chem102 087 lab report vb367

7/25/2019 CHEM102 087 Lab Report Vb367

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V Arvind Balakrishnan

CHEM 102-087

Winter 2015-16

Instructor: Ms. Junyang Xian

Lab 2: Kinetics of Alcohol Oxidation

Introduction:

Chemical Kinetics is the branch of chemistry that deals with the study of reaction

rates and mechanisms. Its imperative that we understand the rate, the factors

controlling the rate as well as the mechanisms of a chemical reaction in order to

understand it fully.

Reactions such as ionic reactions usually take place very quickly. As an example, the

precipitation of silver chloride after the mixing of the aqueous solutions of silver

nitrate and sodium chloride is almost instantaneous. There are some reactions that

take place at a moderate pace, such as the inversion of cane sugar or hydrolysis of

starch. At the other end of the spectrum are extremely slow reactions like the

rusting of iron in the presence of air and moisture.


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Using chemical kinetics to determine the rate of a given chemical reaction involves

the use of the rate law of the given chemical reaction. The rate law for a given

reaction is an expression in which its rate is defined in terms of the molar

concentrations of the reactants with each term raised to a power that may or may

not be equal to the stoichiometric coefficient of the reactant as seen in the

balanced chemical equation of the given reaction. It is impossible to determine the

rate law for a given reaction by merely looking at the balanced chemical equation of

the reaction ; it has to be determined experimentally.

Consider the below shown chemical reaction in the solution phase:

The rate law for this reaction is

where the exponents and may or may not be equal to and , the

stoichiometric coefficients of the reactants. and are the partial orders of the

reaction, that is, order of the reaction wrt A and B respectively. The net order of the

reaction would be equal to the sum of and . is a proportionality constant


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called the rate constant. and are the concentrations of the reactant species

in .

It becomes more tedious to calculate the order of a reaction when there is more

than one reactant involved. This is because each reactant contributes to the

reaction. So, in this case, what can be carried out in order to reduce the complexity

in the math that would arise is called pseudo-order treatment, which involves

determining the order of the reaction wrt each component independently.

The second part of the experiment involves an application of the Beers Law, which

is described in the following paragraph.

Beers Law: Beers Law states how attenuation of light and the properties of the

material through which the light travels are interrelated. According to Beers Law,

where stands for the measured absorbance, is a constant of proportionality

called the molar absorption coefficient, refers to the path length of the solution

that the light passes through and is the concentration of the absorbing molecule.


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In this experiment, b is to be kept constant at 1 cm because of which the above

expression would reduce to

The third half of the experiment deals with the oxidation of alcohols and involves

the Breathalyzer Test which takes place according to the following reaction:

Here, potassium dichromate acts as the oxidising agent. The dichromate reduces

the Chromium (III) ions and the alcohol undergoes oxidation to get converted into

an aldehyde. The orange colour of the dichromate solution is observed to turn

green at the end of the reaction.

The concept of pseudo-order is put into use while finding the order and the rate

constant of the reaction. Given that the concentration of the alcohol used in the

reaction is large, it can be assumed to be of a constant value throughout the

reaction. This thereby enables us to ignore the value of the concentration of the

alcohol while finding the rate. Therefore, the rate of the reaction can be written as:

The order of the reaction is calculate graphically by applying the concept of linear

regression. The order of the reaction would be of that graph with the value of

correlation factor closest to 1.


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Calculations and Discussions:

1. Concentration vs Absorbance

Concentration Transmittance Absorbance

0 13.1 0

0.00203 30.3 0.519

0.001015 51.43 0.289

0.004059 13.11 0.882

0.000203 85.06 0.07


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The slope of the line gives us the value of molar absorptivity ( ).

which is of the form y=mx+c. Therefore, the value of is equal to 216.

Concentration of potassium dichromate for the next part of the experiment is found using the formula


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Time Transmittance Absorbance [K2Cr2O7] ln[K2Cr2O7] 1/[K2Cr2O7]

30 43.64 0.36 216 0.00166666667 -6.39692965 599.999999

60 36.28 0.44 216 0.002037037037 -6.19625896 490.9090909

90 39.25 0.406 216 0.00187962963 -6.276680527 532.0197044

120 42.32 0.373 216 0.001726851852 -6.361455267 579.0884718

150 45.21 0.345 216 0.001597222222 -6.43948927 626.0869565

180 48.07 0.318 216 0.001472222222 -6.520982304 679.245283

210 50.79 0.294 216 0.001361111111 -6.599453919 734.6938776

240 53.62 0.271 216 0.00125462963 -6.680914866 797.0479705

270 56.27 0.25 216 0.001157407407 -6.761572769 864

300 58.78 0.231 216 0.001069444444 -6.840615976 935.0649351

330 61.08 0.214 216 0.000990740740 -6.917057672 1009.345794

360 63.46 0.198 216 0.000916666666 -6.994766656 1090.909091

390 65.62 0.183 216 0.000847222222 -7.073547534 1180.327869

420 67.57 0.17 216 0.000787037037 -7.14723525 1270.588235

450 69.39 0.159 216 0.000736111111 -7.214129484 1358.490566

480 71.1 0.148 216 0.000685185185 -7.285821413 1459.459459

510 72.74 0.138 216 0.000638888888 -7.355780002 1565.217391

540 74.13 0.13 216 0.000601851851 -7.415499236 1661.538462

570 75.42 0.123 216 0.000569444444 -7.470849331 1756.097561

600 76.68 0.115 216 0.000532407407 -7.538101558 1878.26087

630 77.9 0.108 216 0.0005 -7.60090246 2000


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660 78.91 0.103 216 0.000476851851 -7.648304698 2097.087379

690 79.82 0.098 216 0.000453703703 -7.698066208 2204.081633

720 80.79 0.093 216 0.000430555555 -7.750434194 2322.580645

750 81.63 0.088 216 0.000407407407 -7.805696872 2454.545455

780 82.26 0.085 216 0.000393518518 -7.84038243 2541.176471

810 82.92 0.081 216 0.000375 -7.888584532 2666.666667

840 83.62 0.078 216 0.000361111111 -7.92632486 2769.230769

870 84.11 0.075 216 0.000347222222 -7.965545573 2880

900 84.7 0.072 216 0.000333333333 -8.006367568 3000

930 85.09 0.07 216 0.000324074074 -8.034538445 3085.714286

960 85.4 0.069 216 0.000319444444 -8.048927182 3130.434783

990 85.75 0.067 216 0.000310185185 -8.078341067 3223.880597

1020 86.06 0.065 216 0.000300925925 -8.108646417 3323.076923

1050 86.41 0.063 216 0.000291666666 -8.13989896 3428.571429

1080 86.69 0.062 216 0.000287037037 -8.155899302 3483.870968

1110 86.86 0.061 216 0.000282407407 -8.172159822 3540.983607

1140 87 0.06 216 0.000277777777 -8.188689124 3600

1170 87.18 0.06 216 0.000277777777 -8.188689124 3600

1200 87.39 0.059 216 0.000273148148 -8.205496243 3661.016949


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Zero Order Reaction:

Correlation Coefficient =

First Order Reaction:


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Correlation Factor =

Second Order Reaction:

Correlation Factor =


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Observations:

Based on the data obtained from the graphical plots, the oxidation reaction

appears to be of order 2. This is because the vs time graph has the

value of correlation factor R closest to 1.

Calculation of the rate constant for the reaction:

The rate law for a second-order reaction has the following format:

where k is the rate constant and A is the concentration of the reactant.

Using the above rate law, the integrated rate law for a second order reaction can be

obtained as

where is the concentration of the reactant at time t and is the initial

concentration of the reactant.

This integrated rate law resembles a straight line of the formula

where k (the rate constant) is the slope of the line and is the y-intercept.

So, from the graph, the value of k can be obtained to be equal to .


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Also, given that the concentration of the alcohol was observed to be very large in

comparison to that of in the oxidation reaction, its a pseudo-first order

reaction. The pseudo-order rate would therefore give the true order of the reaction.


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Conclusions:

The reaction was observed to be of a pseudo-order equal to 1, and with a

pseudo-order rate constant equal to .

These calculations were obtained from graphical plots which were obtained from

data analyzed with the help of Beers Law.

In this lab, we learnt how to use the Vernier LoggerPro software and collect data

using the program installed on the computer. We also learnt to collect the

absorbance of each reaction and the rate constants.

In terms of theory, we gained a lot of insight into chemical kinetics using Beers Law

and also the pseudo-order treatment of reactions.


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chem102 087 lab report vb367

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