Iodination and bromination

Kinetics study of acid catalyst halogenation of propanone - comparison of iodination and bromination

1. Introduction

1.1. Research question:

To study the kinetics and find the order of the reactants in acid catalysed iodination of propanone and then compare it with the order of the reactants in acid catalysed bromination of propanone under the same conditions.

The rate of chemical reaction indicates how fast the products have been formed measured as the change in concentrations divided by the change in time, so reaction rate has units of mol dm-3 s-1. This is equal to the rate at which the reactants are consumed, so for a reaction:
Reactants à Products, then the rate = Δ[P]Δt= -Δ[R]Δt where [P] is concentration of products, [R] is concentration of reactants and t is the time taken.[1]

The change in the concentration of which reactant or the catalyst in the reaction

I2(aq) + CH3COCH3(aq) à CH3COCH2I (aq) + H+(aq) + I- (aq), Reaction 1

Br2(aq) + CH3COCH3(aq) à CH3COOH2I (aq) + H+(aq) + I- (aq), Reaction 2

We all know that reactions take place at different rates due to various factors. While some reactions take months like the rusting of iron, some reactions are very spontaneous like neutralisation reactions. This is because, in the comparatively fast and quick reactions, more number of effective collisions take place between the reactants. Many collisions are not effective because they don't have enough kinetic energy to overcome the repulsion between the electron clouds.

Factors that affect the rate of reaction are pressure, concentration of the reactants, surface area, temperature and a use of catalyst. Factors affect the collision rate in different rates and proportions which we will talk about in detail in the essay.

The number of effective collisions between the reactants can be increased in a number of ways:

a) Effect of concentration: the more the concentration of the reactants, the more is the rate of reaction as the number of effective collisions increase with increase in concentration increases.

b) Effect of temperature: the higher the temperature, the more is the kinetic energy of the reactants and hence they collide more often and hence the rate of reaction increases with rate of reaction. Also the increase in temperature reduces the activation energy of the reactants and hence the reaction takes place faster.

c) Effect of surface area: as the surface area increases, the reactants colliding with the each other increases and come in contact and hence the rate of reaction increases. But this is only effective when a solid and a liquid or gas reacts and when a liquid and a gas react.

d) Effect of catalyst: catalyst help in increasing the rate of reaction without undergoing an overall change in the reaction. They speed up the reaction by creating an alternative pathway which requires lower activation energy and helps speed up the reaction.

In this experiment I will be changing the concentration of the reactants and will hence determine the rate of the reaction. Also by changing the concentrations of one reactant and keeping the rest constant we will be able to determine the order of the reaction.

We can find the rate of the reaction in various ways:

a) Collection of the gas evolved: if in a reaction gas is evolved, then the gas can be collected and used to measure the rate of the reaction. The more the gas produced in less time, the faster is the reaction.

b) Electrical conductivity: the amount of electricity conducted by a substance depends on the no. of ions present in the substance. If ions are produced in a reaction, then the reaction rate can be found from change in conductivity.

c) Change in the mass of reactant: the mass of the reactant will reduce if products are formed and the faster the mass is reduced, the faster is the reaction taking place. But this experiment can only be used if the reactant is solid like if a block of aluminium metal is made to react with the HCl then the rate of reduction in the block of Al metal block will give us the rate of reaction.

d) Light absorption by the product: if a precipitation reaction takes place, then this technique can be used to determine the rate of the reaction. The time taken to make the marked paper unclear which is kept at the bottom of the reaction vessel can be used to measure the rate of reaction. The more the time taken for the marked paper to become unclear the slower is the rate of reaction.

e) Change in the mass of the reactants: the total mass of the reactants will vary if gas is evolved in the reaction. For this technique to be effective the gas evolved should be having a high molar mass, so as to change the mass significantly.

But for this experiment, we need accurate and precise readings on the rate at which the reaction takes place and hence we will use light absorption method to calculate the change in the colour of the reactant mixture and hence determining the rate. Thus software named Colorimeter is used to measure the rate at which the reaction takes place.

Energy is also needed to break the intermolecular forces and the reactants to react or decompose. For this energy has to be provided. The minimum energy required for a reaction to start is known as activation energy.

A catalyst does not necessarily appear in the stoichiometric equation but it can appear in the rate equation. The other species which are likely to appear are the reactants and so you may assume that the rate expression is:

Rate = k [CH3­­­COCH3]a [I2]b [H+]c

I will be determining the order of reaction with respect to reach reactant by varying the concentration of reach species in turn, keeping the others constant and following the reaction calorimetrically. As the intensity of the iodine colour decreases more light is transmitted through the solution and hence the absorbance decreases.

There are three parts to the both the experiments, i.e. iodination of propanone and bromination of propanone

1) Choosing the right filter for the colorimeter.

2) Calibrating the colorimeter so that the meter readings can be converted to concentrations of iodine,

3) Obtaining values for the concentration of iodine at intervals of time for a series of experiments with the following sets of conditions:

a. Keeping propanone and [H+] and distilled water concentration constant and varying the concentration of the halogen (chlorine in experiment 1 and bromine in experiment 2)

b. Keeping Halogen and [H+] and distilled water concentration constant and varying the concentration of propanone

c. Keeping propanone and halogen and distilled water concentration constant and varying the concentration of [H+]

d. Keeping Halogen and [H+] and propanone concentration constant and varying the concentration of distilled water.

Each set of experiment will give me the rate of the reaction for that concentration of reactants and this will help me calculate the order of reaction of the reactants.

In order to measure the rate of the reaction we had to use coloured reactants which would react to give colourless products. We measured the rate of the reaction using a colorimeter which used to take readings of the transmission through the mixture of reactants and hence which meant that the forward reaction was taking place to give the colourless products and the colour of the reactants was fading off.

Firstly, we prepared all the reactants in the iodination of propanone in fixed concentrations and changing the concentration of reactants while keeping the other concentrations same we did various reactions and hence it helped us to determine the rate of the change in the fading off of the colour and hence the rate of the reaction. The same process was followed for bromination of propanone.

Increase the rate of the reaction to a greater extent, I2 or propanone or HCl or water in the first reaction, and Br2 or propanone or HCl or water in the second reaction and then comparing the rate of reaction between iodination of propanone and brominaton of propanone.

Obtain a rate equation between iodine and propanone and between bromine and propanone by determining the order of reaction with respect to each reactant and to the catalyst (in these cases hydrogen ions) and comparing the two reactions with respect to the rate of the reaction.

Rates of reaction are very important as they have a wide use in the Industries and Manufacturing Business. The Rate of reaction theory is extremely important because they help companies decide the various factors which will help them make the products faster and increase the production rate and hence saving time. Also equilibrium along with rate of reaction helps improve and decide the amount and quality of the products formed and also the rate at which they occur. This also helps decide the cost and determining how the energy can be saved just by making changes to the concentration of the reactants.

Experiment Theory:

As the rate of reaction changes with the change in the concentration of the reactants, we will try to vary different concentrations of the reactants including the catalyst to know the respect to which change in the concentration of reach reactant affects the rate of the reaction the most.

As we know that Bromine is more reactive than chlorine and is brighter than chlorine it will undergo a colour change faster and hence we are expected to see a greater transmittance in the case of Bromine and the reaction taking place much faster as compared to iodine.

Iodine is dark red in colour and the product is colourless in nature and hence the calorimeter will detect the change in the colour after every fixed time interval giving us the rate of the reaction and similarly Bromine is light yellow in colour while the product is colourless and hence we can detect the change in the colour from light yellow to colourless.

Independent variable: The concentration of the reactants and the halogen

Dependent variable: Rate of the reaction.

Controlled variables:

Controlled Variable

Method of control

Room temperature

By making sure that we conduct this experiment in a closed room and at the same place over a short period of time, we can nullify the effect of this variable on the outcome of the experiment.

Calibration of colorimeter

There is always some variation in the calorimeter after taking a reading. Hence after every use, the colorimeter was calibrated using distilled water to avoid errors.

The concentration of the reactants - HCl, Propanone, iodine and bromine

As long as the initial concentration and volume of the HCl are controlled, this aspect of the experiment will have no effect on the outcome of the experiment. The initial volume and concentration can be controlled by using the same HCl for all readings. Similarly propanone, iodine and bromine were covered with black chart paper and also kept in dark to prevent their reaction and hence controlling the concentration. Also a large amount of the reactants are made at once and the same reactants are used throughout the experiment to nullify the effect of uncertainty in the concentration.

The amount of light passing through the colorimeter

We need to calibrate the colorimeter after every experiment as there will be a deflection in the readings of the colorimeter and to avoid a deflection in the readings, hence systematic errors we need to use distilled water to calibrate the colorimeter.

Also as we have compared Br2 and I2 in which bromine is more reactive than iodine and also brighter than iodine we saw that the rate of reaction to be much faster in iodine than in bromine.

The number of effective collisions between the reactants can be increased in a number of ways:

e) Effect of concentration: the more the concentration of the reactants, the more is the rate of reaction as the number of effective collisions increase with increase in concentration increases.

f) Effect of temperature: the higher the temperature, the more is the kinetic energy of the reactants and hence they collide more often and hence the rate of reaction increases with rate of reaction. Also the increase in temperature reduces the activation energy of the reactants and hence the reaction takes place faster.

g) Effect of surface area: as the surface area increases, the reactants colliding with the each other increases and come in contact and hence the rate of reaction increases. But this is only effective when a solid and a liquid or gas reacts and when a liquid and a gas react.

h) Effect of catalyst: catalyst help in increasing the rate of reaction without undergoing an overall change in the reaction. They speed up the reaction by creating an alternative pathway which requires lower activation energy and helps speed up the reaction.

In this experiment I will be changing the concentration of the reactants and will hence determine the rate of the reaction. Also by changing the concentrations of one reactant and keeping the rest constant we will be able to determine the order of the reaction.

We can find the rate of the reaction in various ways:

f) Collection of the gas evolved: if in a reaction gas is evolved, then the gas can be collected and used to measure the rate of the reaction. The more the gas produced in less time, the faster is the reaction.

g) Electrical conductivity: the amount of electricity conducted by a substance depends on the no. of ions present in the substance. If ions are produced in a reaction, then the reaction rate can be found from change in conductivity.

h) Change in the mass of reactant: the mass of the reactant will reduce if products are formed and the faster the mass is reduced, the faster is the reaction taking place. But this experiment can only be used if the reactant is solid like if a block of aluminium metal is made to react with the HCl then the rate of reduction in the block of Al metal block will give us the rate of reaction.

i) Light absorption by the product: if a precipitation reaction takes place, then this technique can be used to determine the rate of the reaction. The time taken to make the marked paper unclear which is kept at the bottom of the reaction vessel can be used to measure the rate of reaction. The more the time taken for the marked paper to become unclear the slower is the rate of reaction.

j) Change in the mass of the reactants: the total mass of the reactants will vary if gas is evolved in the reaction. For this technique to be effective the gas evolved should be having a high molar mass, so as to change the mass significantly.

Plotting all the graphs in one graph to have a better understanding of the graph

From the obtained values I calculated the initial rate of reaction. If a straight line was found then the gradient of the straight line will be the value while if the graphs are curved then I drew a tangent from the origin and measured its slope.

Hence we can determine the order of reaction with respect to the component I had been varying, by comparing the initial rates at different concentrations.

Apparatus Required

1) Colorimeter with a set of filters

2) Twenty test tubes

3) Six 25 ml measuring cylinder

4) 6 droppers

5) Watch glass

Chemicals Required:

1) Distilled water

2) Iodine (I2) of 0.02 M/cm3 concentration

3) Bromine (Br2) of 0.02 M/cm3 concentration

4) HCl of a known concentration

5) Propanone solution

Procedure Followed

1) Firstly for the experiment we had to choose a light from the three given lights in the colorimeter to take the readings. Now we know that iodine is red in colour while bromine is yellow in colour. Referring to the chart, it said that the colours opposite each other must be used for the experiment. Now according to the chart blue colour lies opposite red and yellow and hence we used blue colour light for the experiment.

2) Now, to convert my experimental reading it was important for me to take readings of colorimeter of different concentrations of iodine. For this I had to prepare a calibration curve concentration of iodine and bromine and different concentration levels as shown below in table. To make the calibration curve for iodine, I took different concentrations of iodine in different volumes of water and with the known concentration of the iodine, I found the transmittance in the calorimeter and noted it down as given in the table given below:

Volume of 0.02 M of I2 solution (cm3)

Volume of distilled water (cm3)

Concentration of I2

(mol/cm3)

Transmission %

0.0

10.0

0.0

100

0.25

9.75

0.5 * 10-3

84

0.5

9.5

1.0 * 10-3

73

0.75

9.25

1.5 * 10-3

68

1.0

9.0

2.0 * 10-3

33

2.0

8.0

4.0 * 10-3

26

3.0

7.0

6.0 * 10-3

24

4.0

6.0

8.0 * 10-3

22

5.0

5.0

10.0 * 10-3

20

The same was done with bromine and the readings calculated are shown below in the table:

Volume of 0.02 M of Br2 solution (cm3)

Volume of distilled water (cm3)

Concentration of Br2

(mol/cm3)

Transmission %

0.0

10.0

0.0

100

1.0

9.0

2.0 * 10-3

98

2.0

8.0

4.0 * 10-3

93

3.0

7.0

6.0 * 10-3

90

4.0

6.0

8.0 * 10-3

85

5.0

5.0

10.0 * 10-3

81

Now I will plot the graph of the transmission against concentration of iodine and another graph of transmission against concentration of bromine and we will keep this to use in analysing the results.

3) Now we will make a table and we will see the different concentrations of the reactants that I have to take in the experiment:

H2O

I2

Propanone

HCl

Experiment 1

2

4

2

2

Experiment 2

4

2

2

2

Experiment 3

2

2

2

4

Experiment 4

0

2

6

2

Experiment 5

5

1

2

2

Experiment 6

0

6

2

2

4) Now the experimental procedure will begin. Firstly, let us look at experiment-1. We will measure 2ml of H2O, 4ml of iodine and 2 ml of water using a measuring cylinder and will have a separate measuring cylinder for each of them.

5) Then we will pour each of them in a test tube.

6) Now we will take 2 ml of propanone in a measuring cylinder and pour it into another test tube. Care must be taken for any spillage of the contents in both the test tubes and we must wipe the test tube clean and handle it only from the top of the test tube.

7) Before the readings using the calorimeter can be taken for experiment 1, we need to calibrate the colorimeter first. So we will take distilled water in a filter and put in the colorimeter. Now putting the colorimeter at 0% Absorbance light we will mark the reading as 0 and selecting at blue colour light from the calorimeter we will mark the reading at 100%. Now we are ready to conduct the experiment.

8) We will add the mixture of water, HCl and iodine to the propanone solution and letting them mix thoroughly.

9) Now quickly take a dropper and take 4 ml from the test tube and place it in the filter of the colorimeter and then start taking the readings.

10) Taking the readings for 300 seconds after an interval of every 10 seconds will help us take a better approximation of the rate of the reaction.

11) Now repeating the experiment with the variations in the volume of each reactant as given in the table above and following the same procedure and hence noting down the readings.

12) I recorded the readings found in the table given below:

Time

transmission

Concentration

0

24

.006

10

28

0.0034

20

40

0.0019

30

54

.0017

40

78

0.001

50

97

0.0001

60

97

0.0001

70

97

0.0001

80

97

0.0001

90

97

0.0001

13) Then I recorded the temperature of the room and the temperature of the mixture after the final reading.

Interpretation of the data and the graphs.

1) Using the calibration curve made earlier I converted the meter readings to iodine concentrations and entered these values in the table below :

Experiment 1

0

24.02721

0.619297

3

25.47655

0.593859

6

27.07832

0.567378

9

28.90935

0.538962

12

30.96904

0.509072

15

33.33358

0.477118

18

36.15542

0.441827

21

39.51201

0.403271

24

43.40211

0.362489

27

47.97877

0.318951

30

53.77549

0.269416

33

60.18319

0.220525

36

68.26825

0.165781

39

77.26914

0.111994

42

87.0328

0.060317

45

94.58435

0.024181

48

96.79646

0.014141

51

97.17754

0.012434

54

97.17754

0.012434

57

97.10132

0.012775

60

97.02511

0.013116

63

97.10132

0.012775

66

97.10132

0.012775

69

97.10132

0.012775

72

97.17754

0.012434

75

96.94889

0.013457

78

96.94889

0.013457

81

97.02511

0.013116

84

97.02511

0.013116

87

97.02511

0.013116

90

97.10132

0.012775

93

97.02511

0.013116

96

97.02511

0.013116

99

96.94889

0.013457

102

96.94889

0.013457

105

97.02511

0.013116

108

97.02511

0.013116

111

96.94889

0.013457

114

96.94889

0.013457

117

97.10132

0.012775

120

97.17754

0.012434

123

97.17754

0.012434

126

97.17754

0.012434

129

97.17754

0.012434

132

97.10132

0.012775

135

97.17754

0.012434

138

97.25438

0.012091

141

97.10132

0.012775

144

97.10132

0.012775

147

97.10132

0.012775

150

97.10132

0.012775

0

22.1968

0.65371

3

23.56992

0.627642

6

25.17169

0.599088

9

27.0021

0.568602

12

29.138

0.53554

15

31.35011

0.503761

18

34.02014

0.468264

21

37.22368

0.429181

24

40.73208

0.390063

27

44.92766

0.347486

30

49.50432

0.305357

33

54.84376

0.260873

36

60.64048

0.217237

39

66.81953

0.175097

42

72.61626

0.138966

45

77.42219

0.111135

48

80.70194

0.093116

51

82.53236

0.083376

54

83.5244

0.078187

57

83.90548

0.07621

60

84.05791

0.075421

63

83.90548

0.07621

66

83.90548

0.07621

69

84.05791

0.075421

72

84.21034

0.074635

75

84.43961

0.073454

78

84.36339

0.073846

81

84.28655

0.074242

84

84.36339

0.073846

87

84.28655

0.074242

90

84.36339

0.073846

93

84.36339

0.073846

96

84.36339

0.073846

99

84.36339

0.073846

102

84.43961

0.073454

105

84.28655

0.074242

108

84.28655

0.074242

111

84.28655

0.074242

114

84.36339

0.073846

117

84.21034

0.074635

120

84.21034

0.074635

123

84.21034

0.074635

126

83.98169

0.075815

129

84.21034

0.074635

132

84.21034

0.074635

135

84.13412

0.075028

138

84.13412

0.075028

141

84.05791

0.075421

144

83.98169

0.075815

147

84.05791

0.075421

150

83.98169

0.075815

153

83.98169

0.075815

156

84.05791

0.075421

2) Then I plotted the graph of the concentration of iodine against the time for each mixture in the set of experiment conducted.

3) Plotting all the graphs in one graph to have a better understanding of the graph

4) From the obtained values I calculated the initial rate of reaction. If a straight line was found then the gradient of the straight line will be the value while if the graphs are curved then I drew a tangent from the origin and measured its slope.

5) Hence we can determine the order of reaction with respect to the component I had been varying, by comparing the initial rates at different concentrations.

CONCLUSION

The graph shows theat the rate of the reaction is directly proportional to the initial concentrations of both propane and hydrogen ions, but it is independent of the initial concentrations of iodine. Thus the reaction is first order with respect to propanone, first order with respect to hydrogen ions and zero order with respect to iodine. Hence the rate equation is:

Rate = k [CH3COCH3(aq)] [H+(aq)]

Hence the overall order of the reaction is 2.

Hydrogen ions appear in the rate equation but are not reactants in the stoichiometric equation. On the other hand, iodine is a reactant but does not appear in the rate equation.

The rate of reaction must be determined by a step in the mechanism which involves propanone and hydrogen ions but not iodine.

Rate constant needed to be found at the temperature at which the experiment was conducted.

ERRORS & IMPROVEMENTS

During the experiment there were many errors that took place because of which there were various variations in the reading. The main sources of errors were and the way they could have been avoided

Errors and Effect on the experiment

Improvements

There were a lot of temperature variations when the experiment was conducted. Because the chemistry laboratory has many windows, the experiment was not conducted in a closed room and hence there were various temperature changes and since temperature directly affects the rate of the reaction the temperature variations could have caused error in the readings.

The effect of temperature can be reduced by performing the experiment in a closed room. Also the experiment can be done over a short period of time to nullify the effect the temperature change.

There could have been variation in the light output from the bulb in the calorimeter as there could have been voltage changes causing the intensity of the light to be changing and hence causing an error in the reading of the transmittance.

Controlling this aspect of the experiment is pretty tough to do as we do not have control over the electricity and the voltage changes that occur. If possible a battery run colorimeter can be used to nullify the effect of intensity of the blue light.

Different filter tubes were used for different concentration of mixtures. This could have caused a change in the transmittance of light from the filter tubes and hence causing a variation in the reading. The difference in the filter tubes used in the colorimeter could have caused variation in the transmittance from the tubes due to the following reasons:

a. Different tubes could have different optical characteristics.

b. The same tube being in different positions relative to the bulb and behaving as a lens,

c. Drops of liquid or patches of dirt on the outside of the tube.

The concentration of the HCl, propanone, iodine and bromine prepared for the reaction could not be same due to taking their constituents in the wrong amount and hence this could error in the concentration of the reactants and hence in the rate equation

This is another aspect which is tough to control as the change in the masses will be very insignificant and hence the change won't be noticed in the rate of reaction.

There could I2(aq) + CH3COCH3(aq) -> ch3cooh2i (aq) + H+(aq) + I- (aq)

Determining the order of reaction with respect to each reactant by first varying the concentration of each species and keeping the other constant and in the second experiment by differing the temperature.

0.0201 mol

5ml

Ratio 2:1

2.6 ml

.03865 mol/dm (divide by 2) = 0.01933 - concentration of iodine.

(A * D * 1000)/Molar mass

99 * 0.791 * 1000 / 58.08

13.48 mol/dm

V1 c1 =v2c2

2/13= 14.834 dm3 Z

[1] Chemistry text book

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