Calculating the Results for
Using the Iodine Clock Method to Find the
Order of a Reaction (Activity EP6.4)

The following series of steps is required to complete the
calculations

Calculating the concentration of [I-] in the 5 reaction mixtures:

Mixture 1: [I-] =
Volume of KI x [KI]
Total volume of mixture
=
5.0 x 0.1
10
=
0.5 mol dm-3
For the other mixtures substitute the appropriate volumes into the
above equation
Reactant in excess (part c)

The number of mol of I- in mixture 5 is calculated as follows:
Concentration

= volume of KI x [KI]
=
Total volume of mixture
1 x 1.0 =
10
0.1 mol dm-3
The [S2O82-] in mixture 5 is calculated as follows:
Concentration
=
volume of K2S2O8 x [K2S2O8]
Total volume of mixture
=
2 x 0.04
10
= 8 x 10-3 mol dm-3
The calculation shows that I- is in excess.
Why is mixture 5 used?
It is used because it contains the lowest [KI] used and this can be easily
compared to [K2S2O8] which is kept constant in all reaction mixtures
Total amount of iodine produced (part d)

This is calculated from the reaction between peroxodisulphate(VI)
ions and iodide ions. The reaction for this is:
S2O82- (aq) + 2I- (aq)
2SO42- (aq) + I2 (aq)
The total moles of iodine produced is calculated as:
[K2S2O8] x
Total volume of mixture
1000
= 8 x 10-3 x 10
1000
= 8 x 10-5 moles I2 can be produced
The amount in moles of thiosulphate ions added to
each mixture (part e i)

This is calculated as :
[Na2S2O3] x volume added
1000
= 0.01 X 2
1000
= 2 x 10-5 moles
The amount of iodine used up by
thiosulphate ions (part e ii)

The reaction is:
2S2O32- (aq) + I2 (aq)
S4O62- (aq) + 2I- (aq)
In the reaction 2 moles thiosulphate ions react with 1 mole of
iodine.
The reaction is 2:1 and the moles of iodine used up is calculated to
be:
1 x 10-5 moles
The percentage of the
reaction studied (part e iii)
This is calculated as follows:
% Reaction studied
=
Moles of iodine used x 100
Moles of iodine made
=
1 x 10-5 x 100
8 x 10-5
=
12.5%
Calculating the initial rate

The can be calculated for each mixture as follows:

Rate =
The number of moles of iodine used
Time taken for colour to appear (T)
Or
1 x 10-5
T
Tabulating the results
Mixture
[I-]
1
0.5
2
0.4
3
0.3
4
0.2
5
0.1
Clock time
(s)
Rate
Mol dm-3 s-1
Temp
°C