What is the rate of the reaction between iodine and acetone?
The Question
In this laboratory, the rate law describing the reaction between iodine and
acetone in acidic solution will be determined experimentally using the initial rate
method. To completely determine the rate law, the order of the reaction with
respect to each reagent, and the rate constant at the reaction temperature must
be determined.
What is the rate of the reaction between iodine and acetone?
Introduction
In the study of reaction kinetics, the rate at which a chemical reaction occurs
depends on several factors:
(1)
(2)
(3)
(4)
The chemical properties of the reactants
The concentrations of the reactants
The reaction temperature
The presence (if any) of catalysts.
The rate of a reaction at a particular temperature is generally expressed in a
mathematical equation of the form:
Rate = k[A]m[B]n.
In the rate law above, k is the rate constant; [A] and [B] are the molar
concentrations of reagents A and B; and m and n are the orders of the reaction
with respect to reagents A and B. The effects of temperature and catalysts are
conveyed in the numerical value of the rate constant, k.
The reaction being studied is that between iodine and acetone (as shown below),
which occurs in aqueous solution in the presence of HCl:
Although H+ does not appear as a reagent in this reaction, experimental
observation shows that the reaction does not proceed unless H+ is present in the
initial mixture. Therefore, the order of the reaction will be determined with
respect to iodine, acetone and H+ concentrations.
Iodine has a characteristic brown color in aqueous solution. The rate of the
reaction can therefore be followed by measuring the time it takes for all the iodine
initially present to react- the time it takes for the solution to convert from colored
to clear and colorless. The experimental rate for each run is calculated as
Rate = [Iodine]/t.
The procedure here allows student groups to choose the specific concentrations
of reagents to be used. To get reliable results, it is imperative that the scientific
method be systematically employed. The scientific method assumes that
variables are controlled, and that only one variable at a time is changed.
Therefore, as the initial concentration of components is varied to determine the
rate law, only one of the reagent concentrations should be changed between any
pair of runs.
Use of a systematic approach will allow easy determination of the effect of a
specific change in concentration of a particular reactant on the rate. To simplify
the calculations, consider changing the concentration of a reactant from one trial
to the next by an integer factor (doubling, tripling, halving, etc.). This will make
the subsequent mathematical analysis a lot easier than changing a concentration
by a factor of 2.46, for example.
The Objectives for this experiment are to:
1) design an initial rates procedure applying the scientific method
2) determine the order of the reaction with respect to iodine, acetone and
HCl
3) calculate k, the rate constant, at lab temperature and
4) examine the effect of temperature on k.
What is the rate of the reaction between iodine and acetone?
Why Should I Care?
Chemical Kinetics plays an important role in every process known to man. As
shown in the abstract for this recent article from Geophysical Research letters,
Geophysicists use the methods of chemical kinetics to understand the interactions
between atmospheric and geological features on the earth.
GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L14815, doi:10.1029/2005GL023317, 2005
Initial uptake of ozone on Saharan dust at
atmospheric relative humidities
R. Y.-W. Chang
Department of Chemistry, University of Toronto, Toronto, Ontario, Canada
R. C. Sullivan
Department of Chemistry, University of Toronto, Toronto, Ontario, Canada
J. P. D. Abbatt
Department of Chemistry, University of Toronto, Toronto, Ontario, Canada
Abstract
To better evaluate heterogeneous loss mechanisms for ozone in the atmosphere, the
initial uptake kinetics of ozone on mineral dust surfaces was studied at room
temperature under dry conditions and at elevated relative humidity. Ozone was added
to a static absorption cell containing a glass tube coated with a thin film of Saharan
dust collected from silt deposits on the Cape Verde Islands. Ozone concentrations
were monitored by UV absorption at 254 nm, from which the reactive uptake
coefficient in the first 10 s was calculated. An increase in initial ozone concentration
from 2 × 1012 to 1 × 1014 cm−3 results in a decrease in the initial reactive uptake
coefficient from 6 × 10−6 to 2 × 10−7, respectively, revealing a negative dependence of
initial reactive uptake on initial ozone concentration. Simultaneously exposing the
Saharan dust to ozone and water vapor at 50% and 75% relative humidity does not
significantly change the initial reactive uptake coefficient.
Received 25 April 2005; accepted 20 June 2005; published 23 July 2005.
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Citation: Chang, R. Y.-W., R. C. Sullivan, and J. P. D. Abbatt (2005), Initial uptake of ozone on
Saharan dust at atmospheric relative humidities, Geophys. Res. Lett., 32, L14815,
doi:10.1029/2005GL023317.