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Rate Law Determination of the Crystal Violet Reaction

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Rate Law Determination of the Crystal Violet ReactionName: Candice JosephDate: May 8, 20071.

PurposeThe purpose of this experiment was to determine the order of reaction with respect to crystal violet (CV) for the reaction between crystal violet (CV) and sodium hydroxide (NaOH).2. IntroductionCrystal violet ion (CV+) is a purple colored dye commonly used in super market labeling. When this ion is made to react with hydroxide ion it forms a colorless compound, which can be written as CVOH.

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The reaction between crystal violet and hydroxide ion is presented below: The rate expression for this reaction is          rate = k[CV+]m[OH-]n                                        (2)Where k = rate constant, m is the order of reaction with respect to CV+ and n is the order of reaction with OH-.

To experimentally determine order of reaction (1), with respect to one reactant (here CV+), concentration of the other reactant (here OH-) is intentionally kept much larger than that of the other reactant. In this experiment concentration of OH- was kept 1000 times that of CV+.

Under this condition the rate equation (2) reduces torate =  = k[CV+]m                                     (3)If this reaction is zero order with respect to CV+ i.

e. m = 0, then equation (3) reduces torate =  = k0                                                 (4)If this reaction is 1st order with respect to CV+ i.e. m = 0, then equation (3) reduces torate=  = k0[CV+]                            (5)If this reaction is 2nd order with respect to CV+ i.

e. m = 0, then equation (3) reduces torate=  = k0[CV+]2                                       (6)Using calculus, the rate equations (4) through (6) can be can be converted into linear forms. The linear form of the rate equations are presented in table 1, below:Table 1: Linear relationship between concentration of CV+ and time for reactions of different orders:Reaction OrderLinear relationship between concentration of CV+ and time0[CV+]t = [CV+]t – k0t                                                                                   (7)1ln[CV+]t = ln[CV+]t – k1t                                                                             (8)2(9)Here the subscript 0 and t for [CV+] refers to initial and instantaneous concentrations of CV+ respectively.All these rate equations from (7) through (9) are of y = mx + c form.

If reaction is zero order then plot of [CV+]t vs. t will be linear and slope of this line will be –k0.If reaction is 1st order then plot of ln[CV+]t vs. t will be linear and slope of this line will be –k1.

If reaction is 2nd order then plot of 1/[CV+]t vs. t will be linear and slope of this line will be k2.Beer’s Law states that absorbance of a solution is directly proportional to the concentration. Therefore, in equations (7) through (9), absorbance can be taken as proxy for concentration and ifAbsorbance vs time plot is linear then reaction is zero order with ko being negative slope of line.

ln(Absorbance) vs t plot is linear then reaction is 1st order with k1 being negative slope of line.1/(Absorbance) vs t plot is linear then reaction is 2nd order with k2 being slope of line.In this experiment above three plots were made from the experimentally observed values of [CV+]t and corresponding time. Whichever of these plots was linear gave the order of the reaction with respect to [CV+]t.

3. Materials:IBM compatible computer,  Serial box interface, LoggerPro Software, Vernier Colorimeter, plastic cuvette, 250-mL beaker, 0.02M NaOH, 2×10-5 M crystal violet,  glass stirring rod, two 10-mL graduated cylinders.4.

Procedure:1. Lab coat, gloves and safety glasses were used as safety measure during the experiment as the experiment involved handling of hazards chemicals.2. 10 mL of 0.

02 M NaOH solution was measured using 10 mL graduated cylinder. 10 mL of 2.0×10-5 M crystal violet solution was also measured using the other 10 mL graduated cylinder. 3.

Computer was prepared for acquiring the data during the experiment. The vertical axis had absorbance scaled from 0 to 0.35 and the horizontal axis had time scaled from 0 to 20 minutes. Calibration of the Colorimeter for 0% transmittance (no light) and 100% transmittance (maximum light):5.

The outside of the cuvette was cleaned and dried using a chemwipe and the cuvette was filled 3/4th with distilled water in a manner to avoid bubbles in the solution. The cuvette was placed in the colorimer’s cuvette slot with the reference mark facing the white reference mark at the right of the colorimeter and the opaque side perpendicular to the direction of the light source.6. From the experimental menu calibration was performed.

The blank cuvette was inserted into the colorimeter and the lid was closed. The wavelength nob of the colorimeter was turned to the 0%T position, 0 was typed in the %edit box and keep was clicked.7. For 2nd reading the wavelength knob of the colorimeter was turned to Red position, 100 was typed in % edit box and keep was clicked.

Preparing the reaction mixture and absorbance measurements8. The 10 mL samples of crystal violet and sodium hydroxide was poured simultaneously into a 250 mL beaker. And the reaction mixture was stirred with the glass stirring rod.9.

The cuvette was rinsed twice with the reaction mixture and subsequently refilled to observe absorbance of the reaction mixture. Absorbance was observed at an interval of 1 minute until it reaches a value of 0.34.10.

When the absorbance value was stable and below 0.34, it was recorded by clicking “keep”. The time was also noted.11.

The cuvette was taken out kept for 45 seconds, inserted in the colorimeter again and the stabilized value of absorbance and time was recorded.12. Step (11) was repeated for 20 minutes and the data was saved.13.

The solution was discarded and the data was analyzed.14. Three different plots  – 1. Absorbance vs t, 2.

ln(Absorbance) vs t and 3. 1/(Absorbance) vs t was made. The linear plot of the three was selected and a linear regression was done for that plot. Slope of the linear plot gave the rate constant of this reaction.

5. Results and Analysis:1. The reaction between crystal violet and sodium hydroxide was of first order with respect to the concentration of the crystal violet. This is because the ln(Absorbance) vs.

time plot was linear, while the other two plots Absorbance vs. time and 1/Absorbance vs time were not linear. The ln(Absorbance) vs. time plot is presented below (figure 1):2.

The value of rate constant k will be negative slope of the linear regression line in figure 1 (above). Therefore, value of the rate constant k is            0.0188 min-1 = 3.133×10-4 3.

The correct rate equation for rate of the reaction will berate =  = 1.88×10-2[CV+] min-1 =3.133×10-4[CV+] s-14. The half life of this reaction ist1/2 = 0.693/(3.133×10-4s-1) = 2.212x103s = 36.86 min   

Cite this Rate Law Determination of the Crystal Violet Reaction

Rate Law Determination of the Crystal Violet Reaction. (2017, Mar 16). Retrieved from https://graduateway.com/rate-law-determination-of-the-crystal-violet-reaction/

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