Method of Initial Rates in the Bromination of Acetone PDF

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Required Lab for Physical Chemistry at FIU. Complete paper graded with an A...

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Running Head: Method of Initial Rates in the Bromination of Acetone

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Method of Initial Rates in the Bromination of Acetone Michael Jonathan Blanco

Lab Partners: Ana Sosa Ligia Pinto

Method of Initial Rates in the Bromination of Acetone

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Abstract The purpose of the lab was to find the rate constant and the order of acetone and hydrogen ion on the bromination of acetone reaction. Seven solutions were prepared with a different concentration of acetone and hydrogen ion. Some solutions the initial concentration stay the same except for the hydrogen ion, while the other the acetone is changing in concentration and the other initial concentration is held constant. The reaction order of bromination of acetone was found to be of the 2nd order. Additionally, the rate constant was averaged to be .001361 ± .00007 at 95% confidence limit. Introduction The purpose of the experiment is to find the value of the rate constant for the reaction. Ketones can react with halogens under acidic condition to substitute the alphacarbon (Figure 1) (Bromination of Acetone).

Figure 1 - Bromination of Acetone

Rate laws are the expression of rates in term of concentration of reactants (Chemical Kinetics). The rate law of the above reaction is as follows:

R = k [H+]p[Acetone]q The reaction above is known to be zeroth order with respect to Br 2.

Method of Initial Rates in the Bromination of Acetone

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Simple chemical kinetic can be applied to the bromination of acetone reaction to determine the rate constant and order of the reaction. Chemical reaction rates are the rates change in concentration of either reactants or products (Chemical Kinetics). By comparing the initial rate of reaction and varying the initial concentration of hydrogen ion or acetone while the initial concentration of the other reactant is held constant, we can determine the order of the reaction. Absorbance measurements are measured against time to determine the rate of change of absorbance. Beer’s Law states: A=∈ cl where “A” is absorbance, “ ∈ ” is extinction coefficient, “c” is concentration and “l” is path length. Beer’s law is simply used to find the rate of change in bromine concentration with time. Procedure Refer to Lab Manual: pg. 33-34

Results

Solutio n 1 2 3 4 5

1

2

3

4

0.67 2 0.68 2 0.73 2 0.72 8 0.65 0

0.64 4 0.64 4 0.68 0 0.64 8 0.60 2

0.60 6 0.58 2 0.60 0 0.52 2 0.54 4

0.57 8 0.53 8 0.51 2 0.39 4 0.48 4

Absorbance/min 5 6 0.53 8 0.47 8 0.40 8 0.25 3 0.42 2

0.49 2 0.41 4 0.29 6 0.10 0 0.35 6

7

8

9

10

0.44 6 0.34 1 0.17 1 0.00 5 0.28 6

0.40 6 0.26 3 0.05 1

0.35 3 0.18 0 0.00 6

0.30 5 0.09 4

0.21 8

0.14 0

0.05 4

Method of Initial Rates in the Bromination of Acetone 6 7

0.65 6 0.68 8

0.58 6 0.59 4

0.51 4 0.48 4

0.43 2 0.37 6

0.35 1 0.24 3

4

0.26 0 0.10 7

0.15 4 0.00 6

0.05 0

0.00 8

Absorbance vs. Time (Trial 1) 0.800 0.700

f(x) = − 0.04 x + 0.73 R² = 0.99

0.500

Absorbance vs. Time (Trial 4)

0.400

12.000

0.300 0.200

10.000

0.100

8.000

0.000 0

2

4

6

Time (minutes)

8

10

1

Absorbance

Absorbance

0.600

6.000 4.000 2.000 0.000 0

f(x) 2 =0 4 6 8 R² = 0 Time (minutes)

10

12

Method of Initial Rates in the Bromination of Acetone

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Absorbance vs. Time (Trial 5) 0.700 f(x) = − 0.07 x + 0.74 R² = 1

0.600

Absorbance vs. Time (Trial 7)

0.400 0.300

12.000

0.200

10.000

0.100 0.000 0

2

4

6

8

Time (minutes)

10

1

Absorbance

Absorbance

0.500

8.000 6.000 4.000 2.000 0.000 0

Solutio n 1 2 3 4 5 6

Change in Absorbance over time 0.0411 0.0654 0.0978 0.1262 0.066 0.0849

f(x) 2 =0 4 6 8 R² = 0 Time (minutes)

10

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Method of Initial Rates in the Bromination of Acetone

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(1) Use the slope of the absorbance vs. time to determine initial rate of change in bromine concentration. Use equation 17 L ∈=160 mole− cm L =1.3 cm

Equation (17)

1 dA ( ∈l) dc = dt

dt

Example Calculation: Solution 1 M 1 dc = (.0411 )=.000 1976 dt ( 160 x 1.3 ) min Solutio n 1 2 3 4 5 6 7

Initial Rate of Change of [Br]/min 0.000197596 0.000314423 0.000470192 0.000606731 0.000317308 0.000408173 0.000560096

[H+ ] 0.2 0.3 0.4 0.5 0.2 0.2 0.2

[Acetone ] 0.8 0.8 0.8 0.8 1.2 1.6 2

(2) Calculate the Final concentration of H+ and Acetone used in the solution. Example Calculation: Solution 1 [H+] Use M 1 V 1= M 2 V 2 (1 M HCl)(10.0 mL) = M2(50 mL) M2 = .2 M [Acetone] ( 4 M Acetone)(10.0 mL) = M2(50 mL) M2 = 0.8 M Acetone ∂ ln ( Ri )

, by plotting ln (Ri) vs. ln ([A]i), the ∂ ln (A i ) slope of the line will be equal the order of the reaction with respect to A. (3) According to equation (12)

p=

Method of Initial Rates in the Bromination of Acetone

Ln (Initial Rate) vs. Ln(H+) -1.6

Ln (Initial Rate)

-1.8

-1.4

-1.2

f(x) = 1.24 x − 6.55 R² = 1

-1

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The slope of the line indicates a first

-6.8 reaction order respect to [H+] and -0.8 -7-0.6 [acetone]. -7.2 -7.4 -7.6 (4)Find rate constant -7.8 R = k [H+]p[Acetone]q -8 -8.2 Example Calculation: Solution 1 -8.4 -8.6 0.0411 = k [0.2]1[0.8]1 -8.8 k = .257 M−1·s−1

Ln (H+)

(5) Find 95% Confidence Interval

s

¿

√

n

(x i−x avg)2 ∑ i=1 n−1

Solutions 1 2 3 4 5 6 7 Average Std. Dev. 95% Confidence Interval

95 % C . I =´x ±

Rate constant (k) 0.001235 0.001310 0.001469 0.001516 0.001322 0.001275 0.001400 0.001361 0.0001 7.63488E-05

tS (1.934 )(0.000104 ) M =0.00136 ± .00007 =0.00136 ± s √n √7 Discussion

Method of Initial Rates in the Bromination of Acetone

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The purpose of the experiment is to determine the order of the bromination reaction with respect to hydrogen ion and acetone. After determining the reaction order is possible to find the rate constant value. Seven solutions were prepared. All seven solutions had 0.02 M. There were four conditions where all initial concentration is held constant except for acetone. Similarly, another four solutions were prepared where all initial concentration except for the hydrogen ion is held constant. The absorbance was plotted against time. According to Beer’s law, the change in absorbance over time can be used to calculate the initial rate of change of Br 2. The reaction order was determined by plotting the natural log of Br2 against the natural log of hydrogen ion or acetone. The reaction order was determine to be one for both hydrogen ion and acetone. Thus, the overall order of the bromination of acetone reaction is of the second order. Finally, the rate constant, k, was calculated for each of the seven solutions. The average rate constant was calculated to be 0.001361 with .00007 95% confidence. Possible source of error could be the introduction of light to the bromine solution since bromine is highly sensitive to light. Additionally, the timer used was not precise and thus each measurement at the minute mark is not exact.

References 1. Bromination of Acetone. (n.d.). Retrieved October 11, 2017, from http://chemlab.truman.edu/physical-chemistry/physical-chemistrylaboratory/bromination-of-acetone/

Method of Initial Rates in the Bromination of Acetone

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2. Chemical Kinetics. (n.d.). Retrieved October 11, 2017, from

http://www.science.uwaterloo.ca/~cchieh/cact/c123/chmkntcs.html 3. ChM 3400L Fundamentals of Physical Chemistry Laboratory Manual. Jeffrey A. Joens, Dpt of Chemstry, FIU...