Running Head: CHEMICAL KINETICS LAB 1

Chemical Kinetics Lab

Prepared for: Mrs. Freeman

February 6, 2014

Introduction

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The science of chemistry is, in the simplest context, the formation of substances from

other substances; in other words, the concept of the chemical reaction is at the very core of all

chemistry. In turn, chemical kinetics, the study of the rates these chemical reactions, is also of

paramount importance (Kinetics, n.d.).

Chemical kinetics involves the concepts of reaction rates and rate laws. Reaction rates

can be the rate at which a product/reactant is formed/used in a reaction and rate laws are the

definitions that govern the aforementioned rates of appearance/disappearance. Many factors

affect the rate of a reaction. First, the concentration of the reactants plays an important role, as

increased concentrations usually correlate to a faster reaction. Increased temperature similarly

affects rate, as well as increased surface area of the reactants. Finally, the presence of a catalyst

decreases the activation energy of reaction, in turn speeding up the reaction (Intro to Kinetics,

n.d.)

Furthermore, these concepts are not confined to theoretical use. One example of a real

world application of rate law is found in the use of pesticides. When pesticides are used in an

environment to keep insects away, rainfall and other sources of water, such as sprinklers, will

wash off the pesticides from the plants, and the runoff will contain traces of these pesticides. To

prevent the contamination of freshwater sources, such as rivers, scientists must find and create

pesticides that dissolve into non harmful chemicals when reacted with water, and this rate of

reaction is of utmost importance (Mathematical Prediction, n.d.)

Another use of rate law can be found in enzymes. Enzymes are biological catalysts that

cause biochemical reactions. The rate of these biochemical reactions is very important in many

processes, such as food digestion. Digestive enzymes located in the stomach help break down

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food that is ingested, and the determination of the average rate of this digestion can be used to

develop digestive supplements for those with digestion problems (Chymotrypsin, n.d.).

Finally, the concept of reaction rates are used in nuclear decay and areas that deal with

nuclear decay. For example, the dating of ancient organisms using Carbon-14 is a manipulation

of rate law. All living creatures contain carbon, and this technique of dating is widely accepted

Carbon-14 slowly decays, and using the concepts of decay kinetics and the rate of decay of

Carbon-14, scientists can accurately determine the age of fossils and other decomposed organic

material (Decay Kinetics, n.d.)

Purpose:

In this lab, the pseudo rate law governing the reaction between Crystal Violet (CV) and

Sodium Hydroxide (NaOH) will be derived from the gathered data.

Methods:

1. All the materials needed for the lab were obtained.

2. The colorimeter was calibrated. To calibrate the colorimeter, use a pipette and fill one

cuvette 75% full with distilled water and place the cuvette in the colorimeter. The

absorbance should be 0.00. (Note: The colorimeter was set at 565nm because of the CV’s

purple color).

3. In 11 different test tubes, dilutions of the 24.5 μM solution of CV were created with

distilled water, as shown in Table 1.

4. With 11 DIFFERENT PIPETTES for each test tube, 11 different cuvettes were filled

about 75% full of each of the 11 dilutions of CV solution. The absorbance of each was

collected and recorded.

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5. Then, the concentration of each of the dilutions was found using the formula 𝑀1𝑉1 =

𝑀2𝑉2. (Note: The first dilution was essentially only 24.5 μM solution of CV; this

information was used to calculate the concentrations of the other dilutions as shown in

Table 1.)

6. A chart showing the concentration of CV dilutions vs. Absorbance of CV dilutions was

created, as well as a graph of this collected data (Table 1 and Figure 1). The graph was

created with a trend line.

7. The equation of the trend line and the coefficient of determination (R2) of this line was

then found, as shown in Figure 1. The R2 value should be high (>.90). This high value

ensures that the trend line accurately represents the data plotted. Both the equation and R2

value was recorded on the graph created in the previous step (Figure 1)

8. A separate cuvette was filled about 50% full of the 24.5 μM solution of CV using a

different pipette.

9. Next, with a new pipette, a small amount of NaOH solution was obtained. This solution

was then put in the cuvette filled in the previous step, which started to react with the CV.

The cuvette was placed into the colorimeter.

10. The absorbance of the solution of CV and NaOH was then recorded immediately and

every 25 second interval thereafter until the absorbance shown on the colorimeter is 0.00

(or close to it), (Table 2).

11. The equation found in step 7 is defined as absorbance = m(concentration of CV) (where

m is a definite number). Using this equation, the concentration of the solution of CV at

each of the 25 second intervals was determined. This was done by substituting the

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absorbance at a specific time into the equation and solving algebraically for the

concentration (Table 3).

12. Next, the natural log of the concentration of CV was determined for each concentration

found in the previous step, as well as the reciprocal of the concentration of CV. (Table 3).

13. A graph was then created containing 3 different graphs: time vs. concentration of CV,

time vs. ln(Concentration of CV), and time vs. 1/(Concentration of CV) (Figure 2). These

3 were also graphed individually with linear trend lines for clarity (Figures 3, 4, and 5).

14. The R2 value of each of the individual graphs (Figures 3, 4, and 5) was found in order to

determine the order of CV in the reaction.

15. With the order of CV determined, the pseudo rate law was found.

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Data

Table 1: Concentration of CV Dilutions vs. Absorbance

Dilutions (mL of CV/mL of water) Concentration (μM) Absorbance

10ml/0ml 24.5 1.161

9ml/1ml 22.05 1.12

8ml/2ml 19.6 1.104

7ml/3ml 17.15 1.051

6ml/4ml 14.7 0.967

5ml/5ml 12.25 0.805

4ml/6ml 9.8 0.716

3ml/7ml 7.35 0.546

2ml/8ml 4.9 0.36

1ml/9ml 2.45 0.169

0ml/10ml 0 0

Table 2: Time vs. Absorbance during reaction between CV and NaOH

Time (s) Absorbance

0 1.01

25 0.915

50 0.818

75 0.718

100 0.622

125 0.533

150 0.454

175 0.384

200 0.324

225 0.224

250 0.231

275 0.193

300 0.162

325 0.136

350 0.113

375 0.094

400 0.078

425 0.064

450 0.053

475 0.044

500 0.036

525 0.03

550 0.025

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Table 3: Time vs. Concentration, ln (Concentration), and 1/Concentration of CV

Time Concentration Ln[Concentration] 1/Concentration

0 17.93960924 2.887011075 0.055742574

25 16.25222025 2.78822953 0.061530055

50 14.52930728 2.676167801 0.068826406

75 12.75310835 2.545775034 0.078412256

100 11.04795737 2.402245558 0.090514469

125 9.46714032 2.247826889 0.105628518

150 8.063943162 2.087402663 0.124008811

175 6.820603908 1.919948017 0.146614583

200 5.754884547 1.750048981 0.173765432

225 3.978685613 1.380951517 0.251339286

250 4.103019538 1.411723175 0.243722944

275 3.428063943 1.231995654 0.291709845

300 2.877442274 1.0569018 0.347530864

325 2.415630551 0.881960351 0.413970588

350 2.007104796 0.696693284 0.498230088

375 1.669626998 0.512600247 0.59893617

400 1.385435169 0.326014292 0.721794872

425 1.136767318 0.128188548 0.8796875

450 0.941385435 -0.060402622 1.062264151

475 0.781527531 -0.246504901 1.279545455

500 0.639431616 -0.447175597 1.563888889

525 0.53285968 -0.629497153 1.876666667

550 0.444049734 -0.81181871 2.252

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Figure 1: Concentration vs Absorbance of Dilutions of CV

Figure 2: Time vs. Concentration, ln (Concentration), and 1/Concentration of CV

y = 0.0563xR² = 0.9067

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 5 10 15 20 25 30

AB

SOR

BA

NC

E

CONCENTRATION (MICROMOLAR)

CONCENTRATION VS. ABSORBANCE OF DILUTIONS OF CV

-5

0

5

10

15

20

0 100 200 300 400 500 600

CO

NC

ENTR

ATI

ON

(M

ICR

OM

OLA

R)

TIME (S)

T I M E V S . C O N C EN T R A T I O N , L N ( C O N C EN T R A T I O N ) , 1 / C O N C EN T R A T I O N

Concentration ln(Concentration 1/Concentration

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Figure 3: Time vs. Concentration of CV (0th Order)

Figure 4: Time vs ln (Concentration of CV) (1st Order)

R² = 0.8615

-5

0

5

10

15

20

0 100 200 300 400 500 600

CO

NC

ENTR

ATI

ON

[M

ICR

OM

OLA

R]

TIME (S)

TIME(S) VS. CONCENTRATION OF CV

R² = 0.9962

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

0 100 200 300 400 500 600

LN[C

ON

CEN

TRA

TIO

N]

(MIC

RO

MIL

AR

)

TIME (S)

TIME (S) VS. LN[CONCENTRATION OF CV]

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Figure 5: Time vs 1/Concentration of CV (2nd Order)

R² = 0.7783

-0.5

0

0.5

1

1.5

2

2.5

0 100 200 300 400 500 600

1/C

ON

CEN

TRA

TIO

N (

MIC

RO

MIL

AR

)

TIME (S)

TIME (S) VS 1/[CONCENTRATION OF CV]

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Calculations

Concentrations of Dilutions: (Known information: Concentration of 10 mL/ 0 mL = 24.5

μM)

Using 𝑀1𝑉1 = 𝑀2𝑉2:

Ex) 9mL/1mL: (24.5 𝜇𝑀)(9 𝑚𝐿) = (? 𝑀)(10 𝑚𝐿); (24.5 𝜇𝑀)(9 𝑚𝐿)

10 𝑚𝐿= ?𝑀 =

22.05 𝜇𝑀

Process repeated for each dilution

Equation/ Coefficient of Determination (R2) of Trend Line of Figure 1 (Found on Excel)

y = 0.0563x

R² = 0.9067

Concentration of CV when reacted with NaOH (Table 3)

Using above trend line

Ex) 𝑦 = 𝑎𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒 𝑜𝑓 𝐶𝑉 = 1.01

𝑥 = 𝜇𝑀 𝑜𝑓 𝐶𝑉 = ?

0.0563𝑥 = 1.01

𝑥 =1.01

. 0563

𝑥 = ?= 17.94 𝜇𝑀 𝑜𝑓 𝐶𝑉

Repeated for all absorbances in Table 2

R2 value of Figures 3, 4, and 5

Figure 3 (0th order) - .8615

Figure 4 (1st order) - .9962

Figure 5 (2nd order) - .7783

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Analysis

The purpose of this lab was to find the pseudo rate law of the reaction of Crystal Violet

(CV) and Sodium Hydroxide (NaOH). The reaction of CV and NaOH can be written in net ionic

form as shown below:

𝐶𝑉+ + 𝑂𝐻− 𝑦𝑖𝑒𝑙𝑑𝑠→ 𝐶𝑉𝑂𝐻

This, in turn, means that the rate law of CV and OH is defined as

𝑅𝑎𝑡𝑒 𝐿𝑎𝑤 = 𝑘 [𝐶𝑉]𝑥[𝑂𝐻]𝑦

Where x is the order of CV in the reaction, y is the order of OH in the reaction, and k is the rate

constant that governs the reaction. However, the concentration of OH- ions in the reaction was so

high compared to the CV ions that the value of [OH] y almost remains constant, allowing the rate

shown above to be substituted with a pseudo rate law defined as

𝑅𝑎𝑡𝑒 𝐿𝑎𝑤 = 𝑘∗[𝐶𝑉]𝑥

𝑘∗ = [𝑂𝐻]𝑦

Now, with the pseudo rate law properly defined as an equation, the missing component

that made the pseudo rate law complete was the value of x, the order of CV. To find the order of

a reactant in any reaction, two aspects of the reaction are needed: the time within which the

reaction takes place, and the changing concentration of the reactants at during this time. To find

these aspects, the properties of Beer’s law that connects absorbance and concentration were

utilized. First, as shown in Table 1, dilutions of CV and distilled water were made, and the

absorbance of each was found using a colorimeter. Then, using the known molarity of the 10

mL/ 0mL dilution (24.5 μM) and 𝑀1𝑉1 = 𝑀2𝑉2 , the concentrations of all the dilutions were

calculated (Table 1). This information was then rearranged and graphed as Concentration vs.

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Absorbance of CV dilutions in Figure 1, and the equation of the trend line was found. This

information was crucial later in the lab.

Next, a small amount of CV and NaOH were reacted in a cuvette, and the absorbance of

this solution was measured consistently every 25 seconds. The absorbance fell dramatically, due

to the fact that when the hydroxide ions reacted with the vibrantly purple CV, the solution slowly

became transparent. This data is shown in Table 2. Now, with the equation of the trend line

found in Figure 1, the concentration that correlated with each absorbance in Table 2 was found,

as shown in the calculations section of this report. This means that, after some manipulation of

charts and data, time and concentration of CV, the two components needed to find the order of

the CV, have been found (Table 3).

The final step to finding the order of CV is to determine which manipulation of the

concentration graphed against the time yields the straightest line. These graphs are defined as

time vs concentration of CV (Figure 3), which represents the 0th order, time vs. ln (concentration

of CV) (Figure 4), which represents the 1st order, and time vs. 1/ (concentration of CV) (Figure

5), which represents the 2nd order. To determine which yielded the straightest line, the R2 value

of a linear trend line was found for each graph. Statistically speaking, the higher the R2 value, the

more of the data can be explained by the trend line. The first order graph had the highest R2

value (.9962), which means that 99.62% of the data could be explained by the linear trend line.

In other words, 99.62% of the data was completely in line with the trend line, making it the

straightest data regression of the 3 orders. Therefore, the order of CV was found to be 1.

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Error Analysis

The results of the experiment and the subsequent analysis of the data seem to be very

sound. While there was a rush for time and the reaction could not completely go to completion

(absorbance of CV and NaOH solution is 0), this does not seem to have had a large effect on the

data at all.

Conclusion

The pseudo rate law that governs the reaction between Crystal Violet and Sodium Hydroxide

was found to be:

𝑅𝑎𝑡𝑒 𝐿𝑎𝑤 = 𝑘∗[𝐶𝑉]1

𝑘∗ = [𝑂𝐻]𝑦

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References

Chymotrypsin. (n.d.). - Chemwiki. Retrieved February 5, 2014, from

http://chemwiki.ucdavis.edu/Physical_Chemistry/Kinetics/Case_Studies/Chymotrypsin_

II

Decay Kinetics - variation of decay rate with time. (n.d.). Decay Kinetics - variation of decay

rate with time. Retrieved February 5, 2014, from

http://www.science.uwaterloo.ca/~cchieh/cact/nuctek/decaykinetics.html

Introduction to Chemical Kinetics, First Order Precesses, Half-Life. (n.d.). Introduction to

Chemical Kinetics, First Order Precesses, Half-Life. Retrieved February 5, 2014, from

http://www.chem.arizona.edu/~salzmanr/480a/480ants/kinintro/kinintro.html

Kinetics - The Rate Law. (n.d.). Kinetics - The Rate Law. Retrieved February 5, 2014, from

http://bouman.chem.georgetown.edu/S02/lect2/lect2.htm

Mathematical Prediction of Cumulative Levels of Pesticides in Soil. (n.d.). - Organic Pesticides

in the Environment. Retrieved February 5, 2014, from

http://pubs.acs.org/doi/abs/10.1021/ba-1966-0060.ch010