Kinetics by the Initial Rates Method
Rate laws are mathematical expressions that describe the relationship between reactant concentrations and the rates of reaction, taking the form,
Rate = k[reactant 1]m[reactant 2] n[reactant 3]p… [M/s] (1)
The proportionality constant k is the rate constant; any given reaction has a specific value of k for a given set of conditions, such as temperature, pressure, and solvent; the rate constant, in contrast to the rate of reaction, does not depend on the concentration of the reactants. Exponents m, n, p, …, are the reaction orders, and indicate the degree to which the reaction rate depends on the concentration of the associated reactant. ...view middle of the document...
In this experiment, the way in which the reaction rate at the beginning of the reaction (initial rate) depends on initial reactant concentrations was considered. This is the method of initial rates, and avoids problems of intermediate products building up, or interfering with the reaction. Thus, for any run in this experiment,
Initial Rate = -Δ[I2]/Δt = -([I2]final – [I2]initial)/(timefinal-timeinitial) (5)
Assuming that the final concentration of I2 is zero, because the concentration of I2 that was added was orders of magnitude less than the concentration of both acetone and hydronium ion, and given that the initial concentration of I2 was zero, and start time was zero, Eq. 5 becomes,
Initial Rate = [I2]initial / timefinal (6)
Using the measured initial rate, the reaction orders (with respect to each of the three reactants) may be determined by setting up proportions comparing the different initial rates obtained when the concentration of only one of reactants (the one in question) is changed. For example, the reaction order with respect to acetone may be found, comparing the rate data for the ith and jth runs, as follows,
Ratei = k[CH3COCH3]im[H3O+]i n[I2]ip = [CH3COCH3]im →
Ratej k[CH3COCH3]jm[H3O+]j n[I2]ip [CH3COCH3]jm
ln(Ratei/Ratej)/ln ([CH3COCH3]im/ [CH3COCH3]jm) = m (7)
Where m is the reaction order with respect to acetone, Eq. 3.
The rate constant k can then be experimentally determined using the overall rate law, Eq. 3, and the initial rate from any of the experimental runs,
k = Ratej/[CH3COCH3]jm[H3O+]j n[I2]jp (8)
A comparison with the literature value of k (measured at 25°C) would indicate the precision of the experiment. Knowing the rate law, finally, can suggest the reaction mechanism.
The spectrophotometer was turned on and set up as described in the manual.2 Seven spectrophotometer vials were cleaned using DI water and labeled (on the top to avoid interference with the spectrophotometer measurements). The first was filled with DI water and placed in the spectrophotometer where it was measured as a blank. The wavelength for the spectrophotometer was set to 470 nm because solutions with iodine absorb well in the visible spectrum (400-700 nm), and finally set to measure absorbance every 15 seconds.
Three 50 mL beakers were filled with about 30 mL of each of the stock reagents: 4.0 M acetone, 1.0 M HCl, and 0.005 M I2. Two burets (instead of one) were set up. The first was cleaned with DI water, and conditioned with HCl, and filled with approximately 15 mL of HCl; then the remaining six vials were filled with the volume specified in the manual.2 The second buret was cleaned with DI water, conditioned with the 4.0 M acetone, filled with approximately 15 mL of acetone; then acetone in the volume specified was added to each of the six vials. The (first) buret was cleaned again, conditioned, and filled with DI water; then DI water in the volume specified...