Hi, this is Tom Loschiavo, Chemistry Education Manager at PASCO scientific.
I'm here today to talk about kinetics.
Kinetics are an integral part of any chemistry curriculum,
from intro chemistry with factors that affect the rates of reaction,
to more advanced topics like rate laws and orders of reactants.
Now, when we do experiments or demonstrations with kinetics,
it's nice to have a system that you can study that will be continuously monitored.
In this case, we're going to be looking at a very simple system of blue dye and bleach.
We're going to use the PASCO Colorimeter and SPARKvue software.
Whenever we use the Colorimeter, we want to do a couple things.
We want to blank the Colorimeter,
and we want to select the appropriate wavelength for the colored solution that we're studying.
We're going to do that all from the home screen.
To blank the Colorimeter, I'm going to take a sample of the solvent, in this case, water.
I'm going to put it in the Colorimeter.
I'm going to push the green button.
A little LED light goes on, showing me that it's working.
When the LED light goes off, then I know that it's calibrated.
I can also look at the SPARKvue screen.
This is the great thing about the Colorimeter.
It displays all the transmittances and absorbances at once.
I see red, green, orange, and blue transmittance and the absorbance of those colors.
When it's calibrated, I should see 100% transmittance and 0 for the absorbance for each of those things.
That way I know that my system is blanked.
Now I want to pick the appropriate wavelength for my solution.
I'm using a blue solution. I took some of the blue and put it into the cuvette.
Then I make sure it's clean.
I'm going to put it into the Colorimeter. Close the lid.
What I want is the wavelength that has the highest absorbance.
If I look over here, it's not absorbing any blue, which makes sense, since it's a blue solution.
It's absorbing a little bit of green--very little.
Significant red and lots of orange.
So orange is going to be the color that I'm going to want to study for this particular reaction.
So now on to the kinetics part of this.
I want to monitor the reaction as it happens over time.
I'm going to build a graph of orange absorbance versus time,
and I'm going to hit OK.
Now I have the display that I want.
Now we just have to get my sample ready.
I'm going to take some bleach.
I'm going to add it to one of these test tubes.
These two test tubes both have the blue solution in it.
I'm going leave one alone so that I can see it as the standard,
and I'll have one with the bleach in it so I can monitor the reaction.
I put about 10 drops in.
I'll mix that up, making sure it's nice and mixed.
I'm going to put it into one of the cuvettes.
Make sure it's at least three-quarters full.
Wipe down the sides.
Put it into the Colorimeter,
and we'll hit Start.
We can autoscale this.
Since we have the two things here, we can monitor the reaction here,
and we can observe the absorbance using SPARKvue and the Colorimeter.
OK, so we let the reaction run for about two minutes.
The absorbance went from 0.46 down to about 0.26.
This is a good place to do some analysis.
I am going to stop the data collection.
For an introductory class this might be a good place to have an inquiry lab:
have the students design an experiment to make the reaction happen faster or slower,
changing concentrations or changing temperatures,
or even changing dyes to see if all dyes react in a similar fashion.
For a more advanced class, you want to do some more analysis on the data.
That's where the SPARKvue software comes in.
So I'm going to build a new page.
With kinetics there are three things that we typically look at:
we look at concentration versus time, natural log of concentration versus time,
and one over concentration versus time.
Using the Colorimeter, absorbance is proportional to concentration.
So we can use that as our measurement.
We can actually plot all of those things at once
to see which ones gives us the straightest line.
That will give us an indication of the order of the reactants in the reaction.
I am going to create a couple of data sets.
I am going to create a natural log of absorbance.
We'll do natural log of our orange absorbance because that's what we are measuring.
I'll do with the inverse of absorbance.
So orange absorbance.
And those are two calculations that I need.
So now I am going to take advantage of the multiple y-axis.
I'm going to plot absorbance. If that's straight it's a zero order reaction.
I'm going to plot natural log of absorbance. If that's straight it's a first order reaction.
And inverse absorbance. If that's straight it's a second order reaction.
All of them versus time on a multiple y-axis. I'll hit OK.
Each of them looks fairly straight,
but with this SPARKvue software we can figure out which one is the straightest,
which one has the best fit.
First I'll select my y1, which is absorbance, and I'll do a fit on there.
We're looking for straight lines, so we'll do a linear fit. And R, in this case, is 0.997.
With a root mean square error of 0.00454. It's pretty good.
If we look at the second, y2, that's the natural log.
We can do a linear fit on that one.
The R on this one is one. So very good data.
And the root mean square error is 0.00207. So a smaller squared error.
One for R, indicating a very, very good fit.
Just to be thorough, we'll check the last one.
We'll select y3.
Do the linear fit.
And R is 0.996, so not as good.
Definitely y2 is the straightest line,
indicating that the natural log gives us the best fit,
indicating a first order reaction.
Again, for advanced users, we have these tools available to do those kinds of calculations.
Then from the slope of the line, we can figure out k and do some more analysis of the reaction.
This very simple experiment, bleach and blue dye,
gives us lots of opportunities to study kinetics,
from the very basic to the more advanced, all using our SPARKvue tools and software.
Thank you very much.
This has been Tom Loschiavo.
If you have any questions or comments, feel free to reach me at chemistry@pasco.com.
Thank you.