Chemistry Experiment
Introduction
The rate of the reaction is the measure of how fast a reaction takes place. It is always a measure of the
rate of change of the concentration of reactants or products with time. Thus to study the rate of a
reaction, one would have to measure a change in concentration and the rate at which it is occurring.
There is no simple way the change in concentration can be detected. It is only the change in color or the
formation of a precipitate that helps detect a change in concentration. The rate of a reaction depends
on various factors: temperature, concentration, presence of catalyst etc. To see the effect that one
factor has on the rate of the reaction we must keep the other factors constant.
Research Question
To study the effect of the concentration of hydrochloric acid on the rate of the reaction between sodium
thiosulphate and hydrochloric acid at room temperature.

Independent variable
Variable
Concentration of hydrochloric
acid

Unit
Molarity (mol/dm3)
How is the variable measured
Although there is no direct
way by which this variable can
be measured, we can employ
a quantitative approach.
Assuming that the initial
concentration of HCl is known
to a certain degree of
accuracy, we can calculate the
concentration after dilution
by first calculating the number
of moles in the original
sample and dividing it by the
volume of the diluted
solution. Keep in mind the
number of significant digits in
the (±0.01M)
Unit
Although the unit is actually
mol/dm3, here it would be
seconds, since we cannot
keep a track of the change in
moles.
How is the variable measured
Time can be measured to a
great deal of accuracy using a
stopwatch. (±0.01s). To
account for the moment
when we should stop the stop
watch and maintain
uniformity, we place the flask
Dependent variable
Variable
The rate of the reaction (the
time taken for turbidity to
form)
in which the reaction is taking
place on a white paper with a
cross. The time it takes for the
cross to disappear completely
would be the time that we
would measure on the stop
watch. Since we also need to
account for the random error
involved in this process. This
would be further explored in
the observation section.

Controlled variables
Variable
Amount of sodium
thiosulphate
Unit
Moles (mol)
Temperature of the reaction
Degrees Celsius
The total volume of the
reactants
Milliliters (cm3)
The concentration of
Molarity (mol/dm3)
How is the variable controlled
We ensure that we add the
same number of moles of
Na2S2O3 every time by
ensuring that we put in the
same volume of the Na2S2O3
solution. Since, the
concentration of Na2S2O3
remains the same; the
number of moles we add
remains the same
throughout.
Although, it is highly unlikely
that we can control the
temperature to a very high
degree of accuracy, we can
maintain it within a range. By
performing the experiment in
a corner away from the
sunlight in a temperature
controlled room can help
control the temperature of
the reaction mixture
The total volume of the
reaction is always kept
constant by adding the
required amount of distilled
water to the mixture so that
the volume is always fixed at
50ml. This also helps change
the concentration of HCl is a
uniform manner.
Although this sounds
hydrochloric acid (though the
that used in the experiment
was varied by the use of
distilled water)
redundant it is important to
use the same solution of HCl
for the entire experiment so
that we control the
concentration and thus the
number of moles as well.
Hypothesis
Rate of a reaction is directly proportional to the concentration of the reactants raised to some factor
which is dependent on the type of the reaction.
Also, since a higher concentration results in more number of reactant molecules per unit volume, it
allows more effective collisions to take place and hence results in the faster formation of the product, I
would expect a higher concentration to result in a higher rate of the reaction.
A higher rate of reaction would mean that the solution would turn cloudy more quickly and thus the
cross would disappear even faster. Thus as the concentration increases I would expect the time it takes
for the cross to disappear to reduce.
Diagram (Courtesy:
http://www.sciencephotolibrary.com/images/showFullWatermarked.html/A710024Rates_of_reaction_experiment-SPL.jpg?id=657100024)
At the beginning
At the end
Method
Apparatus list






Conical flask
3 100ml beakers
3 25ml measuring cylinder
Filter paper with a cross marked on it
Blotting paper
Stop watch
Reagent list



150ml 0.20M hydrochloric acid
150ml 0.10M sodium thiosulphate
150ml distilled water
Take a clean measuring cylinder and measure 25ml of hydrochloric acid and pour it into the conical flask
placed on the crossed filter paper. Take another measuring cylinder and measure 25ml of sodium
thiosulphate. Pour it into the conical flask and simultaneously start the stop watch. Be careful while
pouring one solution in another and avoid splashing. Measure the time it takes for the cross to
disappear. However, care must be taken that one must view the cross consistently from a position
vertically above the solution. Any other position would result in erroneous readings.
Repeat the procedure but instead of using 25.0ml of 0.20M hydrochloric acid use 20.0ml and add 5.0ml
distilled water to it. The time should be measured with the same procedure. Perform the same thing for
15.0ml of HCL and 10.0ml of water and 10.0ml of HCL and 15.0ml of water. The results should be
tabulated and graphed
Data Collection
Raw Data
Uncertainty in amount of HCl added = ±0.1ml
Uncertainty in time measured (only systemic) = ±0.01s
Reading
1
2
3
4
Data Processing
Amount of HCL (ml)
25.0 ± 0.1
20.0 ± 0.1
15.0 ± 0.1
10.0 ± 0.1
Time taken (s)
65.43 ± 0.01
74.92 ± 0.01
85.36 ± 0.01
103.12 ± 0.01
The time that we measure can be used as a measure of the rate of the reaction. Since, the more time we
take the slower is the reaction; the relationship between reaction rate and time is inversely
proportional. Thus 1/T would be a good way to approximate the rate of the reaction. However, while
calculating this we need to account for not only systemic errors but also random ones. Although the
uncertainly due to systemic error is about ±0.01s,
𝑚𝑎𝑥𝑖𝑚𝑢𝑚 𝑣𝑎𝑙𝑢𝑒 − 𝑚𝑖𝑛𝑖𝑚𝑢𝑚 𝑣𝑎𝑙𝑢𝑒
2
the uncertainty due to random errors can be much higher. There is no simple way to calculate it. We can
only make an approximation. Since the inaccuracy due to random error is way larger compared to that
of systemic error, we can ignore systemic error altogether and take into consideration only the random
error. To a great deal of accuracy the time that we measure would have an uncertainty of no less than
±1.00 seconds. Therefore, tabulating the results,
Reading
1
2
3
4
Time (s)
65.43 ± 1.00
74.92 ± 1.00
85.36 ± 1.00
103.12 ± 1.00
Rate of the reaction: 1/T (s-1)
0.01528 ± 0.00023
0.01334 ± 0.00023
0.01172 ± 0.00023
0.00970 ± 0.00023
Please note that although the uncertainties decrease as time increases I have accounted for the highest
uncertainty possible. Since the uncertainties in time are approximate, this assumption is fair.
The second parameter that we are measuring is the concentration. This is fairly easy to measure as we
know the uncertainties in measurement and there are fewer chances for random errors.
The concentrations can be calculated by using the following equation:
𝐶𝑜𝑛𝑐 𝑜𝑓 𝑆𝑜𝑙𝑢𝑡𝑖𝑜𝑛 =
(𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝐻𝐶𝑙 𝑎𝑑𝑑𝑒𝑑) ∗ (𝐶𝑜𝑛𝑐 𝑜𝑓 𝐻𝐶𝑙)
(𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝐻𝐶𝑙 + 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝐷𝑖𝑠𝑡𝑖𝑙𝑙𝑒𝑑 𝑤𝑎𝑡𝑒𝑟)
Accounting for the uncertainties we get the following data
Reading
1
2
3
4
Amount of HCl (ml)
25.0 ± 0.1
20.0 ± 0.1
15.0 ± 0.1
10.0 ± 0.1
Conc of HCl (mol/dm3)
0.20 ± 0.01
0.16 ± 0.01
0.12 ± 0.01
0.08 ± 0.01
Therefore, the data after calculation is:
Reading
1
2
Rate of the reaction: 1/T (s-1)
0.01528 ± 0.00023
0.01334 ± 0.00023
Conc of HCl (mol/dm3)
0.20 ± 0.01
0.16 ± 0.01
3
4
0.01172 ± 0.00023
0.00970 ± 0.00023
0.12 ± 0.01
0.08 ± 0.01
Graphing (with error bars)
Concentration of HCl being an independent variable should be on the x axis and the rate of the reaction
measured as 1/T should be on the y axis as it is the dependent variable.
Rate of the reaction vs Concentration
Rate of the reaction -s-1
0.016
0.015
0.014
0.013
0.012
0.011
0.01
0.009
0.008
0.05
0.1
0.15
Concentration of HCl
0.2
0.25
-mol/dm3
The above mentioned graph shows a linear relationship between the concentration of HCl added and
the rate of the reaction. The black line indicates this and passes between the errors bars; not going
outside them. The blue line connects the readings using a curve. Thus the readings compliment the
hypothesis.
Conclusion and Evaluation
This experiment helps establish that the rate of the reaction increases with an increase in concentration.
This is shown within the limits of accuracy by both the graph and the data. Although, we were slightly
lucky in approximating the value of random errors that occur while measuring time; a more accurate
approach would have been to repeat the experiment a few times and take the mean to get an
approximation for the amount of random errors that occur.
Rate of the reaction vs Concentration
Rate of the reaction -s-1
0.016
y = 0.0459x + 0.0061
0.015
0.014
0.013
0.012
0.011
0.01
0.009
0.008
0.05
0.1
0.15
Concentration of HCl
0.2
0.25
-mol/dm3
Also, as shown by the equation of the tread line, the intercept is not zero. This suggests that even at a
zero concentration of HCl we still have a positive value for the rate of the reaction. This is not true.
Therefore, it is not very wise to conclude anything from the gradient of the curve. Moreover, the linear
relationship that holds true for these set of points may not hold true as we move to lower concentration
of HCl. It is important to be aware of these possible limitations of the data that we have collected.
Improvements
Variable
Time
Unit
Seconds (s)
Time
Seconds (s)
Sources
Improvement/s
In the above mentioned
experiment we approximated
the value of the random error.
Although, it turned out be fairly
accurate, to be on the safer side
we can perform the experiment
thrice and take the mean of the
uncertainties to get a measure of
the random errors
Another error that creeps in
while we are measuring time is
that caused by the human eye. It
is difficult to maintain uniformity
while measuring the moment at
which the cross disappears. An
instrument like a colorimeter
might help do this more
accurately.
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Heinemann IB Chemistry textbook
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