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

2. 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]

3. 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.

4. 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+].

5. ( ) = 2m = Rate 2 = k 2[A]m [I2]0 [H+]p Rate 1 = k [A]m [I2]0 [H+]pC 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 k 2[A]m
Rate k [A]m =
= 2m ( )
2[A]
[A] m
What happened to the other terms in these equations?

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

7. ( ) = 2p = Rate 2 = k [A]m [I2]0 2[H+]p Rate 1 = k [A]m [I2]0 [H+]pC 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 k 2[H+]p
Rate k [H+]p =
= 2p ( )
2[H+]
[H+] p

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

9. 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 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