General Chemistry Lab Rates of Chemical Reactions, I: The Iodination of Acetone Experiment 20, p.155

Rate = k [acetone]m [I2]n [H+]p CH3 H3C acetone CH2I + (aq) I2(aq) + H+(aq) + I-(aq) Rate = — [I2] t Rate = k [acetone]m [I2]n [H+]p We want you to determine the value of m, n, and p. Abbreviate [acetone] as [A]

Rate = k [A]m [I2]n [H+]p colored colorless C O CH3 H3C acetone CH2I + (aq) I2(aq) + H+(aq) + I-(aq) colored colorless We can follow the course of the reaction by observing color changes. Rate = k [A]m [I2]n [H+]p The reaction is zero order with respect to I2 (n = 0). So the rate does not depend on [I2] at all, as long as [I2] is not = 0. In the lab, you will make I2 the limiting reagent, present in a large excess of acetone and H+, so the [ ] of these effectively do not change.

C O CH3 H3C acetone CH2I + (aq) I2(aq) + H+(aq) + I-(aq) The rate of the reaction is constant during its course under these conditions, so we can vary the rate by changing the initial [A] and [H+].

( ) = 2m = Rate 2 = k 2[A]m [I2]0 [H+]p Rate 1 = k [A]m [I2]0 [H+]p C O CH3 H3C acetone CH2I + (aq) I2(aq) + H+(aq) + I-(aq) If we double the [A] while holding the [ ] of the other two constant Rate 2 = k 2[A]m [I2]0 [H+]p Rate 1 = k [A]m [I2]0 [H+]p Rate 2 k 2[A]m Rate 1 k [A]m = = 2m ( ) 2[A] [A] m What happened to the other terms in these equations?

So by timing these two reactions, we can discover the order of the reaction with respect to [A] (that is, the value of m).

( ) = 2p = Rate 2 = k [A]m [I2]0 2[H+]p Rate 1 = k [A]m [I2]0 [H+]p C O CH3 H3C acetone CH2I + (aq) I2(aq) + H+(aq) + I-(aq) Next we double the initial [H+] while holding the [ ] of the other two constant Rate 2 = k [A]m [I2]0 2[H+]p Rate 1 = k [A]m [I2]0 [H+]p Rate 2 k 2[H+]p Rate 1 k [H+]p = = 2p ( ) 2[H+] [H+] p

So by timing these two reactions, we can discover the order of the reaction with respect to [H+] (that is, the value of p).

Before you arrive at chemistry lab, read and study the introduction to Lab Exercise 20 (p.155). Then read and re-read the “Experimental Procedure” on pp.157-161 until you completely understand what you will do. Next week you will need to do this for Lab Exercise 21: Rates of Chemical Reactions, II. A Clock Reaction (p.165). This ppt file is available on my web site http://www.evangel.edu/Personal/badgers/Web/