Physical Chemistry Laboratory Course - Section B
BP 21
Experiment 2
To Determine the order of the reaction
2Fe3+ + 3I- 2Fe2+ + I3-
with respect to Fe3+ and I- at 35°C
Ashen Samaranayake
144097
19/09/2016
pg. 1
Date: 12/09/2016
Theory:
This experiment is known as the iodine clock reaction. Chemical kinetic is the study of rates of
chemical reactions, the change of concentration of reactants to products with time, the
following reaction takes place was determined:
2Fe3+ + 3I- 2Fe2+ + I3-
The rate equation can be expressed as:
Rate = k [Fe3+] a. [I-] b Where, k – rate constant
[ ] – Concentration of reactants used
a – The order of the reaction with respect to Fe3+
b – The order of the reaction with respect to I-
The rate constant and the order of reactants cannot be derived from the stoichiometry of the
balanced reaction alone, it can only be determined experimentally. In this experiment the rate
constant and the order was determined using the initial rate method. The results indicates a
dependence between the initial rate and the concentration of reactants, and the dependence
of the reaction by the limiting reagent.
The experiment was performed in two method by varying the concentration of one reactant
and keeping the other constant. The order of ‘a’ with respect to Fe3+ making up reaction
mixtures with varying Fe3+ concentration but keeping the iodide concentration constant. This
can be expressed as:
Rate = k’ [Fe3+] a (where k’ = k [I-] b)
Or
log (rate) = a.log [Fe3+] + constant (where constant = log k’)
log (rate)
Slope = a
Intercept = Constant
pg. 2 Section B Physical-BP21
The initial rate is determined using the time taken for a small amount of I3- to be produced in a
reaction mixture. This is done by keeping the medium with a constant amount sodium
thiosulphate which reacts very rapidly with the I3- formed in the reaction. Starch was used an
indicator. When all the sodium thiosulphate is used up, I3- is produced and the final colour
change is a blue colour solution.
The same method was done with varying the iodide concentration in order to determine ‘b’
with respect to I-, and keeping the concentration of Fe3+.
Reactions for the experiments:
2Fe3+ + 3I- 2Fe2+ + I3- (Slow Reaction)
I2 + 2S2O32- 2I- + S4O62- (Fast Reaction)
Pre-lab questions:
1)
a) The relationship between the rate of a chemical reaction and the concentration
of the reaction at a given temperature.
b) Temperature and Activation energy
2) The molecules in the reaction medium must collide, the collision occurring must have
enough energy and the collision should occur in the correct orientation.
pg. 3 Section B Physical-BP21
Procedure:
X – 0.1 mol dm-3 ammonium ferric sulphate acidified with 5% H2SO4 solution
Y – 0.1 mol dm-3 KI solution
Z – 0.002 mol dm-3 sodium thiosulphate
i. Determination of ‘a’
5 Conical flasks of 250 cm3 labelled 1,2,3,4, and 5. Into each flask the respective volumes were
added with reagents (expect for the Fe3+ solution). 5 drops of starch solution was added into
each flask.
Conical Volume Volume of Volume of Volume of acidified Time taken for the
Flask of 0.1 0.002 mol distilled 0.01 mol dm-3 Fe3+ blue colour to
Number mol dm-3 dm-3 Na2S2O3 water / solution / cm3 (C) develop / min.sec
KI / cm3 / cm3 (Z) cm3
(Y)
1 5.00 20.00 00.00 25.00
2 5.00 20.00 05.00 20.00
3 5.00 20.00 10.00 15.00
4 5.00 20.00 15.00 10.00
5 5.00 20.00 17.00 8.00
I. Immersed all five flasks in a water bath maintained at 35oC.
II. The flasks were shaken frequently to help the contents to reach the bath temperature.
III. In another flask (C) 150 ml of prepared 0.01 mol dm-3 of Fe3+ solution and immersed
into the water bath.
IV. All the flasks containing with the respective volumes and flask C were allowed to reach
at 35oC. Two different thermometers were used to measure the temperature of the
contents in the interested flask and flask C.
V. After the contents reached to 35oC, 25.00 cm3 of Fe3+ solution was pipetted out and
added into flask 1.
VI. The stop watch was started in the midpoint of the delivery. The time was measure from
the first appearance of the blue colour.
VII. The procedure was repeated with the rest of the flask.
pg. 4 Section B Physical-BP21
ii. Determination of ‘b’
The same procedure was done as ‘i‘(without the Fe3+ solution added at first). 5 drops of starch
solution was added into each flask. The time was recorded.
Conical Volume of Volume of Volume of Volume of Time taken for
Flask 0.01 mol 0.002 mol distilled acidified 0.01 the blue colour to
Number dm-3 KI / dm-3 Na2S2O3 water / cm3 mol dm-3 Fe3+ develop / min.sec
cm3 (Y) / cm3 (Z) solution / cm3
(C)
1 25.00 10.00 05.00 10.00
2 20.00 10.00 10.00 10.00
3 15.00 10.00 15.00 10.00
4 12.00 10.00 18.00 10.00
5 10.00 10.00 20.00 10.00
Results:
i.
Conical Volume Volume of Volume of Volume of Time taken Time taken
Flask of 0.1 0.002 mol distilled acidified for the blue for the blue
Number mol dm- dm-3 water / 0.01 mol colour to colour to
3 KI / Na2S2O3 / cm3 dm-3 Fe3+ develop / develop/
cm3 (Y) cm3 (Z) solution / min.sec min
cm3 (C)
1 5.00 20.00 00.00 25.00 4.04 4.67
2 5.00 20.00 05.00 20.00 5.01 5.17
3 5.00 20.00 10.00 15.00 6.10 6.17
4 5.00 20.00 15.00 10.00 13.14 13.23
5 5.00 20.00 17.00 8.00 16.11 16.18
ii.
Conical Volume Volume of Volume of Volume of Time taken Time taken
Flask of 0.01 0.002 mol distilled acidified 0.01 for the blue for the blue
Number mol dm-3 dm-3 water / mol dm-3 Fe3+ colour to colour to
KI / cm3 Na2S2O3 / cm3 solution / develop / develop /
(Y) cm3 (Z) cm3 (C) min.sec min
1 25.00 10.00 05.00 10.00 2.20 2.33
2 20.00 10.00 10.00 10.00 3.07 3.12
3 15.00 10.00 15.00 10.00 4.56 4.93
4 12.00 10.00 18.00 10.00 9.33 9.55
5 10.00 10.00 20.00 10.00 13.15 13.25
pg. 5 Section B Physical-BP21
Calculations
i.
Flask Number t/ min loge (1/t) loge (V) *
1 4.67 -1.54 3.218
2 5.17 -1.64 2.996
3 6.17 -1.82 2.708
4 13.23 -2.58 2.302
5 16.18 -2.78 2.080
Δ𝑦
ii. m=
Δ𝑥
𝑦2 −𝑦1 −2.87−0
m= = = 0.87
𝑥2− 𝑥1 0−(3.3)
According: log (rate) = a.log [Fe3+] + constant
Where ‘a’ is the gradient, therefore a = 0.87 ≈ 1.0
iii.
Flask Number t/ min loge (1/t) loge (V) *
1 2.33 -0.85 3.218
2 3.12 -1.14 2.996
3 4.93 -1.60 2.708
4 9.55 -2.26 2.485
5 13.25 -2.58 2.302
iv.
Δ𝑦
m=
Δ𝑥
(The value of the gradient is inaccurate during to experimental errors (m = 0.82), thereby
the gradient is found with the given values in the table)
𝑦2 −𝑦1 −2.58−(−0.85)
m= = = 1.88
𝑥2− 𝑥1 2.302−3.218
According: log (rate) = b.log [I-] + constant
Where ‘b’ is the gradient, therefore b = 1.88 ≈ 2
pg. 6 Section B Physical-BP21
v. I2 + 2S2O32- 2I- + S4O62-
vi. Statement b), this is about the measurement of time. As soon as the iodide is reacting
with the iron (III), the starch won’t indicate the colour blue, the moment iron (III) is over
the reaction between thiosulphate and iodine takes place and when the blue colour is
observed the time is easily measured.
vii. Rate = k [Fe3+] a. [I-] b a = 1 and b = 2
1 1
Rate = = = 0.214 min-1
𝑡 4.67
Assume the volume of Fe3+ (25.00 cm3) and I- (5.00 cm3) ∝ to their concentration
25 5
0.214 min-1 = k x (1000 ) 1 x ( )2
1000
k = 3.424 x 10-7
viii. Flask 1 : Volume of Fe3+ = 25.00 cm3
1 1
Rate = = = 0.214 min-1
𝑡 4.67
Assume the volume ∝ concentration
Rate = k’ [Fe3+] a
25
0.214 min-1 = k’ (1000 )a Equation 1
Flask 2: Volume of Fe3+ = 20.00 cm3
1 1
Rate = = = 0.193min-1
𝑡 5.17
Assume the volume ∝ concentration
Rate = k’ [Fe3+] a
20
0.193 min-1 = k’ (1000 )a Equation 2
pg. 7 Section B Physical-BP21
𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 1
𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 2
0.214 k′ ×(25 ÷1000)𝑎
=
0.193 𝑘 ′ ×(20 ÷1000)𝑎
1.1088 = (1.25) a
log 1.1088 = log (1.25) a
log 1.1088 = a log (1.25)
a = 0.46
Discussion:
ix.
We are determining the average rate of the reaction and assume as the initial rate. This
is a major error in this experiment.
The intensity of blue colour varies with different people performing the experiment, to
ensure proper colour change hot and freshly prepared starch solution was used.
The experiment was performed at 35°C in water bath to maintain constant temperature
since the rate constant is dependent on temperature.
The solution of KI was slightly yellow in colour before the experiment was conducted. Its
means the iodide has oxidized to iodine and liberation is possible. This can lead to an
error with the experiment.
The rate equation Rate = k [Fe3+] a. [I-] b is actually a 3rd order reaction (a + b = 1 +2 = 3).
The starch solution provided is a warm solution and addition (1 – 2ml) to the contents
can cause small temperature changes and affect the rate constant by change of
temperature.
The reason why the 2nd experiment, the gradient of the graph was inaccurate was due to
the KI was slightly yellow colour (liberation of iodine), the thermostat solution of Fe3+
because it was clamped (random error).
pg. 8 Section B Physical-BP21
Post lab question:
1) Concentration was assumed to be directly proportional to the volume, this won’t effect
on the order of the reaction but it will slightly affect the rate constant of the reaction.
This is because we assume the average rate = the initial rate, but the way the experiment
was conducted and time measured is different due to the midpoint of delivery of Fe3+ can
change from person to person.
2) The measurements can be triplicated, to obtain an average time taken. An error can occur
during the midpoint of delivery from each replication can vary.
3) We assume concentration directly proportional to the volume but it is inversely
proportional.
No.of moles
Concentration =
Volume
This can affect the order of the reaction values.
Reference:
Determining-the-Rate-Law-for-a-Reaction-between-Iron-III-and-Iodide-Ion -
https://www.scribd.com/doc/227422374/Determining-the-Rate-Law-for-a-Reaction-
between-Iron-III-and-Iodide-Ion (accessed Sept 12, 2016).
http://www.chemguide.co.uk/physical/basicrates/orders.html (accessed Sept 12, 2016)
pg. 9 Section B Physical-BP21
