F04037100 GP:19

Experiment 10

Chemical Reaction Rate-Iodine Clock Reaction Introduction: For a reaction as the following:

2I

+ 2S2O

8

2 I

2 + 2SO

4

2

The rate of reaction can be expressed as:

R = [S2O

8

2]/t = [I

]/2t = [I

2]/t = [SO

4

2]/2t

Rate law (or rate equation) describes the relationship of rate and reactants as the following:

R = k[I]

m[S

2O

8

2]

n

where k is the rate constant and m, n is the reaction order for each reactant respectively. Reaction order can be determined only through experiments. If the concentration of each reactant is varied individually and the rate is measured, then the order for each reactant can be calculated. Because the reaction in this case is too rapid to be measured, the limiting agent of thiosulphate ions is introduced in reaction to make it more easily for timing measurements.

I2

+ 2S2O

3

2 2I

+ S

4O

6

2

If the same reaction is processed at different temperatures, the reaction activation energy can also be calculated according to the Arrhenius equation:

k = AeEa/RT

(or logk = Ea/RT + constant) the activation energy can be evaluated through the slope of the linear regression of the curve. Procedure:

1. Pipet 5 mL of 0.005 M Na2S

2O

3 into a 50-mL beaker, add 3 drops of starch solution.

Put a stirrer into the beaker. Pipet the amounts of 0.2 M KCl and 0.1 M (NH4)

2SO

4 (if

required in the chart above) into the beaker. 2. Take two test tubes and pipet the required amount of 0.2 M KI into a test tube, 0.1 M (NH

4)2S

2O

8 into the other test tube.

3. Pour the solutions of the test tubes into the beaker at the same time, record the time (in seconds) the solution takes to turn to blue. 4. Repeat the procedure from reaction No. 1 to 5 of the chart above.

A. Reaction No.

0.20 M KI (mL)

0.20 M KCl (mL)

0.10 M (NH4)2S2O8

(mL)

0.10 M (NH4)2SO4

(mL)

0.0050 M Na2S2O3

(mL)

1 10.00 0.00 10.00 0 5.00 2 5.00 5.00 10.00 0 5.00 3 10.00 0.00 5.00 5.00 5.00 4 10.00 0.00 2.50 7.50 5.00 5 5.00 5.00(H2O) 10.00 0.00 5.00

F04037100 GP:19

B. 1. Take a 1000-mL beaker which is half filled with water and ice cubes. Repeat reaction No. 1 of A at this temperature. Be sure that all the solutions should be 0 in this system before reacting. 2. Repeat reaction No. 1 of A at 40. Put all solutions in thermal tank to equilibrate at 40

before reacting. Use an Erlenmeyer flask in place of the beaker to be the reaction vessel.

Results:

F04037100 GP:19

F04037100 GP:19