Experiment 2
Kinetics of Murexide
Decomposition
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Objective
Determine the rate constant and order of reaction for the
decomposition of murexide using graphical methods.
Murexide (NH4C8H4N5O6, or C8H5N5O6.NH3), also called ammonium purpurate or
MX, is the ammonium salt of purpuric acid.
Murexide in its dry state has the appearance of a reddish purple powder, slightly
soluble in water. In solution, its color ranges from yellow in strong acidic pH through
reddish-purple in weakly acidic solutions to blue-purple in alkaline solutions.
O
O
NH
HN
O
O
N
NH
HN
O
O
NH4+
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Spectrophotometric investigation of the kinetics of decomposition of
murexide in acid solutions.
Ramaiah, N. A.; Gupta, S. L.; Vishnu. Indian
Institute of Sugar Technol., Kanpur, Zeitschrift fuer Naturforschung (1957),
12b 189-95.
A hypothesis referring to the
formation of an intermediate
complex (electrically
uncharged) that decompd.
unimolecularly to give the
products, uramil and alloxan,
was proposed.
O
O
NH
HN
O
O
N
NH
HN
O
NH4+
O
H+/H2O
O
O
HN
NH
O
NH2
+
O
O
HN
NH
O
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O
Background
As seen in experiment one, the rate of reaction can be
described by the equation:
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Background
As seen in experiment one, the rate of reaction can be
described by the equation:
 reactant 
rate 
t
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Background
As seen in experiment one, the rate of reaction can be
described by the equation:
 reactant 
rate 
t
or
product 
rate 
t
recall [ ] = mole/L
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Background
- The rate for this reaction changes as the concentration
changes.
- The chemist is required to stop the reaction and measure
the concentration.
- A better approach would be to describe the rate in terms
of the initial concentration.
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Background
Rate Law
For the reaction:
A (aq) + B(aq)  C(aq) + D(aq)
rate = k [A]q[B]r
k = rate constant
[A], [B] = concentration of reactants
q, r = reaction orders
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Reaction Orders
Order with respect
to reactant
Plot method
Equation of Line
Slope
Zeroth
[A] vs. Time
[A] = -kt + [A]o
-k
First
ln[A] vs. Time
ln[A] = -kt + ln[A]o
-k
Second
1/[A] vs. Time
(1/[A]) = kt + (1/[A])o
k
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Background
Determining Reaction Orders
One method is to run the reaction several times using
different concentrations and observe the rate change.
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Background
Determining Reaction Orders
One method is to run the reaction several times using
different concentrations and observe the rate change.
Consider:
2 I-(aq) + S2O82-(aq)  I2(aq) + 2 SO42-(aq)
I-(aq) (M)
S2O82- (aq) (M)
Initial Rate
(mol/L*sec)
0.080
0.040
12.5 x 10-6
0.040
0.040
6.25 x 10-6
0.080
0.020
6.25 x 10-6
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Background
Determining Reaction Orders
A second method follows the concentration of one
reactant with time. How the concentration decreases
reveals the order of the reaction.
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Background
Determining Reaction Orders
A second method follows the concentration of one
reactant with time. How the concentration decreases
reveals the order of the reaction.
Consider:
2 C4H6(g)  C8H12(g)
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Background
Determining Reaction Orders
2 C4H6(g)  C8H12(g)
Time (seconds)
[C4H6]
195
1.6 x 10-2
604
1.5 x 10-2
1246
1.3 x 10-2
2180
1.1 x 10-2
6210
0.68 x 10-2
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Background
Determining Reaction Orders
- The order of the reaction with respect to C4H6 is
determined by building several graphs.
- The graph that yields the straightest line indicates the
order of the reactant.
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Background
Determining Reaction Orders
- The order of the reaction with respect to C4H6 is
determined by building several graphs.
- The graph that yields the straightest line indicates the
order of the reactant.
Order with respect
to reactant
Plot method
Slope
Equation of Line
Zeroth
[A] vs. Time
-k
[A] = -kt + [A]o
First
ln[A] vs. Time
-k
ln[A] = -kt + ln[A]o
Second
1/[A] vs. Time
k
(1/[A]) = kt + (1/[A])o
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Background
Determining Reaction Orders
2 C4H6(g)  C8H12(g)
Time
(seconds)
[C4H6]
195
1.6 x 10-2
604
1.5 x 10-2
1246
1.3 x 10-2
2180
1.1 x 10-2
6210
0.68 x 10-2
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Background
Determining Reaction Orders
Zeroth Order Plot – [C4H6] versus Time
0.018
0.016
Concentration (M)
0.014
0.012
y = -1x10-06x + 0.0154
R2 = 0.9429
0.01
0.008
0.006
0
1000
2000
3000
4000
Time (seconds)
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5000
6000
7000
Background
Determining Reaction Orders
First Order Plot – ln[C4H6] versus Time
-4.1
-4.3
ln[C4H9]
-4.5
-4.7
y = -0.0001x - 4.1432
R2 = 0.9852
-4.9
-5.1
-5.3
-5.5
0
1000
2000
3000
4000
Time (seconds)
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5000
6000
7000
Background
Determining Reaction Orders
Second Order Plot – [C4H6]-1 versus Time
160
140
120
1/[C4H6 ]
100
80
y = 0.0142x + 59.234
R2 = 0.9995
60
40
20
0
0
1000
2000
3000
4000
Time (seconds)
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5000
6000
7000
Background
Determining Reaction Orders
- The problem is finding a way to determine the
concentration of the reactant.
- If the compound has a color, the intensity of color can be
related to the concentration.
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Colorimeter
• Transmittance (T) – the amount of light that penetrates a solution.
T = I I / I0
T = e-ebc
A = -10log(T) = 10log(1/T)
A = 10log (100/%T)
A = e b c e-molar absorptivity (L/mol cm)
For a solution in a cuvette, with a ct. cell width, A a c
A = Kc (Beer’s law)
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Background
Beer’s Law
A = ebc
A = absorbance
e = molar absorbtivity
b = path length
c = concentration
- The sample holder and solution characteristics are
constant.
- For this experiment, “a” and “b” are both constant.
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Technique - Colorimeter
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Experimental Setup
1. Boot up the computer. When the computer
boots up, turn on the Pasco Scientific interface.
2. Connect the serial plug of the converter to
the serial connection on the interface, then
connect the USB plug to the USB port on the
computer (be sure the colorimeter is plugged
into channel B)
3. Start DataStudio Software.
4. On the opening screen, select “Create
Experiment”.
5. Click and drag the “Colorimeter” sensor to
Channel B of the interface box shown in the
experimental setup window.
6. Double click on the colorimeter icon. Click
on “Slow”, then click on the “+” sign until the
“Periodic Samples” displays “5 s”. Click on
“Ok”.
7. Click on the “Options” button. Click on the
“Automatic Stop”. Click on time and enter “301”
then click “Ok” to exit.
Technique - Colorimeter
Calibration of Colorimeter
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1. Fill a plastic cuvet full of 0.1 M HCl.
2. Double click on the colorimeter icon. Click on the calibration tab.
3. Put the cuvet with the 0.1 M HCl into the colorimeter with the ribbed
side facing toward you. Close the lid.
4. Make sure the colorimeter display reads “Please Calibrate”. On the
computer screen, click on the “Take Reading” in the Low Point box.
5. Press the “Select” and the “Start / Stop” buttons on the colorimeter
at the same time.
6. Press “Select” on the colorimeter.
7. Press “Select” on the colorimeter two more times, or until the digital
display on the colorimeter shows “Green 565 nm”
8.
Press “Start” on the colorimeter. Click on “Take Reading” in the
High Point box. Click on “Ok”.
9. Fill a clean, dry cuvet with the 0.100 g / L murexide solution. Put
the cuvet in the colorimeter with the ribbed side facing toward you.
Close the lid of the colorimeter.
10. The digital display on the colorimeter will display the %T. Record
the data on the report sheet.
11. Drag the table icon from the “Displays” menu and drag it onto the
“% Transmittance, Ch B” icon.
12. Double click on the “% Transmittance, Ch B” icon. Click on the
“Numeric” tab. Click on “Fixed Decimals”. Change the number in
the box from 0 to 1. Click on “Ok”.
Technique - Colorimeter
Running the Experiment
1. Measure out 2.5 mL of murexide solution and put
the solution into a cuvet.
2. The following steps must be done quickly:
a. Add 10 drops of 0.1 M HCl to the murexide in
the cuvet.
b. Quickly cap the cuvet, mix the solution and
insert the cuvet (ribbed side toward you) into the
colorimeter.
c. Close the cover and double click on the “Start”
button.
3. When the timer reads zero the computer has
finished taking data, and you may remove the
cuvet from the colorimeter.
4. Repeat the experiment two additional times.
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Example Data
Determining [murexide]
- molecular formula: C8H8N6O6
- formula weight of murexide: 284.19 g/mol
- solution concentration is 0.100 g/L
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Example Data
Determining [murexide]
- molecular formula: C8H8N6O6
- formula weight of murexide: 284.19 g/mol
- solution concentration is 0.100 g/L
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Example Data
Converting %T into A
- use equation 2.5 pg 19 from the laboratory manual.
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Example Data
Converting %T into A
- use equation 2.5 pg 19 from the laboratory manual.
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Example Data
Determining e
- use equation 2.6 pg 19 from the laboratory manual.
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Example Data
Determining e
- use equation 2.6 pg 19 from the laboratory manual.
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Example Data
Time ( s )
Transmittance
Absorbance
0
4.56
1.341
5
4.78
1.321
10
4.88
1.312
15
4.78
1.321
20
4.88
1.312
25
4.98
1.303
30
4.98
1.303
35
5.19
1.285
40
5.28
1.277
45
5.40
1.268
50
5.60
1.252
55
5.70
1.244
60
5.92
1.228
65
5.92
1.228
70
6.01
1.221
6.32
1.199
75
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Example Data
Use Beer’s Law to determine [murexide] at each time
- use equation 2.5 pg 19 from the laboratory manual or
A=ec
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Example Data
Use Beer’s Law to determine [murexide] at each time
Time ( s )
Transmittance
Absorbance
[murexide]
0
4.56
1.341
0.00033
5
4.78
1.321
0.0033
10
4.88
1.312
0.00032
15
4.78
1.321
0.00033
20
4.88
1.312
0.00032
25
4.98
1.303
0.00032
30
4.98
1.303
0.00032
35
5.19
1.285
0.00032
40
5.28
1.277
0.00032
45
5.40
1.268
0.00031
50
5.60
1.252
0.00031
55
5.70
1.244
0.00031
60
5.92
1.228
0.00030
65
5.92
1.228
0.00030
70
6.01
1.221
0.00030
75
6.32
1.199
0.00030
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ln[murexide]
1/[murexide]
Example Data
Zeroth Order graph
- plot [murexide] versus time
- be sure to include a trendline with the R2 and the linear regression equation
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Example Data
First Order graph
- plot ln[murexide] versus time
- be sure to include a trendline with the R2 and the linear regression equation
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Example Data
Second Order graph
- plot 1/[murexide] versus time
- be sure to include a trendline with the R2 and the linear regression equation
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Safety
• The 0.1 M HCl is corrosive. If you spill some on you,
wash the affected area for five minutes. If the acid gets
in the eyes, wash the eyes for ten minutes. Seek
immediate medical attention! Neutralize any acid spills
with baking soda.
• Murexide is a mild irritant. If you spill some on you,
wash the affected area with soap and water. If the
murexide gets in your eyes, wash the eyes for ten
minutes. Seek immediate medical attention!
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Waste Disposal
All waste materials may be disposed of in the sink, flushed
with plenty of water.
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