Valdosta State University Experiment 2 Kinetics of Murexide Decomposition Valdosta State University.

Valdosta State University

Experiment 2Kinetics of Murexide

Decomposition



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.

NH

NH

O

ONH

NH

O

O

O N

O

NH4+


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


Background

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



Background



t

rate

reactant


Background



t

rateort

rate

productreactant

recall [ ] = mole/L


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.



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



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


Background


Determining Reaction OrdersOne method is to run the reaction several times using different concentrations and observe the rate change.


Background


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


Background


Determining Reaction OrdersA second method follows the concentration of one reactant with time. How the concentration decreases reveals the order of the reaction.


Background


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)


Background


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


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.


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 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


Background


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


Background


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)


Background


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]


Background


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]


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.


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)


Background


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.


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.


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”.


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.


Example Data


Determining [murexide]- molecular formula: C8H8N6O6

- formula weight of murexide: 284.19 g/mol- solution concentration is 0.100 g/L


Example Data


Converting %T into A- use equation 2.5 pg 19 from the laboratory manual.


Example Data


Determining - use equation 2.6 pg 19 from the laboratory manual.


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


Example Data


Use Beer’s Law to determine [murexide] at each time- use equation 2.5 pg 19 from the laboratory manual or

A = c


Example Data


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]


Example Data


Zeroth Order graph- plot [murexide] versus time- be sure to include a trendline with the R2 and the linear regression equation


Example Data


First Order graph- plot ln[murexide] versus time- be sure to include a trendline with the R2 and the linear regression equation


Example Data


Second Order graph- plot 1/[murexide] versus time- be sure to include a trendline with the R2 and the linear regression equation


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!



Waste Disposal

All waste materials may be disposed of in the sink, flushed with plenty of water.


Valdosta State University Experiment 2 Kinetics of Murexide Decomposition Valdosta State University.

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