Quimica-Cinetica

In the Laboratory

An Undergraduate Physical Chemistry Experiment on the
Analysis of First-Order Kinetic Data
M. R. K. Hemalatha and I. NoorBatcha*
Department of Chemistry, The American College, Madurai 625002, Tamilnadu, India

The analysis of first-order kinetic data to estimate the
kinetic parameters is a common exercise in undergraduate
physical chemistry laboratory (1). Severalmethods are
available to estimate the rate constant from first-order kinetic data using linear regression techniques. Conventional
methods (method I) require measurements at initial time
(t = 0) and completion times (t = ∞) in addition to the measurements at various time intervals. However, these measurements at t = 0 and t = ∞ are either difficult to make or
accompanied by large uncertainties.The Guggenheim
method (2) (method II) or a similar method proposed by
Kezdy et al. (3), Mangelsdorf (4), and Swinborne (5) (method
III) does not require measurements at t = 0 and t = ∞. The
latter methods are known as time-lag methods because they
involve grouping kinetic data separated by a fixed time interval, τ. Even though the time-lag methods are well known
in the literature (1–7), verylittle effort has been made to
introduce first-order kinetic experiments utilizing these
methods to the undergraduate students. As a result, students get the impression that initial and final measurements of a kinetic run are absolutely necessary and the approximations resulting from such measurements are unavoidable. Schwarz and Gelb (6, 7) have made use of a simulated data set to evaluatealternative approaches to the
analysis of first-order kinetic data. In the present work we
describe how time-lag methods can be easily implemented
in an undergraduate physical chemistry laboratory by following the kinetics of a simple chemical reaction. By using
this approach the students can be made to realize the extent of error caused by “infinite time” measurements and
how this can becircumvented.
We make use of the reaction

HITACHI 200-10 double beam UV-vis spectrophotometer is
employed. The kinetics is monitored by following the variation of absorbance at 350 nm, which is the wavelength of
maximum absorbance of I3 ions. All reagent solutions are
prepared in double distilled water. The following solutions are
prepared using standard procedures: [K2 S2 O8 ] = 0.001 M;
[KI] =0.1 M; [KCl] = 0.2 M. The pseudo-first-order condition is maintained by having the concentration of KI at least
20 times greater than that of K 2S2 O8 . Ionic strength is
maintained at 0.1 M using KCl solution. The absorbances
are recorded every 60 s for 40 min. The infinite time reading is taken after warming the reaction mixture for a further 20 min at 60 °C and then cooling it to roomtemperature. In the present work, the kinetics is studied by following the concentration of the product (I 3 ) of the reaction.
However, the methods discussed here can also be applied
to studies in which the pseudo-first-order kinetics is followed by measuring the concentration of the limiting reagent (S2O 82 in the present case).
Data Analysis
In terms of the absorbance of I3 , the integrated firstorder rate expression is
(A∞ – At ) = (A∞ – A0) e kt

In method I, a linear plot of ln (A∞ – At) versus t is made
and the pseudo-first-order rate constant is extracted from
the slope. At time equal to t + τ,
( A∞ – At + τ ) = (A∞ – A0) e k(t + τ)

Experimental Procedure
The kinetics of the persulphate–iodide reaction can be
studied by measuring the absorption of I 3 formed during
thecourse of this reaction (9). In the present work, a
*Corresponding author. Present address: Department of
Chemistry, Universiti Malaya, 59100 Kuala Lumpur, Malaysia.

972

(2)

Combining these two equations we get

S 2O 82 + 2I → 2 SO42 + I2
to compare different approaches to evaluating pseudo-firstorder rate constants. The kinetic behavior of this reaction
is well known (8) and the...