Rate of Reaction - Sodium Thiosulphate and Hydrochloric Acid

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Rate of Reaction - Sodium Thiosulphate and Hydrochloric Acid


Investigation, to find out how the rate of reaction between Sodium
Thiosulphate and Hydrochloric acid is affected by changing the


I must produce a piece of coursework investigating the rate of
reaction, and the effect different changes have on them. The rate of
reaction is the rate of loss of a reactant or the rate of development
of a product during a chemical reaction. It is measured by dividing 1
by the time taken for the reaction to take place. There is five
factors which affect the rate of a reaction, according to the
collision theory of reacting particles: temperature, concentration (of
solution), pressure (in gases), surface area (of solid reactants), and
catalysts. I have chosen to investigate the effect of concentration on
the rate reaction. This is because it is the most practical to
investigate. Dealing with temperatures is a difficult task especially
when we have to keep constant high temperatures. Secondly the rate
equation and the constant k changes when the temperature of the
reaction changes. We have no gases and solids involved therefore it is
easy to deal with solutions. Similarly the use of a catalyst
complicates things, and if used incorrectly could alter the outcome of
the experiment.

The theory behind this experiment is that 'Increasing the
concentration can increase the rate of the reaction by increasing the
rate of molecular collisions.'


I will place the reaction mixture on a paper with a black cross-drawn
on it. When the cross is completely obscured, the reaction will have
finished. The time taken for this to happen is the measure of the rate
of reaction. We must do this several times, and change the
concentration of Sodium Thiosulphate.

The rate of reaction is a measure of the change, which happens during
a reaction in a single unit of time. The things that affect the rate
of reaction are as follows:

· Surface area of the reactants
· Concentration of the reactants
· The temperature at which the reaction is carried out
· Use of a catalyst

Reaction equation is mentioned above but rate equation could only be
decided by doing experiments. So the following procedure can be used
to carry out the experiment.



· 2 Measuring cylinders
· Beaker
· Stopwatch
· Paper with black cross on it
· Sodium Thiosulphate (different concentrations)
· Hydrochloric acid (same concentration each time)
· Water (different concentrations)
· Pipette


I predict that the greater the concentration of Sodium Thiosulphate

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solution the faster the chemical reaction will take place. Therefore,
the cross will disappear more quickly due to the cloudiness of the
solution. I predict that as concentration is doubled the amount of
time taken for the reaction is halved. This means that both graphs
drawn up in my analysis will have positive correlation, and will
probably be curved as the increase in rate of reaction will not be
exactly the same as the concentration is increased. This can be
justified by relating to the collision theory.

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. All this can be
understood better with full understanding of the collision theory
itself: For a reaction to occur particles have to 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, or Ea. The size of this activation energy is
different for different reactions.

I think that the concentration of a solution effects the rate of
reaction because 'the rate of reaction depends on how frequently the
molecules of the reacting substances collide. A more concentrated
substance has more molecules for a given volume than a more dilute
substance. Because there are more molecules about, the frequency of
successful collisions is greater, and the reactions happen faster.'


There are a lot of safety issues we must abide by in this experiment
also. We must remember that the substances which we use in this
experiment can be very harmful if used the wrong way.

When we do this experiment, it may be necessary to wear safety
goggles, as things are very unpredictable, and even though it is very
unlikely that the solution would come out of the beaker during the
experiment, one must still be cautious of spills.

We must make sure that coats and bags are all out of the way while
doing the experiment. Ties and hair should be tucked out of the way,
so they do not make contact with any of the chemicals. It would also
be preferable to wear a scientific apron, however this is not

We should also try our best not to spill any chemicals, and we must
not eat or drink in the lab while dealing with these harmful
chemicals, as they can get on to our hands.

Fair Test

To make this experiment a fair test, we need to make sure we do a
number of things. In this experiment we are trying to find the rate of
reaction using concentration as a factor, so there is a number of
things we need to make sure we do to keep it a fair test.

Firstly, we need to keep a chemical at a constant concentration. So,
in this experiment we have chosen to keep hydrochloric acid at a
constant concentration (5cm3). We could have, however, used Sodium
Thiosulphate as a constant, but we had chosen to use Hydrochloric

Next we must make sure that the solution is kept at a constant volume
throughout the experiment. If the volume is different, then it could
give different results to if it was at a constant volume. We must also
make sure that we add both the water and the Sodium Thiosulphate at
exactly the same time (into the beaker with the hydrochloric acid in
it), or it could affect the results of the experiment.

We must start the stopwatch at the exact time as we put the water and
the Sodium Thiosulphate into the beaker. To do this it is a lot easier
if there are two people doing the experiment, so one person can put
the two substances in the HCL, and one person can start the stopwatch.
Also The person timing the experiment will look for the disappearance
of the cross, otherwise there would be a time lapse between seeing the
cross disappear and telling the other person to stop the clock and
then eventually stopping the clock.

It is also important to keep the temperature the same to obtain fair
results, however we do not have to worry about this as the temperature
will be kept at room temperature through out the experiment. It is
also important we keep the colour and size of the cross the same this
will help keep the results fair for the experiment

Another thing we must do is to make sure that the beaker is completely
clean and free of any water or any other substance before we attempt
to start the next experiment.


Firstly, we drew a black cross on a white piece of paper. Then, we put
5 cm3 of hydrochloric acid in a beaker, and then put the beaker on the
black cross on the paper. We then added 50 cm3 of Sodium Thiosulphate
to the hydrochloric acid, and at this time we do not need to add any
water. . Immediately, we started the stopwatch. We then waited for the
black cross to be completely obscured by the mixture. When this
happened we stopped the stopwatch. By this time the mixture was
completely cloudy. Then we recorded what the time was.

We then emptied out the contents of the beaker, cleaned it out and
dried it out. Next, we added 45 cm3 of Sodium Thiosulphate; we kept
the HCL acid constantly at 5 cm3. Now, because we have to keep the
volume the same for it to be a fair test, we added 5 cm3 of water. (We
had to add the Sodium Thiosulphate and the water at the same time for
this to be a fair test also). We then recorded the time for the cross
to become completely obscured.

We repeated this many times, adding different amounts of Sodium
Thiosulphate and water each time, and recording the time taken each


After collecting the results I will be in position to analyse and use
graphical format to evaluate the results. When the results are
collected there are two methods that could be adopted to find the
order of the reactants in order to find the rate equation.

1st Method- Concentration-Time graph

In this we will plot the graph of concentration against time. The time
is the time taken for the reaction to finish at particular
concentration of the solution. Then to find the order we will need to
plot another graph by obtaining information from the
concentration-time graph. Then draw straight line tangents to this
graph at points corresponding to a variety of concentrations. Each of
these tangents has a gradient which is the rate of the reaction at
that concentration. You now have a set of rates and corresponding
concentrations which can be used to plot the second graph to find the
order of the reaction.

The concentration time graph looks something like the one on the
right. The graph has negative curve. Various tangents could be found
on the graph and then we can calculate and plot the rate against
concentration graph to find the order. The rate graph has three curves
which differentiates the three orders the following three graphs show
the three curves.


This method can be used to find rate equation. The rate equation is
equal to

Rate = k [HCl]m [Na2S2O3]n

k= constant

m & n= the order of that particular solution

Rate = k [HCl]m [Na2S2O3]n

I have worked the values for m & n. Both of them have value of 2.

So our rate equation is

Rate = k [HCl]2 [Na2S2O3]2

We can also calculate the units for constant k when we have the full
rate equation. Thus I have calculated the units for the constant k.

mol m-3 s-1 = k [mol m-3]2 [mol m-3]2

mol m-3 s-1 = k mol2 m-6 mol2 m-6

mol m-3 s-1 = k mol4 m-12

mol m-3 s-1 = k

mol4 m-12

mol-3 m9 s-1 = k


My results show that as concentration decreases, time taken for the
solution to go cloudy increases. This can be explained using the
collision theory. The rate of reaction is the speed at which a
chemical reaction takes place. It is usually expressed in terms of the
concentration (e.g.- moles per litre) of a reactant consumed, or
product formed in unit time. Therefore the units would be moles per
litre per second (mol 1-1 s-1). It may be affected by the
concentration of reactants, the temperature of reactants, and the
presence of a catalyst.

Increasing the concentration means there are more collisions and more
successful collisions. Consequently the rate of collision increases.
The time it takes for a change to take place can be measured; the rate
of reaction can be approximated by taking the reciprocal of this time
(1/time). For a reaction to take place two reactants must collide and
the collision must have sufficient energy.

In the higher concentration there are more particles, so there is a
higher chance of a reaction with the necessary amount of energy

My prediction was correct to a certain extent, as I predicted that as
concentration increased the time taken for the solution to go cloudy
decreased. However, I was not entirely correct as I predicted that
time taken would double when concentration halved, but this was
incorrect. When the concentration was 50 (M) the average time was 0.47.07
seconds, but when the concentration was 25 (M) the average time was
1.52.27 seconds. This is not half the time; it had an error margin of
1.05.20 seconds. This may be because my prediction was incorrect, but
it may also be due to human error in the measuring of the liquids or
the timing.


I think most of the experiment I have done has been successful with as
little anomalous results as possible. However, there are still a few
anomalous results which I will now point out.


This is the only anomalous result that I have recorded. The result may
have turned out anomalous because of basic human error, or maybe
because we measured the substances wrong. It may have even been
because we did not clean the apparatus properly.

Apart from this, the accuracy of my experiment has been more or less
accurate. Although there are a number of ways in which we could have
made the results more reliable. For instance, we could have used
better measuring equipment, because the apparatus we used was mainly
basic equipment.

Another thing we could have done to bring more evidence is to have
tried to use the Hydrochloric acid as the variable substance, and used
the Sodium Thiosulphate as the constant substance. This would have
brought more evidence to support the idea that the higher
concentration of a substance, the faster it will react.

As detected one of the main source of the error of time was to measure
and record the time taken for the cross to disappear. To detect when
cross disappears and stopping clock at that sudden instance was very
hard to judge effectively. Also when the lower concentrations were
used it was detected that the cross disappears in some areas of the
flask but is still visible in a corner or in a little area. This made
it very hard to examine and to keep the test to a fair level.

The experiment was done over a period of 5 days; I used the same cross
I used for the first day however the light intensity was different on
each day. This made it hard to keep it fair as it would be easier to
see the cross on some days rather than others.

One of the best methods to work out the rate equation is to find out
the concentration of the solutions after at different time intervals.
We do not know the concentration of the reactants after a given time
interval which was the reason this method was not introduced in the
procedure. A school lab does not have enough sophisticated equipment
which will allow us to measure the concentration of a reactant after a
given period of time when the reaction is still continuing. Carrying
out the experiment in a chemist lab which has more detailed technology
available will make the experiment more reliable, accurate and will
minimise the errors.

I conclude that the more concentrated a reactant is, the quicker the
rate of reaction time will be.I have come to this conclusion because
of several reasons. Firstly, my results give conclusive evidence that
as the amount of Sodium Thiosulphate decreases and the amount of water
in the solution there are less atoms to collide and therefore less
successful collisions causing chemical change so the reaction rate is
slower. In a more concentrated solution, there are more atoms to
collide so the reaction time is quicker.

Overall I think that this experiment was a success as I have proved
that concentration does affect the rate of reaction, and I have also
found that when the concentration is doubled the rate of reaction is
not necessarily halved.

I think I could have improved my investigation by:

· Obtaining more results to get a better overall result.

· Take times for a lower concentration than 5cm3 (we could not due to
time restrictions).

· I used ICT to display my coursework, but I did not use it in anyway
that affected the experiment.

· I would like to do a further experiment to confirm my results.
However I am restricted by time and the available facilities which
means I cannot repeat it.

· Also instead of using a cross on a piece of paper I could use a
single beam of light until it could no longer be seen

· Use of computer to aid analysis of results

· Carry out all of the experiments on same day to improve accuracy

· Calculate more than ten tangents to improve accuracy

List of sources to aid work

Various Textbooks and Internet sites were used in providing background

Cambridge Chemistry 2 Book – Brain Ratcliff, Helen Eccles

Do Brilliantly at ... A2 Chemistry- George Facer

Chemistry A-Level through Diagrams- M. Lewis



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