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

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**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

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**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

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**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

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**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

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**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

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**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 = %

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**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

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**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

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