Experiment 29: Rates of chemical reactions

1. Experiment 29: Rates of chemical reactions Purpose In this experiment you will determine the rate law for an iodine clock reaction. In an iodine clock reaction, the completion of the reaction is signaled by the sudden appearance of the dark color characteristic of the reaction of I2 with starch. The rate of an iodine clock reaction depends on the concentration of the reactants. As a result, the time required for the appearance of the dark color can be adjusted by manipulating the concentration of the reacatants .

2. Clock reaction • Normal chemical reactions occur so quickly that very sophisticated techniques are required to study the speed of reactions. Fortunately scientists can also study a group of reactions that occur very slowly known as clock reactions.

3. Rate law • The rate of exn will depend on the concentration of the reactants thus • Rate = k[A]x[B]y • [A] and [B} are molar concentartions (M) • X and Y describe the rate • And k is the specific rate conatant

4. Rate law • The object of the experiment is to determine the values for the rate constant k and the reaction orders x and y • The overall order of the rxn is the sum of the exponents • The object of the experiment is to determine the values for the rate constant k and the reaction orders q and r.

5. In this experiment you will study the rate of the reaction between iodide ion, I–, and • persulfate (peroxydisulfate), S2O82–. The products of the reaction are I2 and sulfate ion. S2O82- + I- I2 + 2 SO4-2 Rate = k[A]x[B]y Rate = k[S2O82-]x[I-]y Δ [S2O82-] = k[S2O82-]x[I-]y Δt

6. To do this experiment you will need : • to know or measure Δ S2O82- (the initial change in concentration of S2O82- ions), • and Δt, the time elapsed during the change, • as well as the initial concentration of S2O82- • And I- ions.

7. A way to obtain the Δ S2O82- is by coupling another reaction to the one we are studying. • The new reaction is the reduction of I2 by Na2S2O3 (sodium thiosulfate). • I2 + Na2S2O3 2I- + S4O62- When you add sodium thiosulfate a color change will take place (blue-black) Every time a color change takes place 2 x 10 -4 M of S2O82- has reacted

8. Procedures • Prepare 4 solutions ,one at a time as needed

9. Procedures • Prepare 7 test tubes filled with 1 ml aliquots of Na2S2O3 • Prepare soln 1 in a 250 ml erlenmeyer flask • Pipet 25.0 ml of (NH4)2S2O8 into a 100 ml beaker • Pour the 25.0 ml of (NH4)2S2O8 into soln 1 and swirl ( note the time you began mixing to the nearest second!!!) • When you see the blue –black color appear add 1ml of Na2S2O3 and the color will dissappear

10. Procedures • When the color reappears record the time and add then another 1 ml aliquots of Na2S2O3 . • The time being measured is the intervals between the appearance of the blue black color • Repeat this procedure seven times for each aliqot. • Solutions 2,3 and 4 should be treated in the same manner

11. graph • X axis is cumulative times • Y axis total moles consumed • EACH interval is 2.0 x 10-4 moles of S2O82-