Experiment 2 Kinetics of Murexide Decomposition

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Experiment 2Kinetics 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.
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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.

H/H2O

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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

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Background

• As seen in experiment one, the rate of reaction
  can be described by the equation

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 AqBr
• 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
Zeroth A vs. Time A -kt Ao
First lnA vs. Time lnA -kt lnAo
Second 1/A vs. Time (1/A) kt (1/A)o
Slope
-k
-k
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/Lsec)
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 Ao
First lnA vs. Time -k lnA -kt lnAo
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
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Background
Determining Reaction Orders First Order Plot
lnC4H6 versus Time
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Background
Determining Reaction Orders Second Order Plot
C4H6-1 versus Time
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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 II / 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 (Beers law)

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Background
Beers 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
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.
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Technique - Colorimeter

• Calibration of Colorimeter
• 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
• 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.

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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
75 6.32 1.199
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Example Data
Use Beers Law to determine murexide at each
time - use equation 2.5 pg 19 from the
laboratory manual or A e c
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Example Data
Use Beers 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
lnmurexide
1/murexide
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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 lnmurexide 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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