Lab, Spring week 2
In this experiment you will investigate the rate of reaction of household bleach (sodium hypochlorite, NaOCl) and one or more organic dyes used as food colorants (see figure 1).
The color of these dyes results from the large extent of delocalization of π electrons. Sodium hypochlorite is a relatively strong oxidizing agent, and the bleaching reaction typically is one of oxidative cleavage of a double bond, breaking the delocalization and hence the molecule’s ability to absorb light in the visible region, resulting in a colorless solution. You will determine the rate of this reaction based on the equation:
as well as the rate constant k and the order of the reaction with respect to each of the reactants (x and y). You will monitor the reaction by measuring the absorbance of the dye spectroscopically, which is one of the most common techniques used in kinetic studies. Absorbance is related to the concentration by Beer’s law:
where A is absorbance, ε
is the molar
absorptivity (a constant characteristic of the molecule), b is the path length
the light travels through the absorber (in this case, this is the width of the
cuvette, or 1 cm) and c is the concentration.
As the reaction progresses, the amount of colored dye decreases as it is
converted to its colorless counterpart, and so does the absorbance at a
particular wavelength, λmax, which is chosen as the
wavelength at maximum absorbtion.
Theory:
The rate of a chemical reaction usually decreases over time, largely because the number of reactant molecules decreases as the reaction proceeds. Since the rate itself is not constant, it’s not a particularly useful parameter for characterizing a reaction – you can’t measure it at a particular time and tell whether the reaction is fast or slow. The problem of rate variation can be solved by linearizing the data, which can be done by using the integrated rate laws and plotting the appropriate variables (where [A]0 is the initial concentration and [A]t is the concentration after time t):
0th
order:
1st
order:
2nd
order:
For example, for a first order reaction, if we plot ln[A] vs time, we get a straight line with a slope of –k. The rate constant, k, is a good parameter by which the speed of the reaction can be compared to that of other first order reactions.
When more than one reactant is involved in the reaction, linearization of the concentration/time data can be more complicated. One way to deal with this is to add all reactants except one in excess; even if the reaction goes to completion, fractions of the excess reactants that disappear will be so small that their concentrations remain virtually constant throughout the reaction. For example, for a reaction that is first order with respect to [A]:
if we add B in excess, its concentration is essentially constant, and so can be included in a new constant which we’ll call kobs, the pseudo first order rate constant. We can linearize the data by plotting ln[A] vs time, but this time the slope of the resulting line is –kobs which equals k[B]y. In the case of our reactions, we can add bleach in excess, since we can more easily measure the change of [dye] anyways.
We can find the actual k if we know y, the order of the reaction with respect to bleach. You can do this by measuring the slope of the ln[dye] vs time graph obtained at different concentrations of bleach. Say, for example, you dilute the concentration of bleach by half. If the reaction is zeroth order with respect to bleach, the rate, and so the value of kobs, won’t change. If it is first order, the rate and kobs will decrease by a half, and if it is second order kobs will be one-fourth.
Materials:
Procedure:
o Make
sure the fiber optic cable is screwed into the sample holder. This is what measures the light transmitted
by your sample. Turn on the light
source using the toggle switch on the back.
Remove any cuvets that may be in the sample holder.
o Click
on the OOI icon. Once the spectrum
screen appears, you should automatically be in ‘Spectroscope’ mode (shown by
the blue S on the toolbar). If the
spectrum shown is off the chart (just a red line at maximum intensity), adjust
the ‘Integration Time’ on the toolbar to its lowest value (3 msec). You should see what looks like a
spectrum. You want the top of the
spectrum to be near the top, but not off the scale. You can adjust this by changing the ‘Integration Time’ but
if it’s already off the scale at the minimum integration time it’s ok.
o Fill
a cuvet with DI water and place in the sample holder. Since water is the solvent in this reaction, this will act as
your reference cell. Click on the
bright light bulb on the toolbar (store reference current). Turn off the light source using the toggle
switch, then click on the dark light bulb (store dark current). When you turn the light source back on, you
will be able to switch to absorbance mode (blue A on toolbar). Now you can take an absorbance spectrum.
o Place a cuvet containing your sample in the sample holder. Make sure the outside of the cuvet is clean and dry before inserting in the holder! It’s good practice to wipe the cuvet with a Kimwipe every time you put in a new sample. Make sure the transparent side is facing the light source.
o Since we’re measuring absorbance in the visible range, you can change the spectral window using the ‘Set Scale’ function to set the wavelength scale (x-axis) to read 300-750 nm. You want an ideal initial absorption of 1; once you’ve achieved this you can change the vertical scale using the same function to reflect this.
o To minimze the variance (‘noise’) in the spectrum, you can increase the ‘Signal Averaging Control’. This averages the indicated number of successive integrated measurements. The downside of using a higher number is it slows the response time to a small extent. A value of 5-10 is reasonable. Also set the ‘Boxcar’ control to 10 (this is the pictal averaging) and make sure the ‘Correct for Electrical Dark’ is checked.
o You can identify peaks using the cursor. The absorbance and wavelength at the point of the cursor appear at the bottom of the screen.
o You can print you data directly, or save your data (though not necessary for this lab) as a .csv file which can be opened using Excel. (To plot with Excel, highlight data columns and choose ‘X-Y Scatter’ as chart type.)
o You can mix the reactants in a beaker, then fill a cuvet and measure the absorbance, then return the solution to the beaker and remix, etc, recording the time from the start of the reaction each absorbance measurement was taken. The advantage of this is you can stir the solution, but it can be rather tedious, and there’s more time between data points.
o You could, on the other hand, choose volumes whose total will easily fit in the cuvet, then run the reaction directly in the spectrometer, recording the absorbance at periodic time intervals.
If you plot ln k vs 1/T, the slope of the line is –Ea/R.
Calculations and report considerations:
This allows you to calculate the instantaneous rate as long as you know the concentrations of dye and bleach!