Aim :
To study the effect of the concentration of sodium thiosulphate, Na2S2O3, solution on the rate of reaction.
Problem statement : How does the concentration of sodium thiosulphate, Na2S2O3,
solution affect the rate of reaction?
Hypothesis : The rate of reaction will increase if the concentration
of sodium thiosulphate, Na2S2O3, solution increases.
When
the concentration of sodium thiosulphate, Na2S2O3, solution increases, the rate
of reaction increase.
Variables :-
Manipulated variable : Concentration of sodium thiosulphate,
Na2S2O3, solution.
Responding
variable : Rate of reaction.
Controlled variables : Volume and concentration of sulphuric acid,
H2SO4
Operational definition : The time taken for the mark “X” to disappear
from sight when the sodium thiosulphate, Na2S2O3, solution react with sulphuric
acid, H2SO4
The rate of reaction is measured by how fast the
'X' is hidden by the yellow precipitate
Apparatus : 50cm3 measuring cylinder, 100cm3 conical flask,
10cm3 measuring cylinder, stopwatch
Materials : 0.2 mol dm-3 sodium thiosulphate, Na2S2O3,
solution, 0.1mol dm-3 sulphuric acid, H2SO4, distilled water, white paper
marked “X” at the centre
Procedure :-
1.
50cm3 of 0.2 mol dm-3 sodium
thiosulphate, Na2S2O3, solution is measured with a 50cm3 measuring cylinder and
is poured into a conical flask.
2.
The conical flask is placed on the “X”
mark on the centre of a piece of white paper.
3.
Using a 10cm3 measuring cylinder, 5cm3
of 0.1mol dm-3 sulphuric acid, H2SO4 is measured.
4.
The sulphuric acid, H2SO4 is immediately
and carefully poured into the conical flask containing 50cm3 of sodium
thiosulphate, Na2S2O3, solution. At the same time, the stopwatch is started.
5.
The conical flask is shaken slowly
throughout the experiment on the white paper.
6.
The mark “X” is observed vertically from
the top part of the conical flask through the solution as shown in the figure.
7.
The stopwatch is stopped quickly as the
mark “X” on the white paper is no longer visible.
8.
The time taken, t, for the mark “X” to
disappear from sight is recorded
9.
Steps 1 to 8 are repeated four more
times using different volumes of 0.2 mol dm-3 sodium thiosulphate, Na2S2O3,
solution which is diluted with different volumes of distilled water to form 50cm3
of solution as shown in the table. Whereas the other conditions remain
unchanged.
10. The
results are recorded in a table.
Data and observations :-
Interpreting data :-
1.
Based on the results, two graphs are
plotted.
(a)
The graph of the concentration of sodium
thiosulphate, Na2S2O3, solution against the time taken for the mark “X” to
disappear from sight.
(b)
The graph of the concentration of sodium
thiosulphate, Na2S2O3, solution against 1/time.
2.
Based on the graph plotted, determine the
relationships between the concentration of sodium thiosulphate, Na2S2O3,
solution with
(a)
The time taken
-
As the concentration of sodium
thiosulphate, Na2S203, solution decreases, a longer time is needed for mark ‘X’
to disappear from sight. Therefore, as
the concentration becomes lower, the rate of reaction also decreases.
(b)
1/time
-
The rate of reaction is high when the
concentration of sodium thiosulphate, Na2S2O3, solution is high. Concentration
of sodium thiosulphate, Na2S2O3, solution is directly proportional to 1/time or
rate of reaction.
3.
Deduce the effect of the concentration of
a reactant on the rate of reaction.
-
The relationship between the rate of
reaction and the concentration of sodium thiosulphate, Na2S2O3, solution is,
when the concentration of a reactant increases, the rate of reaction increases.
Discussion :-
1.
Write down the ionic equation for the reaction
between sodium thiosulphate, Na2S2O3, solution and sulphuric acid, H2SO4.
-
S2O32-(aq) + 2H+(aq) → S(s) + SO2(g) + H2O(l)
2.
Name the pungent smell which is released
throughout the experiment?
-
The pungent smell produced is
sulphur dioxide gas.
3.
What is represented by 1/time?
-
1/time is a measurement of the rate of
reaction.
4.
Why is the same size of conical flasks
is used throughout the experiment?
-
Conical flask of the same size and shape
are used in this experiment. If a bigger conical flask is used, the time taken
for the mark “X” to disappear from sight becomes longer. This is because bigger
conical flask has a larger base area. The mixture of 50 cm3 solution becomes
shallower. A bigger amount of yellow precipitate is needed to turn the mark “X”
invisible from sight.
5.
Hydrochloric acid, HCl with the same
concentration is used to replace sulphuric acid, H2SO4 in the experiment.
(a)
How does it affect the rate of reaction?
-
If
the experiments is repeated using the same concentration of hydrochloric
acid, HCl, to replace sulphuric acid, H2SO4, the rate of reaction will
decreases.
(b)
Explain your answer in 5 (a).
-
This because
hydrochloric acid, HCl, is a strong monoprotic acid whereas sulphuric acid, H2SO4, is a strong diprotic acid. Although the concentration of acids is the same, the concentration of hydrogen ions in sulphuric acid, H2SO4, is twice the
concentration of hydrogen ions
in hydrochloric acid, HCl.
Conclusion : Hypothesis is accepted. The rate of reaction will
increase if the concentration of sodium thiosulphate, Na2S2O3, solution
increases.
When
the concentration of sodium thiosulphate, Na2S2O3, solution increases, the rate
of reaction increase.
