Reaction kinetics 

 

background [prelab assignment (25 points) due at the beginning of the lab]

The following reaction involving crystal violet and sodium hydroxide (expressed as its net ionic equation):

C25H30N3+  +  OH-  -->  C25H30N3OH

kinetics chem eqn

will be investigated in this laboratory activity. The rate law of the reaction is 

rate = k [Cv+] α [OH-] β                                                                    [equation 1].

When [OH-] >> [Cv+], equation 1 becomes

rate = k* [Cv+] α, where k* = k [OH-] β

which is a psudeo-rate law of the reaction. The value of β and k may be obtained by varying the [OH-].

This laboratory activity is based on investigation 11 in your AP chemistry lab manual; thus, it provides some relevant background for the following prelab questions and this laboratory activity.

1. how could you use a colorimeter to measure the [reactant] as a function of time ? include its basis / rationale.

2. how could you use graphical analysis of the preceding data (do not use the rate of a reaction) to determine the value of the parameters

a. α [hint: see youtube video in below resource section]

b. β and k [hint: see below methods section]

in equation 1 (shown above) ? 

3. what should be the duration of data collection period (as a percent of the initial concentration of the dye) ? include its basis / rationale [hint: use simulation in the below resource section].

4. what is the basis / rationale of the (above) psudeo-rate law ? what is the experimental condition for the psudeo-rate law to be valid ? [hint: use simulation in the below resource section]

review (i) use of the colorimeter / wavelength selection / calibration curve from your earlier Beer's law lab, (ii) how to solve solution preparation problems ! also, the syntax / protocol in using MS Excel to generate the calibration curve and to transforn your (i) absorbance to concentration, (ii) concentration to natural log of the concentration, and (iii) concentration to reciprocal concentration. as appropriate, include sketches / screen shots to clarify your response and use in-text citation.

purpose

materials

colorimeter, cuvette, data collection device, watch (provide your own)

crystal violet (Cv+; ~ 25 µM);  NaOH (~ 125 mM);  water; 100 mL volumetric flask

pipet;  candle, electronic balance

methods [delegate work]

crystal violet reaction

part 1: prepare crystal violet and sodium hydroxide solutions; share solutions among 2 groups at a table. while the sodium hydroxide solution could be made in a single step, recommend preparing the crystal violet solution in two steps. as such, you should be familar in doing these calculations prior to the lab.

part 2:  gather data to generate calibration curve for crystal violet

part 3:  gather data to determine the reaction's rate law

quickly mix solutions of Cv+, NaOH, and water into the cuvette, then put it into the colorimeter

trail 1:  3 mL Cv+, 2 mL NaOH, 0 mL water

trail 2:  3 mL Cv+, 1 mL NaOH, 1 mL water  [ignore effect of solution's ionic strength]

start timer upon mixing the chemicals.   record absorbance at 30 seconds intervals for 30 minutes (or absorbance < 20% initial absorbance; whichever is shorter).  

burning a candle

 student design

 

Content of lab report  [35 points]

graphical analysis  [20 points]

calculation of rate law, i.e. parameters in the appropriate rate law; refer to preceding graphs    [15 points]

 

resources:

CV lab trouble shooting article

lab activity source (examples): site 1  & site 2 [access 4 / 2009]

youtube video:  data analysis hint [access 1 / 2010]

simulations: *.cdf (requires cdf player plugin) or *.nb (requires mathematica):  guided inquiry into determining (i) wavelength to measure absorption (ii) relative [CV] and [NaOH] for valid use of psudeo rate law approximation, and (iii) duration of data collection.

graphical analysis:  use of linearized data (for nerds:  potential issue) & R2