Bromination of acetone rate law

Kinetics Lab Report Turnitin

Determining Rate Laws: The Kinetics of the Bromination of Acetone

The acid-catalysed bromination of acetone in aqueous solution is a

multistep reaction. In this experiment you will determine the order

of the reaction with respect to acetone, bromine and proton (H+)

concentration, and from these, the rate law and observed rate

coefficient. This allows some understanding of the detailed

reaction mechanism and illustrates that the rate law cannot be

predicted from the reaction stoichiometry alone. The rates of many

chemical reactions are measured by following the change in some

physical property (such as electrical conductance, refractive index,

optical rotation, or light absorbance, etc.) and it is possible to relate

the change in that property to the extent of reaction. This applies to

any property that:

(a) changes during the reaction and;

(b) is proportional to concentration.

In this experiment, the progress of the reaction is monitored

directly by following the absorption of visible light by bromine as

a function of time, using a uv-visible (UV-Vis) spectrophotometer.

As bromine is consumed by the reaction the absorption of light by

the solution decreases with time. This change in absorption is

directly related the change in the concentration of the aqueous

bromine. Conveniently, the other species involved in the reaction

do not absorb light in the wavelength range that we will use.

CHEM2201/6201 Analysis and Measurement

2020

Concepts and Skills: • Chemical Kinetics

• Rate Laws

• Steady State

Approximation

• Isolation Method

• UV-Vis Absorption

Spectroscopy

• Beer-Lambert Law

Review Text. • Kinetics: Lecture Notes

• Spectrophotometry:

Harris p436-443

CHEM2201/6201 Determining Rate Laws: The Kinetics of the Bromination of Acetone

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LEARNING OUTCOMES The learning outcomes of this experiment are: • Application of the theory of kinetics, through a practical demonstration; • Exploration of the concepts of order of reaction and how reaction mechanism may

be illuminated by kinetics measurements; • Attainment of an appreciation of the method of graphical interpretation of kinetic

data as an aid in the determination of the order of reaction, employing integrated rate laws;

• Familiarity with some of the practical aspects of kinetics measurements; • An introduction to the technique of UV-Vis absorbance for quantitative

measurements; • Development of general laboratory and research as described in the notes for

Experiment 1.

INTRODUCTION In acidic solution, the bromination of acetone occurs according to the overall reaction: CH COCH + Br → CH COCH Br + HBr

The reaction is really a multistep process which can be presented as the reversible

acid-catalysed keto-enol tautomerisation of acetone (Step 1):

CH C O CH + H 𝑘⇌𝑘 CH C OH = CH + H Acet Enol (S1)

followed by the irreversible addition of bromine to the carbon-carbon double bond

(Step 2): CH C OH = CH + Br → CH C O CH Br + HBr Enol Prod (S2)

CHEM2201/6201 Determining Rate Laws: The Kinetics of the Bromination of Acetone

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This mechanism may be summarised as:

CH C O CH + H ⎯⎯⎯ CH C OH = CH + H (a) CH C OH = CH + H ⎯⎯⎯⎯ CH C O CH + H (b) CH C OH = CH + Br ⎯⎯⎯ CH C O CH Br + HBr (c)

Note here that Step 1, which is a two-way reaction, has been split into the forward (a)

and reverse reactions (b). Strictly Step 1 as not an equilibrium system, even though it

is drawn as though it is, because the right-hand side (product) of Step 1, CH C OH =CH , is used up in Step 2. However, under the right conditions we can consider Step 1 to be an equilibrium. This may be useful in determining an overall rate law.

The rate equation for [Br2] may be obtained by writing down the rate equations for

each species:

𝑑 Acet𝑑𝑡 = −𝑘 Acet H + 𝑘 Enol H (1) 𝑑 Enol𝑑𝑡 = 𝑘 Acet H − 𝑘 Enol H − 𝑘 Enol Br (2) 𝑑 Br𝑑𝑡 = −𝑘 Enol Br (3)

We can use the steady state assumption by declaring that [Enol] is low and constant.

Under these assumptions we make = 0, thus from (2): Enol = 𝑘 Acet H𝑘 H + 𝑘 Br (4)

Substituting (4) into (3) we obtain:

𝑑 Br𝑑𝑡 = −𝑘 𝑘 Acet H Br𝑘 H + 𝑘 Br (5)

The observed rate law will depend on the relative sizes of "𝑘 H " and "𝑘 Br ".

Prelab Exercise: Show how Equation

(4) is obtained from

Equation (2).

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If 𝑘 H ≫ 𝑘 Br , then 𝑘 H + 𝑘 Br ≈ 𝑘 H and we can write:

𝑑 Br𝑑𝑡 = −𝑘 𝑘 Acet H Br𝑘 H

𝑑 Br𝑑𝑡 = −𝑘 𝑘 Acet Br𝑘 = −𝑘 𝑘𝑘 Acet Br = −𝑘 Acet Br (6) and the reaction appears to be 1st order with respect to acetone and bromine, and 0th

order (independent) with respect to H+.

If, on the other hand, the reverse is true, i.e. 𝑘 Br ≫ 𝑘 [H ], then 𝑘 [H ] +𝑘 [Br ] ≈ 𝑘 [Br ] and:

𝑑[Br ]𝑑𝑡 = −𝑘 𝑘 [Acet][H ][Br ]𝑘 [Br ] = −𝑘 [Acet][H ] = −𝑘[Acet][H ] (7) and the reaction appears to be 1st order with respect to acetone and H+, and 0th order

(independent) with respect to bromine.

In this experiment you will determine the order of the reaction with respect to the three

species CH C O CH (𝑎𝑞), H (𝑎𝑞) and Br (𝑎𝑞), and hence determine which of the cases, (6) or (7), applies. Note that, according to both (6) and (7), the rate law does not

follow the reaction stoichiometry.

In a real research situation, the mechanism might initially be unknown; in this case

mechanisms must be postulated, which can be tested against the observed rate laws.

The reaction is followed by measuring the absorbance of Br in a UV-Vis spectrometer. An aqueous bromine solution appears to the human eye to be coloured

orange. This tells us that it has an absorption band in the violet part of the spectrum

(the complementary wavelength range of orange), so by monitoring the absorbance in

this region during the reaction, the bromine concentration can be calculated directly as

a function of time, if the absorption coefficient is known. The visible absorption

spectrum of aqueous Br2 is given in Figure 1. Using the Beer-Lambert Law (Beer’s Law) and given ε = 175 L mol−1 cm-1 at λ = 390 nm, you can calculate the concentration

of Br2(aq).

Prelab Exercise: Calculate the expected absorbance of a Br2 if the solution concentration is 0.00110 mol L-1. Assume you are using the standard 1.000 cm cuvette.

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Figure 1: Visible Absorption Spectrum of Br2(aq) The rate law of this reaction has the general form:

𝑅 = −𝑑[Br ]𝑑𝑡 = 𝑘[Br ] [Acet] [H ] You will be using the isolation method. The concentrations of acetone and protons are

very much larger than that of Br2. This means that for a particular run we will make

the simplification that the concentrations of acetone and protons are constant. This

means that we can write:

𝑅 = −𝑑[Br ]𝑑𝑡 = 𝑘[Br ] [Acet] [H ] = 𝑘[Acet] [H ] [Br ] = 𝑘 [Br ] 𝑅 = −𝑑[Br ]𝑑𝑡 = 𝑘 [Br ] The orders m and n for acetone and protons respectively are determined by

systematically varying the concentrations of acetone (Acet) and protons (H+)

respectively and noting the effect on the measured rate. For j, the order with respect to [Br ], the order of reaction can also be found from the [Br ] vs time data: for zeroth, first and second order the rate laws are given in Table 1.

Table 1: Possible rate laws for the reaction of Br2 in this experiment.

Order Rate Law Integrated Rate Law

Zeroth 𝑑[Br ]𝑑𝑡 = −𝑘 [Br ] = [Br ] − 𝑘 t First 𝑑[Br ]𝑑𝑡 = −𝑘 [Br ] ln[Br ] = ln[Br ] = 𝑘 t Second 𝑑[Br ]𝑑𝑡 = −𝑘 [Br ] 1[Br ] = 1[Br ] + 𝑘 t

0.00

0.05

0.10

0.15

0.20

0.25

0.30

300 350 400 450 500 550 600

Ab so

rb an

ce

λ / nm

Prelab Exercise: Sketch the graph you

would expect to see if

you plotted [Br2]

against time for each

of the three reaction

orders.

CHEM2201/6201 Determining Rate Laws: The Kinetics of the Bromination of Acetone

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Thus Br , ln[Br ] or [ ] will vary linearly with time if the reaction is zeroth, first or second order in [Br ] respectively. You will make plots of each of these equations and determine which one gives the most linear plot.

The rate coefficient, k, can be determined from the measured rate coefficient, kobs,

given the concentrations of acetone and protons and the orders with regard to these

species.

𝑘 = 𝑘[Acet] [H ]

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PROCEDURE

Work in pairs to complete this experiment.

Keep all bromine solutions in the fume cupboard or in stoppered flasks at all times.

You will make up four solutions, each of 100.0 mL, of varying concentrations and

measure the rate of disappearance of Br2 by following the absorbance as a function of

time in the spectrophotometer.

Start by placing approximately 200 mL of the Br2 solution in a stoppered conical flask in the 20 °C bath.

Then put the acetone, nitric acid and major water components into clean 100-mL

volumetric flasks with the volumes given in first three rows of Table 2. The acetone and nitric acid volumes need to be quantitative (accurate). The water volume needs

only be approximate (but not excessive) at this point. Why?

* You must record the actual concentration of the nitric acid.

Table 2: Preparation of Solutions

Component 1 2 3 4

Fi rs

t S te

p Acetone (AR) / mL 10 5 10 10

~1 mol L-1 HNO3 / mL 25 25 10 25

H2O / mL 40 45 55 50

Allow to equilibrate

Se co

nd

St ep

~0.02 mol L-1 Br2 / mL 20 20 20 10

H2O / mL To 100.0 mL mark

These solutions are also placed into the water bath.

While the solutions are equilibrating (coming to temperature), prepare the

spectrometer according to the instruction sheet residing with the instrument. Set it to

“CHEM2201-kinetics” mode. Set the wavelength to the λ = 390 nm. Set the collection

time to 600 seconds and the absorbance range to 0.00 - 1.00. Instructions will be

provided in the laboratory.

Prelab Exercise: What are the t = 0 concentrations of

acetone, H+ and Br2(aq) in

Solution 4, assuming the

stock concentrations are pure

acetone, 1.000 mol L-1

HNO3 and 0.0200 mol L-1

Br2 respectively? You will

need some information that

is given in the “REPORT”

section.

CHEM2201/6201 Determining Rate Laws: The Kinetics of the Bromination of Acetone

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Tip: It will help to do a practice run of the following procedure to ensure all operating parameters are

properly set and that you know what you are doing. Once you add the Br to the main solution, the reaction will not wait for you to fiddle with the spectrometer conditions!

When the solutions are at 20 °C, in the shortest time possible, for each solution in turn:

a. Pipette the required amount of Br solution into the appropriate volumetric flask, make up to the mark with distilled water. Mix thoroughly.

b. Important: start the spectrometer collection at the point that the Br2 solution is added to the volumetric flask. You will need to coordinate with your partner – one

does the pipetting while the other starts the spectrometer.

c. While the data collected before the cuvette is put into the spectrometer will not be useful, by doing this you will have recorded an appropriate zero time, t = 0.

d. Rinse and fill the sample cuvette, dry the outside. Put the cap on the cuvette and

insert it into the spectrometer.

e. Collect the absorbance as a function of time for the sample.

Repeat a. – e. for the next sample until all four are completed.

REPORT – Results, Questions and Conclusions

For your report, complete the following calculations and answer the questions. Your

report should conform to the expected structure for CHEM2201/6201 report. In

addition to the report, you should upload to iLearn the spreadsheets that you construct for your data analysis.

1. Reaction Orders, Rate Equations and Rate Coefficient

(i) From each kinetic run, using the provided extinction coefficient, ε, convert the

measured absorbance to concentration of Br2(aq). This can be left in your

uploaded spreadsheet. It does not need to be included in your report.

(ii) Using the spreadsheet provided on iLearn (CHEM2201_Kin.xls), put in your

concentration and time data. The spreadsheet will plot for you [Br2] against

time, ln([Br2]) against time and [ ] against time. Note, you will need to make decisions on what data points from those you have collected are appropriate to

use. You will need to discard some. The spreadsheet will provide you with the

slope and intercept, with standard error, of these functions. Calculate the relative

standard error (% SE) of the slope, and use this to decide which plot gives the

ε = 175 L mol−1 cm-1

CHEM2201/6201 Determining Rate Laws: The Kinetics of the Bromination of Acetone

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straightest line, and therefore determine the order with respect to [Br2]. Why

must we use the relative standard error and not the absolute standard error?

(iii) Having decided on the order of the reaction with respect to Br2 use the

appropriate plot to determine the rate of loss of Br , [ ], in mol L-1 s-l for each of the solutions. Tabulate these values. In the same table tabulate the initial

concentrations of H , acetone and Br and the observed rate (from (i)) for each of the runs. You may take the density of acetone to be 0.789 g mL-1. [H+]0 and

[Acet]0 can be calculated from the concentrations of the provided reagents,

taking into account to appropriate dilution factors. [Br2] can be obtained from

your plots. How?

(iv) By inspection of your results, determine the order of the reaction with respect to

acetone and H . You can do this by taking the ratio of the two rates of reaction and concentrations where only the concentration of the species of interest varies.

(v) Now that you know the orders with respect to each species, write down the rate

equation for the reaction.

(vi) Determine and tabulate the rate coefficient, kobs, for each run. From kobs, calculate

k for each run. Include these in the table.

(vii) Calculate the mean rate coefficient, k, and the standard deviation of the mean

based on your four measurements.

2. For your discussion

(i) From your results, does either Equation (6) or Equation (7) describe your result?

If so, which one?

(ii) From your results, which step (a), (b) or (c) would be a rate determining step of

the mechanism? Justify.

(iii) In this experiment, the concentration of [Br ] has been chosen to be much less than those of acetone and H+. Why?

(iv) Compare your results with (at least) those from Birk and Walters, J. Chem. Educ.

1992, 69(7), 585. This is your comparison to literature.

3. Conclusion

Write a one paragraph conclusion which describes the important outcomes of your

experiment.

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Assessment Rubric for Determining Rate Laws: The Kinetics of the Bromination of Acetone

Section Assessment Weighting

Pre-lab Pre-lab questions 5

Lab notebook Record the experimental results and write down the observations. Signed off at the end of the lab session.

10

Lab Report

Title: Clearly and concisely inform the reader of the practical

report topic 5

Introduction: general introduction with the aim(s) of

experiment. 10

Material and Methods: Describe what was done in the

experiment. Includes materials used and procedures

followed.

10

Results and Discussion (including Questions): Present,

interpret and explain the findings of the experiment. The

quality of your data will be taken into account.

50

Conclusion: Summarises findings and interpretations. One

paragraph. 5

References: Presents the sources of information used in the

report. 5

100

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Prelaboratory Questions: Kinetics - The Bromination of Acetone

To be checked before the lab session and submitted with your report

Name: Date:

1. Show how Equation (4) is obtained from Equation (2).

2. Calculate the expected absorbance for a 0.00110 mol L-1 Br2(aq) solution

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3. Sketch the plots you would expect to see for [Br2] v time if the order with respect to [Br2] is 0th, is 1st and is 2nd.

4. Calculate the nominal concentrations of acetone, H+ and Br2 at t = 0 for Solution 4.