Samantha Nock
GCSE Science Investigation 2011
Prediction:
For this experiment I must investigate the changes in the rates of reaction between
Hydrochloric Acid and Sodium Thiosulphate, when one variable in the experiment is
changed. The variables that are possible in my experiment are heat, concentration
and surface area and will all affect the rate of reactions.
Since the chemicals that I’m working with are liquids it would be very difficult to
control so I will exclude that method and test concentration and heat in my
preliminary methods.
I predict that when either the heat is increased or the concentration is higher then the
rate of the reaction will be a lot faster. For example as the concentration of Sodium
Thiosulphate decreases the rate of the reaction will do the same because there are
less thiosulphate particles to react with the hydrochloric acid. When temperature
increases the then the rate of reaction will speed up because the particles in the
solution are moving quicker because the heat is giving the particles energy and
therefore are colliding more often.
Scientific Theory:
The rate of a reaction is how fast a chemical reaction happens. Some reactions are
almost instantaneous whilst others can take years to finish. For example, dynamite
exploding is very quick but rusting can take years to happen. We can only calculate
a rate of a reaction by seeing how much of a reactant has been used in a certain
amount of time.
The collision theory: In order for any reaction to take place, particles in a substance
must collide. The faster the chemicals collide the quicker the reaction. There are
several factors that can increase the rate of collisions:
1. Surface Area
If the particle size decreases, this means that the reaction will be faster because the
surface are of the chemical has increased so there is more chemical to be reacted.
Large Pebbles
Small Pebbles
Here are some pebbles
being dissolved in
water. The smaller
pebbles will dissolve
quicker as there is more
surface area of the
pebbles for the water
particles to react with.
Samantha Nock
GCSE Science Investigation 2011
2. Concentration
When substances are dissolved in water like acid we can control the
concentration of the liquid. If there is more acid then then there are more particles
which causes collisions to be more likely with a substance that can be put into the
solution. Therefore the rate of reaction increases.
Here are two beakers
with different
concentrations of
acid. There are
magnesium strips
being reacted in the
acid solutions.
1
2
The magnesium is more likely to collide with the acid in number 2 as the acid is more
concentrated.
3. Temperature
Increasing the temperature can affect the rate of a reaction in two ways:


As the temperature increases then this causes particles to move more quickly
which makes collisions more likely in a certain amount of time.
As the temperature increases the particles gain more energy so collisions are
more likely to occur and result in a reaction.
4. Catalyst
Catalysts are substances that can increase the rate of a reaction. At the end of any
reaction however they remain unchanged. An enzyme is a natural occurring
biological enzyme.
This scientific explanation will be used throughout investigation to explain certain
aspects if the experiments.
Apparatus list:
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Conical Flask
Beaker
Small and Large measuring cylinder
Stirrer
Pipette
Samantha Nock

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






GCSE Science Investigation 2011
Thermometer
Stopwatch
1molar of Hydrochloric Acid
0.1molar of Sodium Thiosulphate
Distilled water
Bunsen Burner
Gauze
Tripod
Black Marker Pen
Preliminary Experiments:
I have carried out preliminary experiments because it helps me to see how to carry
out my experiment according to the variables. For example the amount of each
chemical I should use and how other things affect my results.
I have ruled out the methods of using catalysts as I do not have any to use, and
surface area because since the chemicals are liquids then I cannot control the
surface area in any easy way therefore I tested these two methods of investigating
the rate of reaction:
1. Heat:
In this experiment I kept the amount of hydrochloric acid and sodium thiosulphate the
same each time at 20ml each. But I changed the temperature of the solution each
time. Starting at room temperature of about 20C then increasing by 10C each time
up to 60C. Firstly I drew a cross on the bottom of the conical flask, this will help me
tell when the reaction has finished. Next I used a beaker of water which I boiled to
60C first on a tripod and gauze over a Bunsen flame. I then measured 20ml of each
chemical (making sure to wash the measuring cylinder in between so the chemicals
wouldn’t mix too early) and put them both into the conical flask and then quickly put
the flask into the beaker of water and pressed START on the stopwatch. I waited
until I could not see the cross I drew on the bottom of the conical flask before I
stopped the stopwatch and recorded the time. I repeated this at other temperature
going down from 60C by 10 each time and recorded the results shown below:
Samantha Nock
Hydrochloric acid
(ml)
20
20
20
20
20
GCSE Science Investigation 2011
Sodium
Thiosulphate (ml)
20
20
20
20
20
Temperature (C)
20- Room temp.
30
40
50
60
Time (Minutes:
Seconds)
1:40
0:51
0:45
0:43
0:34
The conclusion I can make from these results is that the higher the temperature the
quicker the rate of reaction. This is because the heat is making the particles in the
solution move quicker and therefore more likely to collide and the reaction happens
faster.
Also I can see that I have used an appropriate amount of each chemical as they are
both the same amount and the reaction took a reasonable amount of time to happen.
However the experiment isn’t very accurate. It is hard to control the temperature of
the water as it is always changing and either needs to be heated up or cooled down
at inconvenient times. This therefore makes my experiment inaccurate as the
temperatures would not have been exact and would have affected my results.
Because of this, heat is not the best method to use for investigating the rate of
reactions as it is difficult to control and affects my results.
2. Concentration:
I started with 20ml of each chemical with no distilled water added. To do this I drew a
cross with black marker on the bottom of the conical flask, measured 20ml of each
chemical with the measuring cylender. Then added them both to the conical flask
and started the stopwatch. I recorded the time it took for the cross to not be seen,
which told me the reaction had finished, to stop the stopwatch and record the time.
The next one I did I added 2ml of water with the two chemicals. I repeated what I did
for the previous test and recorded the time. I repeated it again twice more with 4ml
and 6ml of water added and got these results:
Samantha Nock
Hydrochloric acid
(ml)
20
20
20
20
GCSE Science Investigation 2011
Sodium
Thiosulphate (ml)
20
20
20
20
Distilled water
(ml)
0
2
4
6
Time (minutes:
seconds)
2:22
2:37
2:08
2:37
For the first two tests I stirred the solutions and the second two I did not. Here it
seems to show that this makes the reaction go faster. Maybe I have doing the
experiment incorrectly and not knowingly changing a variable but it is definitely
something to consider.
The successful aspect about this experiment is that I can see the effects in the times
as the distilled water is added, this shows that the experiment is viable.
But however I have changed more than one variable in this test. In this experiment I
have to keep the total amount of liquid in the reaction the same by reducing the
amount of thiosulphate each time as I add distilled water to change the concentration
of the solution. This is the way to keep the experiment fair. Therefore the
hydrochloric acid will stay at the same volume of 20ml but the sodium thiosulphate
will change whilst the distilled water increases so together they will make 20ml in
order for the total volume of liquid to remain at 40ml.
This is the best method which I will use for this investigation as it is efficient to
control and usually accurate.
Method:
Firstly I drew a cross on the underside of the conical flask which would be used to
see whether the reactions have completed. Next I measured 20ml of sodium
thiosulphate using the measuring cylinder then added it to the clean conical flask.
Then I washed the measuring cylinder and measured 20ml of hydrochloric acid. This
was then added quickly to the conical flask with the sodium and the stopwatch was
started as soon as possible. I stirred the solution with a stirrer also. I watched the
solution until I could no longer see the cross through the liquid which went a cloudy
pale yellow. At which point I stopped the stopwatch because I knew the reaction had
completed. I recorded the time on a table.
Samantha Nock
GCSE Science Investigation 2011
I then washed all the equipment before starting the next test. Here I reduced the
amount of sodium thiosulphate to 17.5ml and added 2.5ml of water to it. Which
reduces the concentration of sodium and the amount of liquid I use still equates to
40ml altogether. I then added 20ml of hydrochloric acid, remembering to wash the
apparatus in between chemicals, and started timing. I recorded the total time for that
test.
I carried on this same process but decreasing the amount of sodium thiosulphate
and increasing the amount of distilled water each time in this order. 20:20, 17.5:2.5,
15:5, 12.5:7.5, 10:10, 7.5:12.5, 5:15, 2.5:17.5. During this the amount of hydrochloric
acid stays the same in order to make the experiment a fair test by only changing the
one variable. I also repeated the whole experiment two more times in order to make
sure I get the most accurate results possible.
Results:
Hydrochloric
acid (ml)
Distilled water
(ml)
Time
(seconds)
Average time
(seconds)
20
Sodium
Thiosulphate
(ml)
20
0
91
20
17.5
2.5
20
15
5
20
12.5
7.5
20
10
10
20
7.5
12.5
20
5
15
20
2.5
17.5
101
84
89
108
120
110
119
119
117
118
164
204
266
187
243
426
416
295
511
435
446
537
548
507
The highlighted times are outliers
113
118
185
255
421
441
531
Samantha Nock
GCSE Science Investigation 2011
Graph:
The graph of my results shows a steady negative correlation. This shows the relation
between the concentration and the time taken for the reaction to take place; which is
that as the concentration decreases, the time taken for the reaction to happen
increases. This is because as the concentration decreases there are less
thiosulphate particles to react with the hydrochloric acid which slows the rate of
reaction down significantly with each reduction in concentration of thiosulphate.
My graph also shows several outliers (circled results) which may have occurred due
to faulty equipment or a mistake in the recording of results. The error bars show the
range of what each record could fall in. This gives me an idea of how results can
vary with each test and that the rates of reactions cannot be accurately predicted.
Conclusion:
My results show a correlation between concentration and rates of reaction but not a
definite cause of concentration can either speed up or slow down a reaction.
I can see that my experiment had many outliers. This shows that my experiment was
probably not very accurate. These outliers were probably caused by faulty
equipment or incorrect measuring of chemicals or recording the results incorrectly.
Because of this my experiment may not show an accurate link between time and
concentration.
I could improve this investigation by taking more time to measure out my chemicals
and to make sure that I have waited long enough to see that the reactions have
finished so as the results are true and accurate. I also need to take special care in
recording my results to make sure I don’t make a mistake. I should check that all of
my equipment works and are in the correct units if I’m measuring something. This
will reduce the amount of outliers I get and will probably create a more acceptable
graph of results.
Overall my conclusion is that as the concentration decreases in volume, the rate of
the reaction also decreases. This is caused by the lack of sodium thiosulphate
particles to collide with the hydrochloric acid particles to create a quick reaction. So
concentration affects the rate of reaction significantly and is shown as a negative
correlation on a graph.