February
2003
Overview:
In this lab, you will attempt to measure the rate of a reaction under
various conditions. Your goal will
be to experimentally determine the order of
the reaction with respect to the reactants as well as the rate
constant for the reaction at room temperature.
Theory:
The
reaction that you will be running is as follows:
H2O2
+ 2 I-
+ 2 H+
®
I2 +
2 H2O
Your
objective will be to determine the order of the reaction with respect to I-
and H2O2. You
will be using the method of initial rates.
In order to determine the rate of the reaction, you will be using a
“reporter” reaction that will make a distinct color change when a defined
number of moles of I2 have been produced.
When you see the color change, you will note the time, and you will
therefore be able to calculate a reaction rate in units of moles/liter·sec.
By varying the concentrations of reactants, you should find that the rate
is affected in mathematically simple ways, indicative of zero-order,
first-order, or second-order kinetics.
Materials:
You will be provided with the following “stock” solutions.
·
0.50 M KI
·
3% H2O2
(3 g H2O2 per 100 ml solution—you can calculate
this to be _____ M)
· A reaction “cocktail”, which contains Na2S2O3 (sodium thiosulfate, part of the “reporter” reaction), starch solution (part of the “reporter” reaction)
·
0.33 M H2SO4
(provides H+ for the
reaction)
Each
reaction you do will be run in a final volume of 50 mL.
In order to make this 50 mL volume each time use 20mL of the “reaction
cocktail” and 15 mL of the 0.33 M H2SO4 stock solution.
To these components add the mL of 0.5 M KI and 3% H2O2
necessary to make the desired final reaction concentrations of KI and H2O2. You will need to top off your mixture with a small amount of
water so that the sum of each reaction mixture’s volume adds up to 50 mL. Be
sure to start timing the reaction when you combine the solutions containing KI
and H2O2. I suggest you run your first
reaction using roughly the following concentrations in your reaction beaker:
0.050
M I-
0.050
M H2O2
Your
second, third (and subsequent reactions) will be done using varying
concentrations of I- and H2O2.
Fill
in the blanks in table below for the first reaction so that the final
concentrations will come out to be 0.050 M I- & 0.050 M H2O2.
Then make reasonable suggestions for quantities to use for experiments 2
& 3
(model
this on problem #23 in chapter 12 of your textbook).
In all cases, make the total volume
come out to 50 mL.
|
Reaction
# |
mL
of 0.5 M KI |
mL
of 3% H2O2 |
mL
of water, acid solution + cocktail |
Total
Volume |
|
1 |
|
|
|
50
mL |
|
2 |
|
|
|
50
mL |
|
3 |
|
|
|
50
mL |
WRITE-UP: In your lab notebook construct two
tables. The first table should describe the contents of the reaction
solutions that you are combining. Be
sure to show and label the necessary dilution calculations that you make.
The second table should be a data table containing the reactant
concentrations in each trial plus the times for the reactions.
You may add a column of the calculated value 1/t as related to the rate
of the reaction.
Following
your tables, briefly demonstrate how you determine from experiments 2 & 3
whether the reaction is zero-order, 1st-order, or 2nd-order
with respect to I- and H2O2.
Calculate the rate constant based on the assumption that rate = 1/t.
Write the overall rate equation including units.
