Reaction Between Hydrochloric Acid and Sodium Thiosulphate

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Reaction Between Hydrochloric Acid and Sodium Thiosulphate


I will be conducting two experiments to determine how two factors
affect the rate of reaction in the reaction between hydrochloric acid
and sodium thiosulphate. It is a precipitation experiment. The
equation allows us to see how this experiment will help us find how
rate of reaction changes

Sodium thiosulphate + Hydrochloric acid ---- Sodium chloride + Sulphur
+ Sulphur dioxide + Water

Na2S2O3 (aq) + 2HCl(aq) ---- 2NaCl(aq) + S(s) + SO2(g) + H2O(l)

The main factors that affect the rate of reaction of any experiment
are -Pressure. By reducing the volume in which the same amount of
particles exists the pressure is increased. Once the same number of
particles is in a smaller area there is less space in which to move
and so the particles are more likely to collide each other.

Using a catalyst is another method I could use. A catalyst is a
substance, which lowers the activation energy of a reaction without
being chemically altered.

Energy. By giving the particles extra energy, as heat, they will move
faster. This means that they cover more ground and are therefore more
likely to collide with each other which in turn makes the reaction
faster. (We have to take into consideration the face that not all
collisions are successful as they may not react with the amount of
energy required (activation energy)). The best way to give energy to a
reaction is heat.

Concentration. Just as increasing the pressure will increase the
number of particles colliding, so will the concentration. By putting
more particles into the reaction, the chance that they collide is
increased, thus increasing the rate of reaction.

Surface area. Particles can only collide when the two sorts can meet.
Therefore a reaction can only occur on the surface of a material.
Therefore by increasing the surface area (e.g. cutting the substances)
of the material, which is available to collide, the speed of the
reaction will increase.

In my experiment I will be investigating two factors, the

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concentration and the energy. They will be done separately. The method
I will use will be detailed further on in my planning.

I predict that as the temperature is increased, the rate of reaction
will increase as they are proportional. I also predict that if the
concentration is doubled, the rate of reaction will almost double as
well. If you double the number of particles you can almost double the
amount of successful collisions. The Collision Theory can justify
this. When the temperature is increased the particles will have more
energy and thus move faster. Therefore they will collide more often
and with more energy. Particles with more energy are more likely to
overcome the activation energy barrier and thus react successfully.
[IMAGE]If solutions of reacting particles are made more concentrated
there are more particles per unit volume. Collisions between reacting
particles are therefore more likely to occur. We can also consider the
Kinetic Theory, which states that if there is double the heat, there
will be double the energy so almost doubling the rate of reaction.
There is the Q10 factor which states raising the temperature by 10°C
usually doubles the rate of reaction. In terms of concentrations, I
predict that it will have a greater effect on the rate of reaction
because the reaction is exothermic. Thus even while I am testing
concentration, heat will be given out by the reaction which will give
more energy to the particles and so cause them to reach their
activation energy sooner. Although this is true, the solutions I will
be using will be so dilute that not that much heat will be provided
from the exothermic reaction to give enough energy to help it overcome
the activation barrier. This can all be understood by looking at the
Collision Theory. For a reaction to occur particles must collide with
each other. Only a small percent result in a reaction. This is due to
the energy barrier to overcome. Only particles with enough energy to
overcome the barrier will react after colliding. The minimum energy
that a particle must have to overcome the barrier is called the
activation energy. The size of the activation energy depends on the
experiment. If the frequency of the collisions is increased, the rate
of reaction will increase. However the amount of successful collisions
remains the same. To increase it you need to change the concentration.
We also need to take into consideration the point that when there is
2.5 cm³ of sodium thiosulphate, there will not be many particles to
collide, so the rate will a lot slower.


I have drawn a diagram of the experiment with the apparatus on another
piece of paper. The apparatus that can be used is a light meter, but
as I am not allowed that amount of time, I will not be using one,
along with a burette, to measure the hydrochloric acid and sodium


As I am doing two experiments, testing two different factors that
affect the rate of reaction, there will be two different methods, but
some things will remain the same. The first experiment changes the
concentration of the sodium thiosulphate as this in a surplus quantity
then the hydrochloric acid. I will measure 10 cm³ of hydrochloric
acid, which will remain the same amount throughout the varying
concentrations. The water will be distilled water and will be measured
in a larger beaker, 50cm³, along with the sodium thiosulphate. The
concentrations I will be using are, water to sodium thiosulphate, in
the ratio 0:40, 5:35, 10:30, 15:25, 20:20, 25:15, 30:10, 35:5,
37.5:2.5. These concentrations will be used after a preliminary test
was taken to allow myself to see how long it took for the
concentration, which was the most dilute. Here is a table, which
allowed me to see which concentrations were the best.

Hydrochloric acid (HCl)/cm³

Water (H2O)/cm³

Sodium thiosulphate (Na2S2O3)/cm³

Reaction time/sec

















The total amount of solution must not exceed 50 cm³. A cross will be
drawn on a piece of paper, from which the same person will be able to
tell when the reaction has been completed. The sodium thiosulphate
will be in the beaker and the hydrochloric acid added. The time for
the precipitate to form will be noted.

In the second experiment, we will see how temperature affects the
rate. Heating the sodium thiosulphate along with the water will do
this. It is too dangerous to heat the hydrochloric acid. The solution
will be heated to the required temperatures, which are 20ºC, 30ºC,
40ºC, 50ºC, 60ºC, 70ºC, 80ºC, and 90ºC. A preliminary experiment was
taken to determine these temperatures. Here it is:









The concentrations of the liquids will be kept the same at 10 cm³ for
hydrochloric acid, 30 cm³ of water and 10 cm³ of sodium thiosulphate.
The sodium thiosulphate and the water mixture will be heated and then
when the required temperature has been reached, the acid will be
added. The time taken for the precipitate to from will be noted. In
both experiments, the stopwatch was started the second the acid
touched the sodium thiosulphate solution. No control will be used, but
the fact that the same person will be watching the cross ensures

To make the experiment fair, I will use the same cross with each
experiment, the same person will be watching the cross, the same size
beaker will be used, the same person will measure the solutions and
the same size beakers will be used. Only distilled water and the same
acid will be used in both experiments. For the concentrations
experiment, the same temperature will be used to ensure fairness. The
temperature will be kept at room temperature, 20°C. I will conduct the
experiment in the same room and in the same part so the outside
factors that might affect the experiment will all be the same. The
apparatus will be washed out with distilled water so that the new
experiment is fair. I will do the experiment twice to get an average
and a more accurate result. To allow reliability, the repetition of
the experiment will allow me to see if there are any anomalous
results, which I will repeat. Also I will draw a best-fit line on my
graphs to see if there are any anomalous results, which will show me
if the experiment is reliable.

To ensure safety in the experiment, I will wear goggles to protect my
eyes. I will not heat the acid, as this is dangerous and I will make
sure that the acid is not spilled, as it is corrosive. The sodium
thiosulphate will not be spilled either, as it is dangerous. When the
temperature experiment is undertaken, I will wear some rubber
protectors so that I do not burn myself. I will stand up so if the
beaker was to fall I will be able to move out of the way. Stools will
be kept under tables to stop people from falling over.

In my results I will put the rate of reaction. This was measured by
dividing time by 1.


Looking at my results I can see that they follow trends that will
allow me to ascertain whether my prediction follows or not. The
concentration graph gave me a very nice best-fit line, which allowed
me to prove that as concentration doubles, rate of reaction doubles
(see lines on graph). This fits in with my prediction, so backing it
and also fits in with my theories. The Collision Theory works best in
this, as there are more particles per square unit, meaning a greater
chance of a collision, but also a greater chance of a successful
collisions as there is double the amount of particles. At a low
concentration, 2.5 cm³, the graph shows that there is barely a rate of
reaction, but my results increase at a steady concentration, allowing
me to draw an almost proportional best-fit line. At a concentration of
40 cm³, we see that the best-fit line is starting to level off. This
means that the acid is becoming too saturated; there are too many
sodium thiosulphate molecules in ratio to the hydrochloric particles.
This means that the hydrochloric particles will all have reacted, but
not all of the sodium thiosulphate particles will have reacted,
leaving a surplus. The Collision Theory or Kinetic Theory can do
nothing to cause the rate of reaction to increase. From my graph, I
can see that at 40 cm³, there were the most amounts of collisions, but
at 2.5 cm³, there were so few particles of sodium thiosulphate, there
was a lower chance of a collision between the molecules. This followed
my prediction, as I said that at 2.5 cm³ the rate of reaction would be
much lower. I have explained before how my results prove the theories
behind my prediction for this experiment. The heat given off by the
experiment did nothing to change the rate of reaction, as the
reactants were so dilute. The particles in the solution had to hit in
a particular manner, and with enough energy to cause a successful
reaction, otherwise the rate went down.

There are two anomalous results, which I have circled. They do not
follow the pattern set by the best-fit line. The reason for them is
that in both repeats of the experiment, it came up with the same
inequalities, so in my results table there are no anomalous results,
but once I plotted the average onto my graph, they showed up. It could
be due to method carried out and the accuracy of the apparatus used.
As we used measuring cylinders, it was extremely hard to come up with
the same measurements all the time. This allowed discrepancies to crop
up in the graph rather than the results table.

My second graph again follows a similar trend, allowing me to draw a
best-fit line. I have drawn lines on showing that the rate of reaction
doubles after 10ºC increase in temperature. This fits in with my
prediction and also my theories, especially the Q10 factor, which
states that raising the temperature by 10°C, usually doubles the rate
of reaction. This also ties in with the Kinetic Theory, which states
that if you double the heat you double the energy. It is known that if
there is an increased amount of energy on particles there is going to
be greater ratio of successful collisions. My graph shows that at low
temperatures such as 40ºC, the rate is only 0.029s-1, but at 90ºC it
has increased to 0.014s-1. Thus showing that with increased energy the
rate will increase. This also ties in with the Collision Theory, which
shows that with this extra energy they are more likely to overcome the
activation barrier inhibiting it form fully reacting and causing the
rate to increase. Unlike concentration there is nothing to stop the
rate from increasing, so the graph is directly proportional and does
not taper off. My graph has followed my prediction, and has also
followed the theories backing my prediction.

There is one anomalous result that I have circled. This is likely to
be the cause in a discrepancy caused by the apparatus as I have stated
in the first part of my conclusion. As there are not many factors to
affect temperature, so the line will go on indefinitely. Also we did
not manage to get to the temperature we wanted, as the acid brought
the temperature down extremely, so the rate may be of a lower

The results I attained followed my predictions so it proved my theory,
but they were not necessarily accurate as the apparatus used were not
accurate enough to give me a precise result. As there were anomalous
results it is true that the experiment was not that accurate otherwise
all the results would follow a trend line. My results in my tables
were reliable as they were all at least within three seconds of each
other and they allowed me to draw an easily understandable best-fit
line which allowed me to analyse my results, so I could attain the
information to prove my prediction were almost correct. In both of my
results, they were not that different that meant I could repeat them,
but as I came up with the graph after the time I was allotted to do
the experiments, I did not have enough time to repeat them.

I conclude that although my results were not entirely accurate, they
did follow a trend that backed my prediction. I obtained enough
information to see the affects of temperature ands concentration on
the rate of reaction of this particular experiment.


The method I used was suitable, although I did get a few anomalous
results in my graph. These anomalous drew me to the conclusion that
the method was suitable, but it did have some major discrepancies. I
followed the method accurately, but sometimes we were thrown by the
rapidity of the experiment and were not prepared for the time to be
noted. These anomalous results proved that the experiment was flawed,
as I should have got perfect results. The personal reasons for why I
may have got these results are because we used different acids and
sodium thiosulphate over the days, so we were not sure whether we got
the same result. There were some times when it was necessary to repeat
the experiment as we did not note the time, this may have altered the
results as then the beaker may not have been cleaned thoroughly. The
inaccuracies, which may have occurred because of the method, were the
fact that we did not use accurate measuring devices, of the acid,
sodium thiosulphate or the light. The methods that were used led to
inaccurate and unreliable results.

If I were to improve the results I would ensure that we used a burette
to measure the acid and the sodium thiosulphate. This would add
accuracy to the experiment and the results. Also I would use a light
meter, as this would give an even more accurate result. I have
enclosed a graph using a light meter on different concentrations. It
gives a very detailed graph, which can easily show a similar trend to
prove my theory. We can see where the reaction goes fastest and where
it has finished. It also gives an accurate amount of light
concentration tha6t has been stopped rather than the cross which was
not always accurate as the person looking could not always be sure.

My results were reliable as they allowed me to come up with a suitable
best-fit line and firm conclusions from this. They were not accurate
as they contained anomalous results that were caused by the inaccuracy
of the method. I have talked more about this in my conclusion. I took
enough results to tell me whether there were any anomalous results,
but it would have been more accurate if I had taken three results
rather than two as this would ascertain whether the two results were
inaccurate or not. The range I used was good enough as well as I did
it from the preliminary experiment, which showed me which experiments
took over a certain amount of seconds. This gave me the range, which
ranged from extremely low concentrations to high enough showing how
the rate is affected by the concentration after a certain amount. Also
the temperature allowed me to come up with valid results from my
graph. My trends allowed me to see that a high concentration will have
no affect on the rate of reaction, but a high temperature will
increase it.

If I had more time I would see if I used a burette and other accurate
measuring devices, how they differed from my results. I could also do
three results to attain an accurate rate of reaction. I would also
extend the temperature to see how slow the rate is at a very low
temperature and how extremely high temperature affects the particles
and if they will react differently.

If I extended this experiment I would see how other factors affect the
rate. I would see how catalysts affect it, increasing pressure and
increasing surface area. This would be done by either putting the
gasses into smaller spaces or cutting the solids into smaller pieces.
I would to different sizes to see if a very small piece of substance
would have an differing result from a piece of substance slightly
larger. Also I could see if the rate of reaction differs between
states, solids, liquids and gases. I could test the situation it is
in, if gravity has any effect on it, or if a high amount of pressure,
like under the sea. This experiment helped me to learn a lot about
rate of reactions and the factors. It also helped me to understand how
to improve the method, as it was not entirely accurate.

It is important to learn about rates of reaction as they come into
everyday life. They can allow rusting to occur at a faster rate, but
they also help in making chemicals like in plastics, medicines and
explosives. Also catalysts are used to crack kerosene into octane and
ethanol so there is a higher percentage of gasoline for companies,
from crude oil.

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