1. Calculating the Results for Using the Iodine Clock Method to Find the Order of a Reaction (Activity EP6.4)

2. 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
= x 0.1 10
= mol dm-3
For the other mixtures substitute the appropriate volumes into the above equation

3. Reactant in excess (part c)
The number of mol of I- in mixture 5 is calculated as follows:
Concentration = volume of KI x [KI] = 1 x 1.0 = mol dm-3
Total volume of mixture
The [S2O82-] in mixture 5 is calculated as follows:
Concentration = volume of K2S2O8 x [K2S2O8] = x 0.04
Total volume of mixture
= 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

4. 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
= 8 x 10-5 moles I2 can be produced

5. The amount in moles of thiosulphate ions added to each mixture (part e i)
This is calculated as :
[Na2S2O3] x volume added
1000
= X 2
= 2 x 10-5 moles

6. 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

7. 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
= x 10-5 x 100
8 x 10-5
= %

8. 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

9. Tabulating the results
Mixture
[I-]
Clock time
(s)
Rate
Mol dm-3 s-1
Temp °C 1
0.5 2
0.4 3
0.3 4
0.2 5
0.1