Kyle Baptista
Chemistry Practical
How is the rate at which sulfur is formed as a precipitate, and
therefore the rate of the reaction, affected by the
concentration of the reagents?
Research Question
How is the time taken in seconds for the sulfur precipitate produced, to obscure the cross,
affected by the concentration of sodium thiosulfate at 0.02, 0.04, 0.06, 0.08 and 0.1 M, in the
chemical reaction between sodium thiosulfate and hydrochloric acid?
Scientific Information
The rate of a reaction depends on the frequency of collisions between particles which are
successful. However, not all collisions are successful. The two main factors that influence this
is whether or not the particles have the minimum value of energy or the activation energy for
the reaction to take place, and whether or not they collide with the correct collision geometry.
Therefore, any variable that uses these factors to change the frequency of successful collisions,
should affect the rate of a reaction. The variable that this experiment will investigate is the
concentration of one of the reagents. The effect of concentration of the reactant on the rate
of a reaction can be studied easily by the reaction between sodium thiosulfate and
hydrochloric acid. In this investigation, sodium thiosulfate will react with dilute hydrochloric
acid to produce sulphur dioxide, sulphur and water, represented by the chemical equation,
Na2S2O3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + SO2(g) + S(s). Sulphur dioxide is a soluble
gas and sodium chloride is also a soluble salt, both of which dissolves completely in an
aqueous solution. The sulphur formed however is insoluble and exists in the mixture as a
white or pale yellow precipitate that gives a milky appearance and makes the solution opaque.
Therefore the rate of the reaction can be studied by monitoring the opaqueness of the
reaction. This can be easily done by measuring the time taken (s) for forming a certain
amount of sulphur. In this experiment, the reaction will be carried out by mixing the
reactants in a conical flask, and placing the conical flask on the top of a piece of white paper
that has a cross mark on it. Before the reaction starts, the mark is clearly visible from the top
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of the conical flask through the solution. However, as the reaction proceeds, the sulphur
precipitates will make the solution more opaque until eventually the mark gets completely
obscured. A stopwatch will be used to record the time taken for the precipitate to be produced
to this quantity. Hence, the time taken will determine the rate of the reactions relative to each
other, with the shortest time representing the fastest reaction rate. The independent variable
of this investigation will be the concentration of the sodium thiosulfate solution ranging from
0.02, 0.04, 0.06, 0.08 and 0.1 M. This experiment will also include the order of the reaction
which is the relationship between the concentration of a species and the rate of the reaction.
For example, doubling the concentration could either have no effect, double or quadruple the
rate of the reaction depending on its order of reaction.
(Sites used have been cited in bibliography)
Prediction
The greater the concentration of Sodium thiosulfate, the shorter the time, taken for the cross
to be obscured, should be. This is because by increasing the concentration of sodium
thiosulfate and keeping the volume constant, the number of sodium thiosulfate molecules
present would increase. Therefore, the frequency of collisions between the increased number
of reactant particles should increase, hence causing the frequency of successful collisions to
also increase leading to a faster rate. Moreover, previous experiments similar to this one have
also shown results that support this outcome, therefore the results of this experiment are also
predicted to be similar.
Concentration of Sodium Thiosulfate (M)
Time taken for Cross to be completely Obscured (s) 2
Variables
Independent Variable -
The concentration of sodium thiosulfate that will react with the hydrochloric acid, is going to
be changed. The different concentrations that going to be used are 0.02, 0.04, 0.06, 0.08 and
0.1 M.
Dependent Variables -
The time taken between the start of the reaction and for sulfur to be produced, at a sufficient
quantity to completely obscure the image of the cross, will be recorded. The time will be
recorded in seconds and will done so using a stopwatch.
Controlled Variables -
• The total volume of the solutions will be held constant. Especially since the concentration
of the solutions are kept constant during each trial, the total volume will not affect the rate
of the reactions at all or significantly. Although, to make it a fair test, the total volume of the
solutions will be the same and will be done so, through the use of a measuring cylinder.
• The distance between the experimenter's eyes and the cross mark on the sheet of paper will
be kept the same. Since the time taken for the cross to be obscured depends solely on the
observer’s judgement, the distance between them and the cross could potentially affect their
vision and therefore the time recorded, leading to inaccurate results. Hence, the observer
will look the conical flask from approximately 10 cm above it which can done with the aid
of a ruler.
• The same cross on the sheet of paper will also be kept constant. Since the size, shape or
intensity could affect how easily the experimenter sees the cross, the time recorded would
also be affected. Hence to prevent any inaccuracies, the cross will be drawn once on a sheet
of paper to be reused throughout the experiment.
• The same shaped and sized conical flask will be used. The shape and size will not
significantly affect the rate of the reaction, however to make it a fair and valid experiment,
similar conical flasks will used. Moreover, the size of the flask could also affect the
experimenter’s vision due to change in the distance between them and the cross.
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Uncontrollable Variables -
• Room temperature - The temperature of the surroundings would affect the temperature of
the solutions. This causes the rate of the reaction to also be affected, as an increase in
temperature would mean an increase in average kinetic energy of the particles. Hence, not
only would more particles have the activation energy which increases the rate of the
reaction, but they would also move at greater speeds, colliding and therefore reacting more
frequently. In terms of controlling the room temperature, an air conditioner can be used to
maintain the temperature. Although it is still possible for others to open the doors of the lab
which would cause a fluctuation in the temperature.
• Volume of each solution throughout the experiment - Even tough the volume of each
solution used can be initially measured and kept constant, the water can still evaporate
leaving the solutes behind and therefore making the solution more concentrated. A random
increase in concentration that is not accounted for, can lead to more inaccurate readings.
This could be reduced however, by increasing the pace of the experiment to minimise the
amount of water evaporated of each solution. It could also be monitored by using a control
which consist of the same concentration as each of the solutions. Its mass can be measured
before and after each trial to calculate the loss of water and hence new and higher
concentrations.
Chemicals
• Hydrochloric acid 1 M (150 ml)
• Distilled water (600 ml)
• Sodium Thiosulfate 0.1 M (450 ml)
Materials
• Clamp Stand (1)
• Clamps (2)
• Burette (1)
• Timer (1)
• Measuring cylinders (2)
• Conical Flask (5)
• Permanent Marker (1)
• Sheet of Paper (1)
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Safety
Hazard Risk How can it be Emergency Action Level of risk
minimised
Chemicals (Sodium Hydrochloric acid Perform this Immediately go to Low
Thiosulfate and solution is corrosive demonstration in a the clinic
Hydrochloric Acid) to eyes and skin. It well-ventilated lab
is moderately toxic only. Avoid contact
by ingestion and of all chemicals
inhalation. Sodium with eyes and skin.
thiosulfate solution Wear goggles,
is a body tissue gloves and lab coat
irritant. The sulfur
dioxide gas
produced is also a
skin and eye
irritant.
Any Glass Materials Can fall over and Always keep Slowly walk away Medium
shatter into smaller equipment at the from the mess and
shards of glass center of the table get a broom to
which can cause sweep the glass to
potential injuries one side
Any Heavy Can fall and cause Always keep Slowly walk away Low
Apparatus (for injuries or break equipment at the from the mess and
example, Clamp other objects center of the table get a broom to
Stand) sweep the glass to
one side
(Sites used have been cited in bibliography)
Preliminary Testing
Being unable to access a science laboratory has meant that information from the internet such
as past experiments and simulations has been used to aid in this investigation. Sites such as
amrita.olabs.edu.in, which is an online simulation, has been used to influence some of the
values that were needed to be used, such as the volume and concentration of the chemicals.
Sites such as bbc.co.uk and flinnsci.com has also been used to help with the method of this
experiment, as it provided ideas for easier and efficient ways to determine the differences in
the rates of the reactions.
(Sites used have been cited in bibliography)
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Method
1. Set up equipment as shown in fig 1.1 and 1.2
2. Take five clean conical flasks and label them as A, B, C, D and E respectively.
3. Add 10, 20, 30, 40 and 50 ml of 0.1M sodium thiosulfate solution to the flasks A, B, C,
D and E respectively.
4. Then add 40, 30, 20 and 10ml of distilled water to the flask A, B, C and D respectively
so that volume of solution in each flask is 50ml.
5. Add 10ml of 1M hydrochloric solution to the conical flask A.
6. When half of the hydrochloric solution has been added to the conical flask, start the
stopwatch immediately.
7. Shake the contents of the conical flask and place it on the sheet of paper that has a
cross mark at the centre.
8. Continue to observe from the top of the flask and stop the stopwatch when the cross
marks becomes completely obscured, and record the time.
9. Repeat steps 5 - 9 by using flasks B, C, D and E respectively, instead of flask A.
10. Repeat steps 1 - 9, 2 more times.
Where the
will be
Fig 1.2
Fig 1.1
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Bibliography
Scientific Information
• https://chem.libretexts.org/Bookshelves/
Physical_and_Theoretical_Chemistry_Textbook_Maps/
Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Kinetics/
03%3A_Rate_Laws/3.03%3A_The_Rate_Law/3.3.03%3A_Reaction_Order
• https://www.britannica.com/science/collision-theory-chemistry
• https://chem.libretexts.org/Bookshelves/
Physical_and_Theoretical_Chemistry_Textbook_Maps/
Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Kinetics/
06%3A_Modeling_Reaction_Kinetics/6.01%3A_Collision_Theory/
6.1.06%3A_The_Collision_Theory
Safety
• https://www.epa.gov/sites/production/files/2016-09/documents/hydrochloric-acid.pdf
• http://science.cleapss.org.uk/resource/SSS035-Sodium-sulfites-thiosulfate-and-
persulfate.pdf
• https://www.lung.org/clean-air/outdoors/what-makes-air-unhealthy/sulfur-dioxide
Preliminary Testing
• http://amrita.olabs.edu.in/?sub=73&brch=8&sim=142&cnt=4
• https://www.bbc.co.uk/bitesize/guides/zyb4h39/revision/7
• https://www.flinnsci.com/api/library/Download/78da6c8204aa48a294bd9a51844543ad
