Chemical Kinetics of hydrolysis of crystal violet Introduction: In this

Chemical Kinetics of hydrolysis of crystal violet
Introduction:
In this experiment, the kinetics of the reaction between crystal violet and sodium hydroxide will be
studied. The change in concentration is monitored by the means of changing the color intensity of
the crystal violet (CV+) solution, where the UV-Visible spectrometer will be used to monitor the
crystal violet concentration as a function of time.
All of the reactants and products, shown in scheme 1, are colorless except for crystal violet which
has an intense violet color. Thus, during the course of the reaction, the reaction mixture color
becomes less and less intense, ultimately becoming colorless when all of the crystal violet has been
hydrolyzed and consumed.
The crystal violet color is due to the highly conjugated π-system which extends over all three
benzene rings and the central carbon atom. Upon the reaction with hydroxide ion, The OH group is
attached to the central carbon atom which breaks the conjugation and hence the material is no
longer colored.
Scheme 1
The rate of the crystal violet with NaOH reaction is given by the following generalized rate law:
𝑅𝑅𝑅𝑅 = 𝑘 [𝑂𝑂 − ]𝑥 [𝐶𝐶 + ]𝑦
(1)
where k is the rate constant for the reaction, 𝐶𝐶 + is an abbreviation for crystal violet (C25H30N3), x
is the reaction order with respect to 𝑂𝑂 − , and y is the reaction order with respect to 𝐶𝐶 + . The
values of x and y will be determined experimentally. Possible x values are 1 or 2 (first order or
second order). Possible y values are also 1 or 2. In this experiment, the initial [𝑂𝑂 − ] is made much
greater than the initial [𝐶𝐶 + ]. Thus, the [𝑂𝑂 − ] change, during the time that the 𝐶𝐶 + is consumed, is
negligible. For this reason, [𝑂𝑂 − ]x can be treated as a constant and Equation 1 can be rewritten as
follows,
𝑅𝑅𝑅𝑅 = 𝑘 ′ [𝐶𝐶 + ]𝑦
(2)
where 𝑘 ′ = 𝑘 [𝑂𝑂 − ]𝑥 . 𝑘 ′ is termed as pseudo or apparent rate constant.
The integrated form of the pseudo rate law (Equation 2) depends on the reaction order with respect
to 𝐶𝐶 + . The integrated rate laws for y = 1 and 2 are given in Equations 3 and 4. Compare each with
the general form of a linear equation, y = mx + b.
𝑙𝑙[𝐶𝐶 + ]𝑡 = 𝑙𝑙[𝐶𝐶 + ]0 − 𝑘 ′ 𝑡
1
[𝐶𝐶 + ]𝑡
1
= [𝐶𝐶 +] + 𝑘 ′ 𝑡
0
(3)
(4)
In Equations 3 and 4, [𝐶𝐶 + ]0 is the concentration of crystal violet in the reaction mixture at time
zero, before any reaction has occurred; [𝐶𝐶 + ]t is the concentration at any time t during the course of
the reaction. If a plot of ln [𝐶𝐶 + ]t versus time is linear, y = 1 and the reaction is first order in 𝐶𝐶 + .
Similarly, a linear plot of 1/[𝐶𝐶 + ]t versus time indicates a second order reaction in 𝐶𝐶 + . Only one
of these plots will be linear. For the one that is linear, the resulting straight line slope (its absolute
value) equals the pseudo rate constant, 𝑘 ′ .
In order to do the graphing just described, we need to have data showing how the 𝐶𝐶 +
concentration changes with time. This data will be obtained using the spectrometer by measuring
absorbance of λmax of 𝐶𝐶 + at different times. Crystal violet solutions obey Beer’s law. Thus, the
relationship between the observed current and the 𝐶𝐶 + concentration is given by:
𝐴𝑡 = 𝑙𝑙𝑙
𝐼0
𝐼
= 𝜖 𝑏 [𝐶𝐶 + ]
(5)
In Equation 5, At is the reaction solution absorbance at any time t, I0 is the blank photocell current
(observed for pure water); It is the current observed for the 𝐶𝐶 + /𝑂𝑂 − reaction mixture at time t, 𝜖 is
the 𝐶𝐶 + molar absorptivity (5.0×104 L.cm-1.mole-1), b is the cell path length (1.00 cm) and [𝐶𝐶 + ] is
the molar concentration of the crystal violet at time t. Thus, Beer’s law can be used to calculate
[𝐶𝐶 + ]𝑡 from each photocell current reading (It) during
Ultimately, The measured absorbance of the crystal violet can be translated into concentration of
crystal violet using a calibration curve constructed be taking the absorbance of five standard crystal
violet solutions.
Experimental:
Glassware: Five 100 mL volumetric flasks.
Two 150 mL E. flask with grounded quick fit stoppers.
10 mL graduated cylinder or pipet.
100 or 50 mL graduated cylinder
Two or more cuvettes
Equipment: Spectrophotometer.
Thermostatic water bath set at temperature higher that the room temp by at least 5 oC.
Solutions: 7.5×10-5 M crystal violet solution (the provided solution may have concentration other
than that but still close to it )
0.10 M sodium hydroxide solution
Procedure
1. Using the 10 mL graduated cylinder (or the pipet) measure out carefully 10 ml of the dye
solution and 40 ml of distilled water into a clean and dry flask.
2. In the same way dilute 5 ml of sodium hydroxide to 50 ml and place it in the second flask.
3. Prepare the spectrophotometer by setting it at 590 nm (ask you lab instructor how to do that).
4. Fill two clean cuvettes with distilled water, place them in the sample and blank cuvette holders
in the spectrophotometer and set the reading to zero.
5. Start the reaction by pouring the sodium hydroxide into the dye (and not the reverse, why?). at
the same time start a stopwatch. Mix the dye solutions well by swirling the flask.
6. Draw a sample of the reaction mixture using a dropper or pipet, pour it in the sample cuvette,
and then place the cuvette in the sample holder. Obtain the absorbance and the time passed
simultaneously.
7. Reject the sample. Wash the cell with water and then rinse twice with ethanol to remove any
dye adsorbed on the cell walls. Dry the cell in an air stream.
8. In a similar manner continue to take absorbance measurements on samples of the reaction
mixture a intervals of about 5 minutes for a total of 45 minutes. Note the corresponding
readings of the clock. Before each absorbance reading check the setting of the instrument.
After each reading reject the sample and clean the cell as before.
9. Repeat steps 1-8 for a new reaction mixture prepared at the water bath temp.
Calibration curve
10. Prepare the following solutions 1.5×10-6 , 3.0×10-6 , 4.5×10-6 , 6.0×10-6 and 7.5×10-6 M for the
crystal violet calibration curve by diluting 2.00, 4.00, 6.00, 8.00 and10 mL of the stock crystal
violet solution into 100 mL volumetric flasks.
11. Measure the absorbance for each solution.
Calculations:
Chemical kinetics is the study of the change in conc. with time. Therefore the Abs measurement
must be converted into conc. using the calibration curve. Plot Abs. vs conc. measurements obtained
from the five standard crystal violet solutions. Do least squares to obtain the best line that fits the
data points and obtain the linear equation for that line. Use this equation to convert the Abs into
conc for all abs measurements obtained from the kinetics part.
In order to determine the reaction order with respect to crystal violet (reaction order with respect to
sodium hydroxide is zero. WHY?). The student must examine the plots of ln[CV+] vs. time and
1/[CV+] vs. time. If the reaction is first order the ln[CV+] vs. time would be linear and the other is
not. In the opposite, if the reaction is second order the 1/[CV+] vs. time would be linear and the
other is not. From the graph with the linear relation obtain the slope and calculate the rate constant.
Finally, two experiments at two different temperatures will give two rate constants by which the
activation energy and Arrhenius constant can be evaluated by the relation:
𝑙𝑙
𝑘1 −𝐸𝑎 1
1
=
� − �
𝑘2
𝑅 𝑇1 𝑇2
Once Ea is calculated one can use Arrhenius equation to calculate Arrhenius constant as
𝑘1 = 𝐴𝑒 −𝐸𝑎 ⁄𝑅𝑇1
Your report must include
-
Signed data sheet
-
Table shows Abs. and t values
-
Calibration curve
-
Table shows Abs. and the corresponding concentration
-
Two graphs: ln[CV+] vs time and 1/[CV+] vs time
-
Predicted reaction order
-
Calculated rate constants
-
Calculated activation energy
-
Calculated Arrhenius constant
-
Error analysis
Chemical Kinetics of hydrolysis of crystal violet
Data sheet
Name :
Partner’s name:
Lab section:
Absorbance vs time measurements.
o
Room temperature=
Time (t)
in seconds
(±
)
C
Water bath temperature=
Time (t)
in seconds
(±
)
Absorbance
(±
)
Calibration curve.
Absorbance
(±
)
Conc.
Ambient temperature:
Ambient pressure:
o
C
torr
Instructor’s signature:
Date:
o
C
Absorbance
(±
)