Experiment 22-Crystal Violet LabQuest

Experiment 22
Rate Law Determination of the Crystal Violet Reaction
In this experiment, you will first determine the optimum wavelength absorbed by crystal violet in
the Spectrovis Vernier Sensor and then you will observe the reaction between crystal violet and
sodium hydroxide. The objective is to study the relationship between concentration of crystal violet
and the hydroxide with respect to the time elapsed during the reaction. You will also determine the
Ea for the reaction by changing temperatures at which the reaction takes place, finding the rate
constant for each temperature and then graphing the rate constant as a function of temperature. The
equation for the reaction is shown here:
N(CH 3) 2
N(CH 3) 2
OH
+
C
–
N(CH 3) 2 + OH
C
N(CH3) 2
2
N(CH3) 2
N(CH3) 2
Figure 1
A simplified (and less intimidating!) version of the equation is:
CV+ + OH–(aq) 
CVOH

(crystal violet) (hydroxide)
The rate law for this reaction is in the form: rate = k[CV+]m[OH–]n, where k is the rate constant for
the reaction, m is the order with respect to crystal violet (CV+), and n is the order with respect to the
hydroxide ion. During the procedures in Part 1, the concentration of crystal violet will be more than
5000 times larger than the hydroxide ion concentration. This will mean during the procedures in
Part 1 the [OH–] will not change appreciably during Part 1. Thus, you will find the order with
respect to crystal violet (m), in what is called pseudo rate reaction and then find the order with
respect to hydroxide ion (n).
As the reaction proceeds, a violet-colored reactant, CV+, will be slowly changing to a colorless
product, CVOH. Using the Spectrovis Vernier Sensor you will monitor the absorbance of the crystal
violet solution with time. We will assume that absorbance is proportional to the concentration of
crystal violet (Beer’s law). Absorbance will be used in place of concentration in plotting the
following three graphs:

Absorbance vs. time:
A linear plot indicates a zero order reaction (k = –slope).

ln Absorbance vs. time:
A linear plot indicates a first order reaction (k = –slope).
 1/Absorbance vs. time:
A linear plot indicates a second order reaction (k = slope).
Once the order with respect to crystal violet has been determined, you will also be finding the rate
constant, k, and the half-life for this reaction. Determine the activation energy, Ea for the reaction
between crystal violet and sodium hydroxide.
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Experiment 22
MATERIALS
Equipment/Materials
Spectrovis sensor and LabQuest#2
10 ml pipette
stop watch
thermometer
stirring rod
cuvettes
50 mL beaker (2)
100-mL beaker
0.10 M NaOH
2.5 X 10–5 M crystal violet solution (This must solution must be made from a stock
solution)
ice
600 or 1000 mL beaker
PROCEDURE (PART I)
1.
Obtain and wear goggle.
1.
Calibrate the Spectrovis and then use the sensor to determine the order with respects to the
crystal violet. Analyze the data graphically on the LabQuest2 to decide if the reaction is
zero, first, or second order with respect to the hydroxide ion. Print a copy of each graph.
2.
Make sure you print out all three graphs and then use these graphs to support the order of
crystal violet with respects to this reaction. You should also determine AND RECORD the
temperature of the reactants in this part
PROCEDURE (PART II)
2.
Repeat Part I, except use 10.0 mL of 0.050 M NaOH (this solution will need to be made
from the 0.10 M NaOH used in part 1). Make sure you use the same cuvette used in part I.
3.
YOU DO NOT NEED TO RECORD THE TEMPERATURE!
4.
Use the sensor to determine the order with respects to the hydroxide. Analyze the data
graphically on the LabQuest2 to decide if the reaction is zero, first, or second order with
respect to the hydroxide ion. Print a copy of each graph.
PROCEDURE (PART III)
1.
Repeat Part I at two temperature lower than room temperature and two temperatures higher
than room temperature, but no trials should be lower than 10C and none higher than 40C.
2.
Analyze the data graphically on the LabQuest to determine the rate constant for each
temperature trial, including the trail from Part I. Print a copy of each graph.
3.
Then determine Ea by plotting a graph of the ln of k vs. T (in K)-1on an Excel Spreadsheet.
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Rate Law Determination of the Crystal Violet Reaction
Lab 22: Rate Law Determination of the Crystal Violet Reaction
Name: _____________________________
Item
Points/Out of
/1
/1
/1
/5
/1
Name & Partner
Date Experiment Started
Title of Experiment
Appropriate Purpose
Appropriate Procedure
Graphs
Graphs of Part 1
Graphs of Part 2
Graphs of Part 3 (various temp)
Graphs of changing 1/temp vs. ln k
Calculations
[CV+] order determination (from Graph)
[OH-] order determination
Ea
Summary
Summary of Results (Order for @ reactant, Ea)
Signature
/12
/12
/12
/12
/3
/5
/6
TOTAL
/8
/1
/80
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