14 Determining Reaction Order
Background
Kinetics is the area of chemistry that deals with how quickly or how slowly reactions
take place. By studying the rate of a reaction, valuable information can be gained
about how the reaction proceeds (the reaction mechanism).
In general, the rate of a reaction depends on the concentration of the reactants and
can be expressed mathematically as the rate law.
The rate law for a chemical reaction is an equation that relates the rate of
disappearance of reactants to the rate of appearance of products for the reaction.
Generally, the rate law is expressed in terms of the concentrations of reactants and
products in the form of:
m
n
Rate = k  reactant 1   reactant 2  ...
where k = the rate constant and the exponents m and n are individual reaction orders.
The individual reaction orders (the exponents) in the above equation must be
determined by experiment because they are not based on the mole ratio in the
balanced equation.
The order of a reaction is often zeroth, one, or two. A zero-order reaction is one in
which the rate is independent of the concentration of reactants. A first order reaction
is one where doubling the concentration of one reactant will double the rate of the
reaction. A second order reaction can be one where the reaction depends on the
concentration of two different reactants, or where the concentration of one reactant
has a more powerful effect, so that doubling the concentration of that reactant would
quadruple the rate of the reaction.
A graph of concentration data versus time is one method that can be used to
determine the order of a reaction with respect to a particular reactant. If the reaction is
zero-order, a plot of Concentration versus Time will result in a straight line with the
slope = -k. If the reaction is first-order, a straight-line plot will result from a graph of
Natural Log of Concentration versus Time with the slope = -k. If the reaction is
second-order, a plot of 1 / Concentration versus Time will result in a straight line with
the slope = -k. The following table summarizes the way the graphical method will be
used in this experiment.
Order with Respect to Blue No. 1
0
[concentration] versus time
Plot that Gives a
Straight Line
1
ln [concentration] versus time
Slope Versus Rate Constant
2
1/ [concentration] versus time
This experiment studies the reaction of household bleach with the dye FD&C Blue No.
1, commonly used in many food products (blue food coloring). When FD&C Blue No.
1 reacts with the sodium hypochlorite in bleach, the blue color fades as colorless
products are produced:
NaOCl + Blue No. 1  Colorless product
And the rate can be expressed by:
x
Rate = k  NaOCL  Blue No. 1
y
In this experiment the amount of bleach used will be in excess to such a large extent
that its concentration will essentially remain constant. By doing this, the order of the
reaction with respect to the Blue No. 1 will be determined, and the new rate equation
will be:
Rate = k  Blue No. 1
y
In the above reaction a new “pseudo” equilibrium constant ( k ) is established
because it is the constant for just the Blue No. 1, and not the overall reaction.
Spectroscopic or colorimetric analysis allows the concentration of a reactant or
product to be measured: the greater the concentration of a colored substance in
solution, the more light the substance will absorb. So, in this experiment, the reaction
order with respect to the concentration of Blue No. 1 can be established by graphical
analysis.
Purpose
Analyze reaction rate by determining the order of the reaction with respect to the dye
FD&C Blue No. 1when it reacts with household bleach.
Materials
PASCO & Other Equipment
PASPORT Xplorer GLX
beaker, 50-mL
PASPORT Colorimeter
stirring rod
PASPORT Extension Cable
protective gear
graduated cylinders (2), 10-mL
Consumables
vial of the dye FD&C Blue No. 1 (blue food coloring), 3 drops
household bleach: sodium hypochlorite (NaOCl), 5.0 mL
water, distilled, 20.0 mL
Safety Precautions
Bleach will stain clothing and burn skin — use caution! Avoid breathing vapors.
•
Wear safety glasses and follow standard laboratory safety procedures.
Pre-Lab Questions
Will the rate of the reaction change (slow down or speed up) as the concentration decreases (as the bleach reacts with
it?
1)
Based on your answer to question #1 above, what do you predict the order of the
reaction will be with respect to the dye FD&C Blue No. 1?
2)
Why would knowing the rate constant and reaction orders be important to a
chemist?
Procedure
Equipment Setup
Calibrate the Colorimeter. Use the instructions found in the Sensor Information Sheets included in this manual.
1)
It is important when recording data to act quickly — so make sure solutions and
glassware are organized within reach.
Xplorer GLX Setup
Connect the PASPORT Extension Cable to Port #1 on the Xplorer GLX.
1) Connect the Colorimeter to the PASPORT Extension Cable.
2) Turn on the Xplorer GLX.
3) From the Home screen, open the Sensors screen (
). Use the arrow keys to
scroll through the list of measurements. Using the
key to toggle the Visible/Not
Visible field, set Orange (610 nm) Absorbance to visible and all other
measurements to Not visible.
4) From the Home screen open the Graph display (
Orange (610 nm) Absorbance is displayed.
). Make sure a graph of
Record Data
Measure 3.0 mL of distilled water into a 10-mL graduated cylinder. Add 3 drops of Blue Dye No. 1 and then fill to the
5-mL mark with more distilled water.
1) Using the second 10-mL graduated cylinder, measure 5.0 mL of sodium
hypochlorite (household bleach). Add distilled water to the 10-mL mark.
2) Quickly pour the two solutions into the 50-mL beaker and stir with the glass rod.
3) Fill a cuvette ¾ full with the mixed solutions and place the cuvette into the
Colorimeter and tightly close the cuvette cover.
4) Press
to begin data recording.
As the reaction proceeds, observe the remaining solution in the beaker and record your observations.
5) Record data for about 6 minutes. Then press
again to end data collection.
Analyze
Since absorbance is linearly proportional to concentration, plotting absorbance against
time will result in a graph that will demonstrate qualitative differences in order of a
reactant.
The Zero Order function is plotted as Absorbance versus Time.
Use the calculator to plot:
ln (Absorbance) versus Time
•
1/Absorbance versus Time
For each of the three graph displays, do the following:
Auto Scale the graph.
1)
In the Tools menu (
), select Linear Fit. A line which best approximates a
linear fit to the data will appear on the graph.
2) A box containing the values of the curve fit parameters will appear on the graph.
The value for m is the slope of the best-fit line. This value is related to k, the rate
constant. Determine which of the three graphs results in the most linear plot, and
refer to the Background section above to identify the order of reaction with respect
to Blue Dye No. 1.
3) Run the experiment as needed to verify results.
Data Table
Time
Observation
Analysis and Synthesis Questions
What was the reaction order with respect to the dye FD&C Blue No. 1?
1)
What is the value of the rate constant for the dye FD&C Blue No. 1?
2)
Based on your response to question #1 in the Analyze section above, how would
the observations you made in the Data section be different if 6 drops of the dye
FD&C Blue No. 1 were used instead of 3 drops?
3)
If the rate of the reaction with respect to the bleach was found to be 1, what would
the overall order of the reaction be?