the extra HCL needed only 6 collisions can occur. Even if
there are more sodium Thiosulphate particles no more collisions can
occur. They need to be paired and there isn't the same number of each
so the Sodium Thiosulphate has extra single particles that can't react
with the HCL because there aren't any free, unpaired particles of HCL.

The acid restricts the number of collisions.




Investigating the effect of changing the concentration of hydrochloric


2HCL + CaCO3 »» CaCl2 + CO2 + H2O

(aq) (aq)


We measured the volume of carbon dioxide produced every 10 seconds for
120 seconds. We measured 10cm3 of acid (100%) 5cm3 acid with 5cm3
water (50%) 2.5 cm3 acid with 7.5cm3 of water (25%)

Results of preliminary experiment.

Concentration of acid

Time (secs) 100% 50% 25%

10 3 1 0

20 5 2 0

30 7 4 0

40 10 5 0

50 13 7 0

60 17 10 0

70 21 13 0.5

80 25 16 1

90 28 19 1

100 32 21 1.5

110 35 23 2.5

120 39 25 4

These results support the prediction as the concentration of HCL acid
decreases so does the rate of reaction.


To investigate the effect of changing the concentration of Sodium
Thiosulphate on the rate of reaction between sodium Thiosulphate and

NA2S2O3 + 2HCL »» S + 2NaCl + SO2 + H2O

PREDICTION: the higher the concentration of sodium Thiosulphate the
quicker the rate of reaction


Conical flask 250ml


Paper with cross drawn on it inside a plastic wallet

50ml measuring cylinder

10ml measuring cylinder




Tuck in tie

Hair tied back

Stools and bags tucked under tables

Wear goggles



1) Set up apparatus as shown in diagram above.

2) Put the correct concentration of s.thio in flask. As you add 10cm3
of HCL to flask start stopwatch.

3) Watch for the solution to cloud over. When the cross disappears
stop the stopwatch.

4) Record results and repeat 1-3 times for other concentrations.

3 concentrations of sodium Thiosulphate were investigated

solution time

thio : water 1 2 average

50:0 26.06 26.06

25:25 53.63 52.75 53.19

10:40 3.00.72 180.72

rate = 1/time to get an easier number to plot multiply this by 100

Time taken for cross to disappear

As the concentration of sodium Thiosulphate decrease the time taken
for the cross to disappear increases. This is an inverse relationship.

This is what a graph would look like if the results for this were







Rate = concentration of Thiosulphate / time

From my preliminary experiments I have learned that I can improve my
method and improve the accuracy of my results.

I have found that my range of concentrations doesn't give me good
enough results to back up my prediction. I only used 3 concentrations
and only repeated them once each. In the final experiment I will use a
range of 5 concentrations and repeat each 3 times. This will allow me
to plot more points on any graphs I make from the results.

Also the accuracy of measurements may be the cause of poor results. To
gain precise volumes I use a pipette and the right size-measuring

Also the temperature of the room can affect the rate of reaction. To
see this I can take the temperature at the beginning and end of the

To investigate the effect of changing the concentration of sodium
Thiosulphate on the rate of reaction between sodium Thiosulphate and
hydrochloric acid.

Apparatus and diagram (see preliminary experiment No.2)


Wear goggles to prevent damage to eyes from splashes

Tie back hair and tuck in tie

Bags and stools under tables to prevent falling over.

Report spillages and breakages to a teacher

Concentrations that are going to be used:

Vol : vol ratio

Thio : Water

50 : 0

40 : 10

30 : 20

25 : 25

20 : 30

10 : 40

0 : 50


1. Set up apparatus as in preliminary experiment.

2. Record the temperature of the room.

3. Add the first of the concentrations of sodium Thiosulphate to the
flask. As you add 10cm3 of HCL and start the stopwatch

4. Watch the solution as it clouds over. Once the cross has
disappeared stop the clock.

5. Record the time in a results table

6. Repeat the above steps for the other concentration of sodium
Thiosulphate. Repeat the experiment 3 times for each of the

7. Record all results in a table and work out the rate by dividing 1
by the average time for each.


Concentration of thio



Volume of reactants HCL (10cm3) = thio (50cm3)

Catalyst + or -

Height of eye above cross

Same person to watch over cross each time

Precision of measurements

Constant variables

Volume of reactants

Height of eye above cross


Results (see table)


Graph 1 shows that as the concentration increase so does the rate of
reaction. This is a directly proportional relationship. As one
increases so does the other. The graph is quite accurate. The points
plotted lie on or are close to the line of best fit.

Graph 2 shows that as the concentration on Sodium Thiosulphate
increase the time taken for reaction to take place decreases. This is
an inverse relationship.


As the concentration of Sodium Thiosulphate increases the rate of
reaction does so too and the time taken for a reaction decreases. This
is because as the concentration increases you are increasing the
number of reactions that can take place in a certain time. The
increase in the number of Thiosulphate particles results in more
particles to collide and react with the acid. This means more product
is made in a shorter time( see particle theory in plan) the conclusion
does agree with my prediction made. My experiment wasn't as accurate
as others as the equipment did not include things such as a light
sensor. Overall the experiment proved a success and the results
support my answer. I was able to produce graphs and draw a line of
best fit.


I data I obtained from the experiment was as accurate as it could be
and was fairly reliable. My method could have been made more reliable
by using a light sensor.


Time taken to get to (X) conc.





The light sensor is used instead of an eye. It reads the amount of
light coming through the solution in the flask.


You find out at what light level the sensor records when solution is
completely clouded over. You then do the experiment and record the
time taken for it to reach this point and repeat for all

My results could be slightly off due to bad measuring. I tried to use
cylinders of correct sizes for each volume, as it would be stupid to
use a 10cm3-measuring cylinder to measure 50cm3. Water would be lost
or trapped and so the accuracy would be poor. For measuring a 50cm3
you would use a 50cm3-measuring cylinder.

My graphs were accurate and all points were close to or on the line of
best fit. I had no clearly anomalous results. This could be due to
good measuring and the consistency of fixed variables. The temperature
of the room could also effect them as heat can speed up reaction
times. To check for changes I recorded the room temperature at the
beginning and end of the experiment. There was no considerable
different between them. The temperature of the water could also make a
difference. I didn't measure it but it could be an improvement I could
make in future. The water was from the tap and may alter in
temperature. To prevent this I could use a beaker with water in so
that the water would be room temperature or use a thermometer
controlled water bath.

My results weren't very reliable because on each repeat of the
concentration I got a different time outcome. None of the readings
were exactly matched but then there weren't any clear anomalous
results. The end point of was hard to judge so we tried to prevent
this each time by using the same person to watch the solution. I could
have used a light sensor to improve this further.

I used a clean dry flask for each of the repeat and concentration as
water droplets may dilute the solution more preventing accurate
results. Ideally I would repeat the experiment again with the
improvements in place and use higher concentration range of 8
concentrations. This pattern in the data supports my prediction.

How to Cite this Page

MLA Citation:
"The Rate of Reaction Between Sodium Thiosulphate and Hydrochloric Acid." 20 Apr 2014

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