EXPERIMENT
1:
REACTION
KINETICS
OF
BROMINATION
OF
ACETONE
OBJECTIVE
1. To determine the rate, order and activation energy of the bromination of acetone.
EXPERIMENTAL PROCEDURE
A. Determination of reaction rate at room temperature
1. 10 mL of 4M acetone is filled into a boiling tube labeled as A,followed by 10 mL
of 1.0M HCl and 20 mL of distilled water.
2. 10 mL of 0.02M bromine solution is measured using graduated measuring
cylinder and poured into another boiling tube (B).
3. Both boiling tubes is immersed in water bath at room temperature about 5 minutes
until the solution have reached the temperature of water bath.
4. The two solution is mixed together as the stopwatch started to record the exact time
of mixing.
5. The time,t is recorded when the reaction is complete, that is when the solution
becomes colourless. A test tube filled with distilled water is used for colour
comparison.
6. Step 1 to 5 is repeated twice to ensure that reproducible results are obtained. The
average time is used to calculate the rate of reaction at that temperature.
B. Determination of the reaction order with respect to acetone and hydrogen ion
1. The procedure outline in Part A is repeated using the volumes of reactants given in
Table 1.
Table 1: Volumes of reactants used in experiments
Volume of solution to be used (mL)
Experiment
Test tube B
Test tube A
Bromine
HCl
Acetone
Distilled water
1
10
10
10
20
2
15
10
10
15
3
10
20
10
10
4
10
20
15
10
C. Determination of the activation energy of the reaction
1. The procedure outline in Part A is repeated at different temperature (40℃,45℃ and
50℃).
2. The order of the reaction is determined with respect to each of the reactants.
3. The rate equation of this reaction is determined.
4. The rate of reaction at each temperature studied is calculated and determined the
activation energy of the reaction from the graph of ln rate vs 1/T. The reasons of
graph used to determine the activation energy is ln rate vs 1/T is explained.
RESULTS
PART A
Time taken to become colourless (s)
Experiment
Temperature (℃)
First reading
1
23
1217
Second
reading
783
Average
1000
PART B (CONSTANT TEMPERATURE)
Time taken to become colourless (s)
Experiment
Temperature (℃)
First reading
Second
reading
Average
1
23
1217
783
1000
2
23
895
3420
2158
3
23
450
308
379
4
23
385
377
381
PART C (CONSTANT VOLUME)
Experiment
1
Temperature (℃)
Time taken to become colourless (s)
40
95
45
83
50
39
CALCULATIONS
PART A & B
Calculation of new concentration of reactant
Exp 1 : Acetone : Concentration (new) = Concentration of reactant × Volume
Total volume
= 4.0 M (10 mL)
50 mL
= 0.8 M
Acetone + Br2 + H+ → Acetone-Br + H+ + Br-
Experiment 1
Rate = d [Acetone-Br]
= - d [Br2]
dt
dt
Rate = 0.004M
1000 s
Rate = 4.000 × 10-6 M s-1
Experiment
[Acetone],M
[H+],M
[Bromine],M
Rate, Ms-1
1
0.8
0.2
0.004
4.000×10-6
2
0.8
0.2
0.006
2.780×10-6
3
0.8
0.4
0.004
1.056×10-5
4
1.2
0.4
0.004
1.050×10-5
PART C
Determination of order of reaction
From equation of reaction,
Rate law = k[Acetone]x[Br2]y[H+]z
Experiment 3
:
k[0.8]x[0.004]y[0.4]z
k[0.8]x[0.004]y[0.2]z
Experiment 1
=
1.056×10-5
4.000×10-6
2z
=
2.64
log 2z
=
log 2.64
z log 2
=
log 2.64
z
=
log 2.64
log 2
Experiment 2
:
z
=
1.4
z
≈
1
k[0.8]x[0.004]y[0.2]z
k[0.8]x[0.006]y[0.2]z
Experiment 1
=
2.780×10-6
4.000×10-6
(3/2)y
=
0.695
log (3/2)y
=
log 0.695
y log (3/2)
=
log 0.695
y
=
log 0.695
log (3/2)
Experiment 4
Experiment 3
:
y
=
-0.90
y
≈
-1
k[1.2]x[0.004]y[0.2]z
k[0.8]x[0.004]y[0.2]z
=
1.050×10-5
1.056×10-5
(3/2)x
=
0.994
log (3/2)x
=
log 0994
x log (3/2)
=
x
=
log 0.994
log 0.994
log (3/2)
x
=
-0.015
x
≈
0
⸫ Rate law = k[H+][Br2]-1
Temperature (K)
1/T (10-3. K-1)
Rate (M s-1)
ln Rate
296
3.38
4.00×10-6
-12.43
313
3.19
4.21×10-5
-10.08
318
3.14
4.82×10-5
-9.94
323
3.10
1.03×10-4 ×10-3
-9.18
ln rate vs 1/T
0
3,05
3,1
3,15
3,2
3,25
3,3
3,35
3,4
-2
ln rate
-4
-6
-8
3,1; -9,18
3,14; -9,94 3,19; -10,08
-10
3,38; -12,43
-12
-14
1/T ( ×10-3K-1)
ln k
=
-Ea + ln A
RT
Compared with straight line equation
y
=
mx + c
y
=
ln k
m
=
x
=
1/T
c
=
ln A
where
Ea
R
Determination of activation energy, Ea
y2-y1
Gradient of the graph,
m
=
x2-x1
-12.43-(-10.08)
=
=
(3.38-3.1) K-1
-12.37 K
Ea
From gradient,
m
=
R
The negative value is neglected as it indicates the shape of the graph
Ea
=
mR
=
12.37K (8.314 J K-1 mol-1)
=
102.84 J mol-1
DISCUSSION
The bromination of acetone in acid solution proceeds according to
Acetone + Br2 + H+ → Acetone-Br + H+ + BrThe reaction is catalysed by hydrogen ion. The rate is determined by
Rate = d [Acetone-Br]
dt
= - d [Br2]
dt
because Br2 is the limiting reactant. The rate for this particular reaction at 23°C is 4.00×10-6
M s-1. The rate law is assumed to be of the form
Rate law = k[Acetone] x[Br2] y[H+] z where k is the rate constant. The exponents x, y, and z
indicate the order of the reaction with respect to acetone, bromine, and hydrogen ion,
respectively.
From the result and calculation, we can determine the reaction rate at room
temperature, the reaction order for each of the reactants and the activation energy. We
know the order of each reactant which is 0 for Acetone, - 1 for Bromine and 1 for hydrogen
ion. When a partial order is negative, the overall order is usually considered as undefined.
In this experiment, although the rate equation found is Rate law = k[H+][Br2]-1 , the rate
equation is more complex than that of a simple order reaction.
Rate of the reaction is also determined at several temperatures in this experiment. By
plotting ln rate vs 1/T, the activation energy is calculated from the graph which is
102.84 J mol-1. We use graph of ln rate against 1/T which corresponds with the
Arrhenius equation because it involves the relationship between rate of reaction and
temperature.
CONCLUSION
The rate, order, and activation energy of the bromination of acetone is determined.
REFERENCE
Atkins, P., & de Paula, J. (2006). Physical Chemistry, Eighth Edition. New York: Oxford
University Pres
McCormick, J.M., (2003). Bromination of acetone, University of Kansas,
http://chemlab.truman.edu/physical-chemistry-bromination-of-acetone
Warkentin, J. (2011). Kinetics of bromination of acetone, bromoacetone and
1,1-dibromoacetone, www.researchgate.net