Chemistry Case

Investigating the rate of reaction between Peroxodisulphate (S2O82-) and Iodide (I-) ions

AIM : To find the order of reaction in the reaction between S2O82- and I- with respect to I-

To find the order of the reaction with respect to I- ions, the initial rates method was adopted. Five solutions were prepared by varying the concentration of I- ions while keeping the concentration of S2O82- constant. Appropriate volume of water was added to all the solutions such that the total volume always remained 100cm3 . The time taken for the solution to turn blue is recorded.

DATA COLLECTION AND PROCESSING

TABLE 1 : Time taken for the color of the reaction to turn blue

Volume used / cm3

Volume used / cm3

Total Volume / cm3 Time taken for blue color to appear / s

Expt. 0.25 mol/ dm3 S2O82-(aq) 0.5 mol/ dm3 I-(aq) 0.01 mol/ dm3 Na2S2O3 (aq) Starch solution Water

A 4 60 20 5 11 100 50

B 4 50 20 5 21 100 79

C 4 40 20 5 31 100 109

D 4 30 20 5 41 100 141

E 4 20 20 5 51 100 354

Find the moles, concentration and rate of reaction of I- ions

For example;

Moles of 60cm3 I- = Molarity * (Volume / 1000)

= 0.5 * (60 / 1000)

= 0.03 moles

Concentration of 60cm3 I- = ( Moles of 60cm3 I- / Total volume of reaction )

= (0.03 / (100/1000))

= (0.03 / 0.1)

= 0.3 mol / dm3

Initial Rate of reaction (for 60cm3 I-) = 1 / time taken for color of solution to turn blue

= 1 / 50

= 0.02 mol / dm3 / s

Using the above equations, the moles and concentration of all volumes of I- ions used is found out. These results are summarized in TABLE 2