Reaction rates can be determined experimentally by measuring the
concentration of a reactant and/or product over time.
LEARNING OBJECTIVE [ edit ]
Design experiments that use either chemical or physical properties to measure reaction rates
KEY POINTS [ edit ]
Physical measurements can be performed on a system while it is reacting. These measurements
have the advantage of not disturbing the system, and they can be taken during the reaction.
Examples of these measurements are changes involume, temperature, and absorption of
a solution.
Chemical methods of measurement yield concentrationsdirectly. A small sample is extracted from
the reacting mixture, and the reaction is halted by dilution, cooling the mixture long enough to
measure concentrations, or adding another reagent to stop the reaction.
For zero-order reactions, graph concentration vs. time to get a line with the slope ­k. For firstorder reactions, graph thelogarithm of concentration vs. time to get a line with the slope ­k.
For second-order reactions, graph the reciprocal of concentration vs. time to get a line of slope ­k.
TERMS [ edit ]
Beer's law
Relates the attenuation of light to the properties of the material through which the light is
traveling.
absorbance
A logarithmic measure of the amount of light that is absorbed when passing through a substance.
Give us feedback on this content: FULL TEXT [edit ]
Experimental Methods for Measuring Reactions
In order to experimentally
determine reaction rates, we need to
measure
Register for FREE to stop seeing ads
the concentrations of reactants and/or products over the course of a chemical reaction. If we
know the order of the reaction, we can plot the data and apply our integrated rate laws. For
example, if the reaction is first-order, a plot of ln[A] versus t will yield a straight line with a
slope of -k.
There are two main ways to measure the concentrations of reactions: by measuring the
changes in an observable physical property, or by taking samples of the reaction solution and
measuring concentration directly.
Physical Measurements
Physical measurements can be performed on the system while it is reacting. These
measurements have the advantage of not disturbing the reacting system, and they can usually
be measured quickly. For example, if the total number of moles ofgas changes during a gas
reaction, the course of the reaction can be measured by monitoring the change in pressure at
a constantvolume. Other physical measurements include optical methods, such as measuring
the change in light polarization, the change in the refractive index of the solution, or quite
commonly, the change in the color of the solution, and therefore the absorptionspectrum.
Common electrical methods include changes in the conductivity of a solution, the electrical
potential in a cell, andmass spectrometry. Other methods include thermal
conductivity, viscosity, heats of reaction, and freezing points.
Chemical Measurements
Chemical methods yield concentrations directly. A small sample is extracted from the
reacting mixture, and the reaction is halted either by dilution, by cooling the mixture, or by
adding another chemical reagent to stop the reaction. One disadvantage of this method is
that removing a part of the reacting system, or adding an additional reagent to it, gradually
changes it over time. Also, there is a time lag between when the sample is taken and when the
reaction is measured, which makes the measurement less accurate.
Using Beer's Law to Measure Concentration Over Time
One oxidized form of buckminsterfullerene (C60) is C60O3. When a solution of C60O3 is
warmed, the compounded decomposes, releasing O2 and creating C60O. The reaction is given
by:
C 60 O 3
→
O 2 + C 60 O
The rate of this reaction can be monitored by measuring theabsorbance of the solution. The
absorbance is given by Beer's law:
A = abC
In this equation, a is the absorptivity of a given molecules in solution, which is a constant
that is dependent upon the physical properties of the molecule in question, b is the path
length that travels through the solution, and C is the concentration of the solution. We're
interested in determiningC.
By running the reaction of interest inside a spectrometer, the absorbance of the solution can
be measured over time. The data can then be plotted.
Oxidized buckminsterfullerene absorbance
The absorbance is proportional to the concentration of the C60O3 in solution, so observing the
absorbance as a function of time is essentially the same as observing the concentration as a function of
time.
By Beer's law, the absorbance of the solution is directly proportional to the concentration of
the C60O3 in solution, so observing the absorbance as a function of time is essentially the
same as observing the concentration as a function of time. In this case, the rate law is given
by:
rate = k[C 60 O 3 ]
Therefore, a plot of the rate versus the absorbance will yield a straight line with a slope of k.
Reaction rate vs. absorbance
The absorbance is directly proportional to the concentration, so this is simply a plot of the rate law, rate
= k[C60O3], and the slope of the line is the rate constant, k.
Plotting Integrated Rate Laws to Determine Reaction Rate
As discussed in a previous concept, plots derived from the integrated rate laws for various
reaction orders can be used to determine the rate constant k. Recall that for zero-order
reactions, a graph of [A] versus time will be a straight line with slope equal to -k. For firstorder reactions, a graph of ln[A] versus time yields a straight line with a slope of -k, while for
asecond-order reaction, a plot of 1/[A] versus t yields a straight line with a slope of k.