Kinetics: Crystal Violet

Kinetics: Crystal Violet
LAD E.2 (pg 1 of 3)
Name___________________Per___
Concentration vs Time Analysis
Reaction kinetics is defined as the study of the rates of chemical reactions and their mechanisms. Reaction rate is simply defined
as a change in a measurable quantity divided by the change in time. In chemistry, the “measurable quantity” is usually molar
concentration or absorbance. Consider the generalized chemical reaction equation:
R→P
∆[R]
m
. The differential rate law is: Rate = k[R]
∆ time
The rate constant, k is dependent on temperature and the presence of a catalyst. Both the value of k and the order of the reaction,
m, must be experimentally determined, they can not be known from just looking at a reaction. In LAD E.1 we gathered
concentration and rate data to determine the order of the reaction, however in this experiment we will gather concentration and
time data, then use the integrated rate law and graphical analysis to determine the order of the reaction and the rate constant.
Further, if the reaction is run at different temperatures, graphic analysis will allow the calculation of the activation energy, Ea.
Symbolically the rate can be represented as: Rate =
The Reaction
In this experiment you will investigate the reaction of crystal violet (C25H30N3Cl). The structure of the molecule is shown below.
In the lab it will be reacted with sodium hydroxide. Crystal violet, in aqueous solution, is often used as an indicator in
biochemical testing. As its name suggests, it is a deep purple colored molecule, and the reaction with hydroxide is enough of a
change to convert the molecule to colorless. The reaction of this organic molecule with sodium hydroxide can be simplified by
abbreviating the chemical formula for crystal violet as CV.
CV+(aq) + OH−(aq) → CVOH(aq)
224
INVESTIGATION
As
the reaction11proceeds, the violet-colored CV+ reactant will slowly change to a colorless product, CVOH. The color change will
be measured by spectrophotometer set at the wavelength at which the CV exhibits maximum absorption. The exact wavelength at
which the reaction will be tested will be selected by running a scan of absorbance vs wavelength as shown below. As you learned
in LAD B.1, according to Beer’s Law, absorbance is directly proportional to the molar concentration of a colored chemical in
solution. We will use this Law and the changing absorbance over time to measure the changing concentration over time.
light
(2)
The Spectrophotometer
source
0.375
Any solution that is colored absorbs light at a particular wavelength of
Detector
meter
visible light. An instrument that is used to measure the absorbance of
(4)
sample
diffraction
(5)
light by a sample is called a spectrophotometer. A general schematic
(1)
grating
(3)
diagram for a spectrophotometer is shown to the right.
It begins (1) with a light source or a light bulb. For the instrument that is used in this lab, the light bulb emits visible light of
wavelengths ranging from 340 nm to 700 nm. The light then travels to a diffraction grating (2). This device as the name indicates,
separates the light into its individual wavelengths so that light of a single particular wavelength shines towards the sample. A
monochromator is a polished diffraction grating. Next the light of a particular wavelength and intensity, is passed through the
sample (3). As the light passes through the sample, part of the light may be absorbed. This absorption lowers the intensity of the
Figure
The visible spectrum
of a 25 μM CVintensity.
solution
light. The detector
(4)1. measures
this decreased
The spectrophotometer expresses the amount of light as related to the
concentration
of path
the length
colored
molecule
in theonly
solution
in one of two ways at the meter (5). The first way of measuring the
If we still keep the
fixed,
but now choose
one particular
concentration
of the
solution
thethereby
percent
transmittance,
wavelength of light
to pass
throughis
thecalled
solution,
fixing
the absorptivity this is a measure of how much of the light gets through. The
second
way
of quantifying
thehow
concentration
ofofthe
is called absorbance, which is a measure of the amount of light
constant,
students
can then observe
the absorbance
lightsolution
at that wavelength
changes as they change the concentration of CV. Under these conditions, Beer’s
absorbed.
law describes a straight-line relationship for a graph of absorbance versus solute
Beers
Law A = abc (on your formula sheet)
concentration whose slope is simply the product of the molar absorptivity constant
224
INVESTIGATION 11
and
path
• “a” islength.
molar absorptivity which is constant for any particular molecule
of length,
CV and sodium
(see Figure
dye’s
color
will cuvettes each time
• In the
“b”reaction
is path
whichhydroxide
is constant
since 2),
wetheuse
the
same
as it reacts with sodium hydroxide. A colorimeter (or spectrophotometer)
• fade
Thus
“A”
absorption
is
directly
proportional
to
“c”
concentration
will be used to follow the disappearance through time of CV by measuring the
absorbance of a solution of CV during its reaction with NaOH. The raw absorbance
measurements from the colorimeter (or spectrophotometer) can be transformed to
molar concentration of CV via the use of a Beer’s law calibration curve.
CH3
H3C
H
C
N
HC
CH HC
C
H
C
+
C
H
HC
CH
HC
CH
H3C
N
CH3
(purple)
CH3
CH3
H
C
N
H3C
CH3
N
HC
CH
+
OH–
H
C
CH HC
C
H
C
OH H
HC
N
CH3
CH
CH
HC
H3C
CH3
H
C
CH
N
CH3
(colorless)
Figure 2. Chemical structures in the reaction in this laboratory activity
adapted from AP® Guided-Inquiry Experiments Lab Manual
Figure 1. The visible spectrum of a 25 μM CV solution
If we still keep the path length fixed, but now choose only one particular
wavelength of light to pass through the solution, thereby fixing the absorptivity
constant, students can then observe how the absorbance of light at that wavelength
changes as they change the concentration of CV. Under these conditions, Beer’s
law describes a straight-line relationship for a graph of absorbance versus solute
LAD E.2 (pg 2 of 3) Kinetics Crystal Violet
Rate Laws
The differential rate law (most commonly just called the rate law) is shown below the reaction
CV+(aq) + OH−(aq) → CVOH(aq)
rate = k [CV+]x [OH−]y
0 order Rx
6
4
2
20
[reactant] vs time
a linear plot indicates zero order
ln[reactant] vs time
a linear plot indicates first order
1
vs time
[reactant]
2nd order Rx
8
It is easy enough to see the different orders of these three
hypothetical reactions all together on one graph, but it would
0
0
be difficult to determine the order of the reaction (0th, 1st, or
nd
2 ) when looking at the graph of just one particular reactant
for one particular reaction. Through the use of calculus it is
easier to determine the order of the reaction (0th, 1st, or 2nd) by
using the integrated rate laws and graphing the data three different ways:
and
1st order Rx
10
[Reactant]
Note that the differential rate law describes the rate of a
chemical reaction as a function of the concentration of the
reactants. In the rate law shown, k is the rate constant for the
reaction, x is the order with respect to crystal violet (CV+), and
y is the order with respect to the hydroxide ion. The “order” is
the exponential factor to which the concentration will affect
the rate. In this lab, instead of observing rates at various
concentrations as we observed in LAD E.1, in this experiment
we will observe the changing concentration over time as
shown in the graph of three different hypothetical reactants,
zero, first and second order.
40
60
80
100
Time
a linear plot indicates 2nd order
Whichever graph is most linear (use the R2 value to be sure) will indicate the corresponding order of the reaction.
Swamping − Isolation of One Reactant
It is difficult to observe the order of each individual reactants for a reaction
involving two different reactants. We can simplify this experiment by measuring
the reaction rate as a function of the changing concentration of only the crystal
violet, not for the OH−. To do this, we will use a “trick” called “swamping.” In this
technique, we will use a very high concentration of hydroxide compared to the
concentration of the crystal violet. Since the hydroxide ion concentration is so
much more (~4,000 times more) than the concentration of crystal violet, the [OH−]
will not change appreciably during course of the trial. This technique is often
referred to as “swamping” and is a method of isolating one of the reactants at a
time. Thus, you will find the order with respect to crystal violet (x), but not the
order with respect to hydroxide (y). Therefore, the rate constant you will determine
from the slope of the linear graph is only a pseudo rate constant.
Conc vs Time for CV + OH−
0.125 M
[OH−]
OH−
0.000025 M
[CV+]
CV+
time
Procedure Overview
Measurement of absorbance vs wavelength data to determine the wavelength for maximum absorbance for CV.
Collect absorbance vs time data at room temperature and then use graphical analysis as indicated in the process the data section to
determine the order of the reaction with respect to the crystal violet, from which the rate constant can be determined.
adapted from AP® Guided-Inquiry Experiments Lab Manual
LAD E.2 (pg 3 of 3) Kinetics Crystal Violet
Materials − up front for demonstration
•
•
•
0.10 M NaOH solution
0.000025 M crystal violet solution
2× plastic pipets
• (1 marked CV, 1 marked OH)
•
•
•
30 ml beaker
2× square plastic cuvettes
2× 100 ml tall cylinders and 1 stopper to fit
Procedure
A. Handle the cuvettes with care. DO NOT touch the smooth surface, hold the cuvette at the top between your fingers only
touching the ridged sides. Immediately following a trial, rinse your cuvette with tap water several times and then follow with
a rinse of deionized water.
B. Calibrate the spectrophotometer. Then run a test of just the CV solution. Measure Absorbance vs wavelength. From this scan,
the wavelength of maximum absorbance should be obvious. The scan is shown at the bottom of page 1 of this LAD sheet.
Use this to select the optimum wavelength for running the Absorbance vs Time collection of data. Set the spectrophotometer
to collect the maximum number of absorbance measurements per second.
C. Mix a pipet of CV+ and a pipet of OH− in the 30 ml beaker, swirl, and then quickly pour the mixture into the cuvette, then
quickly place the cuvette into the spectrophotometer. Absorbance vs time data will be recorded. An abbreviated version of
this data shown below is posted on a google sheet so that you will not need to type it in.
To save typing, this data
is available on the
shared Google sheet.
All on the same Google sheet, (in your Google LAD data shared document) construct three graphs of
You can cut and paste to
the data shown to the right to determine the order of the reaction with respect to CV+. (Make sure
your Google LAD
that you graph with a scatter plot NOT a line graph.)
document.
time
abs
a. absorbance vs time
0
0.573
b. ln absorbance vs time
10
0.514
20
0.473
1
c.
vs time
30
0.432
absorbance
40
0.391
2
50
0.343
A trend line for each graph along with the R value will determine which graph is linear, which allows
you to make a conclusion as to the order of the reaction with respect to crystal violet.
60
0.316
70
0.280
Once you determine which graph produces the most linear line, use the slope to determine the rate
80
0.251
constant. Label this on your graph, and report such conclusion on your cover sheet.
90
0.218
Be sure your graphs are properly labeled; titles, axes, etc.
100
0.190
110
0.164
120
0.152
130
0.136
To be turned in: (stapled in the following order)
140
0.121
150
0.110
1. Typed cover sheet:
160
0.098
• Name, LAD #, title,
170
0.088
• the reaction (Take the time to learn to make superscripts.),
180
0.081
Process the Data
2.
•
the purpose,
•
a brief summary of how/why the spectrophotometer worked as a method to collect concentration/time data,
•
conclusions from your graphs (Comment on how the R2 value helps with your conclusions.),
•
determination of the rate constant (with information labeled clearly on the appropriate graph as well)
One printed sheet:
•
3.
with all three graphs one single sheet,
(Be sure the graphs have correct descriptive titles, labeled axes, trend line, and the equation for trend line.)
This LAD protocol.
adapted from AP® Guided-Inquiry Experiments Lab Manual