valdosta state university experiment 2 kinetics of murexide decomposition valdosta state university
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Experiment 2Kinetics of Murexide
Decomposition
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Valdosta State University
Objective
Determine the rate constant and order of reaction for the decomposition of murexide using graphical methods.
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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.
NH
NH
O
ONH
NH
O
O
O N
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.
NH
NH
O
O
O NH2
NH
NH
O
O
OO
NH
NH
O
ONH
NH
O
O
O N
O
NH4+
+
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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Valdosta State University
Background
As seen in experiment one, the rate of reaction can be described by the equation:
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t
rate
reactant
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Background
As seen in experiment one, the rate of reaction can be described by the equation:
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t
rateort
rate
productreactant
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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Valdosta State University
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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Valdosta State University
Reaction Orders
Order with respect to reactant
Plot method Equation of Line
Zeroth [A] vs. Time [A] = -kt + [A]o
First ln[A] vs. Time ln[A] = -kt + ln[A]o
Second 1/[A] vs. Time (1/[A]) = kt + (1/[A])o
Slope
-k
-k
k
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Background
Valdosta State University
Determining Reaction OrdersOne method is to run the reaction several times using different concentrations and observe the rate change.
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Background
Valdosta State University
Determining Reaction OrdersOne 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 SO4
2-(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
Valdosta State University
Determining Reaction OrdersA 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
Valdosta State University
Determining Reaction OrdersA 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
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Determining Reaction Orders2 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
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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
Valdosta State University
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 reactantPlot 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
Valdosta State University
Determining Reaction Orders2 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
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Determining Reaction Orders Zeroth Order Plot – [C4H6] versus Time
y = -1x10-06x + 0.0154
R2 = 0.9429
0.006
0.008
0.01
0.012
0.014
0.016
0.018
0 1000 2000 3000 4000 5000 6000 7000
Time (seconds)
Co
nce
ntr
atio
n (
M)
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Background
Valdosta State University
Determining Reaction Orders First Order Plot – ln[C4H6] versus Time
y = -0.0001x - 4.1432
R2 = 0.9852
-5.5
-5.3
-5.1
-4.9
-4.7
-4.5
-4.3
-4.1
0 1000 2000 3000 4000 5000 6000 7000
Time (seconds)
ln[C
4H
9]
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Background
Valdosta State University
Determining Reaction Orders Second Order Plot – [C4H6]-1 versus Time
y = 0.0142x + 59.234
R2 = 0.9995
0
20
40
60
80
100
120
140
160
0 1000 2000 3000 4000 5000 6000 7000
Time (seconds)
1/[C
4H
6]
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Background
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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-bc A = -10log(T) = 10log(1/T) A = 10log (100/%T)A = b c -molar absorptivity (L/mol cm)For a solution in a cuvette, with a ct. cell width, A cA = Kc (Beer’s law)
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Background
Valdosta State University
Beer’s LawA = bc
A = absorbance = molar absorbtivityb = path lengthc = 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 Setup1. 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 - ColorimeterCalibration of Colorimeter1. 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”.
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Technique - Colorimeter
Running the Experiment1. 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
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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
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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
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Converting %T into A- use equation 2.5 pg 19 from the laboratory manual.
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Example Data
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Converting %T into A- use equation 2.5 pg 19 from the laboratory manual.
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Example Data
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Determining - use equation 2.6 pg 19 from the laboratory manual.
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Example Data
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Determining - use equation 2.6 pg 19 from the laboratory manual.
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Example Data
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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
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Use Beer’s Law to determine [murexide] at each time- use equation 2.5 pg 19 from the laboratory manual or
A = c
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Example Data
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Use Beer’s Law to determine [murexide] at each timeTime ( 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
ln[murexide] 1/[murexide]
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Example Data
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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
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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
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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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