CRYSTAL VIOLET LABInvestigation 11

Prior to lab review:

Serial dilution

Safety

CV will stain skin and clothes

NaOH is a corrosive skin irritant

Wear goggles and aprons Clean stained glassware

with isopropyl alcohol.

Prelab 1

A) using wavelength of 540 nm should result in an absorbance high enough to provide an acceptable range of absorbance values during the reaction of CV and NaOH while ensuring it is not too high to interfere with reliability of the instrument.

1bYour xs should be at the absorbance values given by the equation A=(abs from Fig 1 at chosen wavelength)*(diluted conc of CV)/(stock conc of CV)

CV in micro M Predicted absorbance

5 .26

10 .52

15 .78

20 1.04

25 1.30

2

Use dilution equation M1V1=M2V2 M1=25 microM V1 is unknown M2 is each desired final M specified in

question V2 is 10ml

CV in microM Volume of CV Volume of Water added

5 2 8

10 4 6

15 6 4

20 8 2

25 10 0

3

Absorbance should decrease because the amount of a reactant like CV decreases over time so more light passes through cuvette.

Note that by first calibrating / zeroing instrument with blank cuvette the instrument reads just the absorbance due to whatever was not in the blank which in this case is the CV

4a

In 1:1 ration nothing is left of either and goes to completion

In 1:1000 ration CV is limiting so none left.

Assume 1 mol of CV initial

CV NaOH

I 1 1000

c -1 -1

e 0 999

4b

When the concentration of one chemical is several orders of magnitude higher than another its concentration hardly changes.

To within few sig figs we can assume it is constant

So [NaOH]z is practically the same at the beginning as at the end of rxn and can be bundled in with k as new constant k*

5. Graph 3 plots to test 0, 1 or 2 order

Fill in the following table

Order with respect to CV

Integrated rate law

Plot that is linear

slope Half life

6

For a total of 10mL combined volume the following combination works well: 6mL 25microM CV with 4.00mL of 0.200M Na OH and blank cuvette of 6mL of distilled water combined with 4 mL of NaOH

Skip 7

8a

Fig 3 shows three plots each for different chem rxn. Looking at fig 3 one can see that the reaction that is 0 order (blue) produces a straight line for a graph concentration vs time. The graphs for a different rxn which is first order (red) and one for second( yellow) appear to relatively linear until concentration dips from starting value of 10M to roughly 1 or 2M

8b

For quite a while all there follow linear relationship. You have to collect data for a sufficient amount of time. So don't collect data for a set period of time. Instead collect until the value of the absorbance of CV and its reaction with NaOH has dropped to roughly 10%-20% of initial value. At that point only 1 graph is linear of those three different chemical reactions.

Simulation on Integrated Rate Law

http://www.chm.davidson.edu/vce/kinetics/integratedratelaws.html

Procedure

Typical cuvette ranges from 4mL to 6mL You can make 10Ml of solution and

pouring it into cuvette

For the first reaction of CV with NaOH a blank could be made from 6.00 mL distilled water + 4.00 mL 0.2M NaOH. This has everything but CV to zero the absorbance reading

Then to perform actual rxn of CV and NaOH combine 6mL 25.0micro M CV with 4 mL of 0.200 M NaOH

Calibration curve sample data for CV

Concentration in Micro M Predicted absorbance

12.5 0.742

10 0.639

7.5 0.508

5 0.332

2.5 0.159

Extension activity-write this after your lab conclusion

Design the experiment to determine the kinetics of candle burning. Some ideas that would influence the kinetics are: a) Temp of airb) Different brands or types of waxc) Humidity of the aird) Shape of one type of candle possibly

modified via whittling with kids scissorse) Length of the wick exposedf) Still air vs wind blowing (eg. Fan turned on a

low setting)