Iodine Clock Reaction Kinetics 3m3i4l

 Dionee Liefman 2008

Iodine Clock Reaction Kinetics Introduction The aim of this experiment is to determine the effect of the reactant concentration and temperature on the rate of a chemical reaction. This reaction is the oxidation of the iodide ion (I-) to molecular iodine (I2) using sodium persulphate (Na2S2O8): 2I-(aq) + S2O82-(aq)  I2(aq) + 2SO42-(aq)

(1)

Ordinarily upon the completion of this reaction, the solution would become a brown colour, denoting the presence of I2 molecules. However, thiosulphate ions were also added to this mixture (S2O32-) causing the reaction below to occur instantaneously as I2 is produced: I2 + 2S2O32-  2I- + S4O62-

(2)

This second reaction is so fast that the brown colour of I2 is unable to develop, as the thiosulphate ions reduce the iodine back to iodide ions instantaneously. As the I2 is in excess and the thiosulphate ions are a limited reagent, the fixed amount of thiosulphate added will soon be used up. Thus, the excess I2 molecules that were not reduced by the thiosulphate will react with the I- ions produced and the starch indicator added will react with the subsequent I3- in the solution, displaying a blue colour. The initial concentration of thiosulphate is known and the duration of time for the thiosulphate to be consumed was measured, so the rate of reaction 1 can be determined. Reaction 2 is known as the ‘iodine clock’ reaction. By repeating this experiment with different concentrations of S2O82- and I-, the effect of these changing persulphate and iodide ion concentrations on the initial rate of reaction 1 can be determined. Procedure 150 mL beakers, stirrer bead, 2 measuring cylinders Solution A: 0.20M potassium iodide solution Solution B: 0.005M sodium thiosulphate solution Solution C: 0.10M sodium persulphate (1) The two graduated measuring cylinders were labelled – one cylinder labelled I- and the other labelled S2O82-. A beaker was filled with 120mL of iodide solution and another was filled with 120mL of persulphate solution. (2) 10mL of thiosulphate (same for all mixtures) was measured into a 150mL beaker, 3 drops of starch was added and a stirring bead was placed inside. (3) As one person poured in the correct volumes of iodine and persulphate (volume specific to trial), the other person started the timer. The exact time the solution turned blue was measured.

 Dionee Liefman 2008

Results Data Table 1: Proportions of mixing Solution A (I-) (mL) 20.0 15.0 10.0 5.0 20.0 20.0 20.0

Mixture 1 2 3 4 5 6 7

Water (mL) 0.0 5.0 10.0 15.0 5.0 10.0 15.0

Solution B (S2O3-) (mL) 10.0 10.0 10.0 10.0 10.0 10.0 10.0

Solution C (S2O82-) (mL) 20.0 20.0 20.0 20.0 15.0 10.0 5.0

Data Table 2: Observed results Mixture

Temperature (°C)

1 2 3 4 5 6 7

20 19 19 20 20 20 20

Time for colour to appear (s) 88 136 227 506 124 193 427

Average class time of reaction (s) 89 ± 3 131 ± 4 220 ± 3 493 ± 5 125 ± 5 198 ± 5 421 ±

Data Table 3: Calculating approx. initial rate Trial 1 2 3 4 5 6 7

[I-] 8x10-2 6x10-2 4x10-2 2x10-2 8x10-2 8x10-2 8x10-2

[S2O82-] 4x10-2 4x10-2 4x10-2 4x10-2 3x10-2 2x10-2 1x10-2

Approx. initial rate 5.6x10-6 3.8x10-6 2.3x10-6 1.0x10-6 4.0x10-6 2.5x10-6 1.2x10-6

n(I2) used/consumed = 5x10-5 mol/2 = 2.5x10-5 mol If this I2 hadn’t been removed from the solution by the 2S2O32- the concentration of I2 would have gotten up to: [I2] = n/v = 2.5x10-5/50x10-3 = 5x10-4M Approx. initial rate of reaction = [I2]/∆t = 5x10-4/89 = 5.6x10-6 M-1s-1 Calculations Calculating the rate law equation for this reaction: R1/R3 = k [I-]a [S2O82-]b / k [I-]a [S2O82-]b 5.6/2.3 = [I-]a / [I-]a 2.43 = 2a

 Dionee Liefman 2008

a = 1.22 R1/R2 = k [I-]a [S2O82-]b / k [I-]a [S2O82-]b 5.6/3.8 = [I-]a / [I-]a 1.47 = (4/3)a 1.47 = 1.33a a = 1.35 R1/R4 = k [I-]a [S2O82-]b / k [I-]a [S2O82-]b 5.6/1.0 = (8/2)a 5.6 = 4a a = 1.24 R1/R5 = [S2O82-]b / [S2O82-]b 5.6/4.0 = (4/3)b b = 1.17 R1/R6 = [S2O82-]b / [S2O82-]b 5.6/2.5 = 2b b = 1.16 R1/R7 = [S2O82-]b / [S2O82-]b 5.6/1.2 = 4b b = 1.14 The average calculated value for b = 1.14 Thus, rate = k [I-]1.25 [S2O82-]1.14