Reaction Kinetics
Experiment
3
Decomposition of Hypochlorite (Bleach)
2 OCl-(aq)  2 Cl-(aq) + O2(g)
The rate of a reaction may be measured by following the disappearance of reactants or
the appearance of products. The rate expression is as follows:
rate  
[ Reactants] [ Products]

time
time
The rate of a reaction is affected by a number of different factors. Two of the factors that
will be studied in this experiment are reactant concentration and temperature.
The reaction is catalyzed by cobalt oxide formed when Co(NO3)2 solution is added to the
bleach. The reaction rate will be monitored by measuring the volume of water displaced
by the O2 gas produced. On day 1, various initial concentrations of the bleach will be
used to determine the reaction order by the “Method of Initial Rates”. The initial rate is
the linear slope of the plot of concentration vs time. The rate law is expressed by the
following equation:
rate = k[OCl-]x
On day 2, the activation energy, Ea, will be determined by elevating the temperature of
the reaction at a constant initial concentration of bleach and plotting of ln k vs 1/T.
PROCEDURE
Day 1 – Determination of reaction order.
1. Set-up the apparatus as shown in the Figure 1 on page 3. Fill the Florence flask with
water and insert the stopper assembly. Fill the tube connecting the flask and the
graduate by blowing into the glass tube, and then close the pinch clamp.
2. For each trial, measure 5 ml of Co(NO3)2 solution into a shell vial. The volumes of
bleach and water to be used are shown in the table below. Tighten all stoppers and
remove the pinch clamp. If everything is tight, only a drop or two of water should
drip into the cylinder.
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3. At time zero, tip the flask and mix the two solutions. Keep swirling the contents of
the flask at a uniform rate as the reaction proceeds. Record the volume of water in
the graduate at 30 second intervals until a volume of 100 mL is obtained.
4. Repeat the process for each of the dilutions shown in the table for A-D. Record the
temperature for each of the trials.
Dilution Table
Trial
Temp. (C)
mL bleach
mL H2O
mL
Co(NO3)2
Total Volume
(mL)
A
room
25
none
5
30
B
room
25
30
5
60
C
room
25
60
5
90
D
room
25
90
5
120
Day 2- Determination of Activation Energy.
5. Using the concentration of bleach and water in trial D warm this solution to 10
degrees higher than the temperature previously recorded and repeat the rate study.
6. Repeat the measurements for 20 and 30 degrees above room temperature.
Day 3 - Computer Graphing of Data and Analysis.
You may chose to use your own laptop or one of the computers in the computer lab to
plot the data for analysis. Each group may plot the group data together. Bring a flash
drive to save the data plots.
REPORT
1. Graph the results for each of the concentrations (trials A, B, C and D) on the same
plot. Using a computer graphing program (excel), plot [O2] on the vertical axis
versus time (s) on the horizontal axis. Determine the initial rate for each trial (M/s).
How does the rate vary with the concentration of OCl-. Use the equation below to
calculate the concentrations of O2 at various time intervals.
P = barometric pressure (atm)
Vt = volume (L) of O2 gas evolved at time t.
2. Determine the rate order for [OCl-] using the initial rate method. Calculate the rate
constant for this reaction.
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3. Plot the results of the temperature variation on a separate graph showing how the
same concentration gives different rates at different temperatures. How does the rate
vary with the temperature increase that was measured?
4. Determine the rate constants for the reactions at the various temperatures and plot
them as ln k (y-axis) vs 1/T in Kelvin (x-axis). Calculate the activation energy
(kJ/mol) for this reaction from the following equation of a straight line.
R = 8.314 J/mol K
T = Kelvin
Figure 1. Kinetics apparatus set-up.
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