Reaction Rates for a Homogeneously Catalyzed Reaction
Howard Nechamkin, Elhannan Keller, and Jerome Goodkin Trenton State College Trenton. New Jersey 08625
T h e teaching of kinetics on t h e freshman level has become eeneral and sonhisticated durine recent vears. Such concents as t h e rate eq;ation a n d reaction order Hre included in mbst modern texts. I n addition, t h e role of t h e catalyst has been modernized t o reflect i t a s a reactant rather t h a n as a simple accelerator of t h e reaction. Many freshman laboratory experiments t h a t deal with kinetics neglect the influence of t h e catalyst on t h e rate equation. T h i s experiment is a n example of a homogeneously catalvzed reaction t h a t can easily he performed in t h e laboratory without t h e use of any s o p h & i c ~ e dequipment and is within t h e area of competency of t h e college freshman.
The Experiment Reaction of KMnO&with hydrogen in acid medium proceeds according to the equation Mn04- + Hz HsOt + Mn2+ (1) (purple) (colorless) Hydrogen can be generated simply by in situ reaction of a strong acid with an active metal; i.e. sulfuric acid and zinc. The reaction proceeds slowly under normal circumstances but is considerably accelerated by the homogeneous catalyst, nitrate ion? according to the equation
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The general rate equation R = k [A]"'[B]" . . . expresses the speed of the reaction in moles per second as a function of the concentrations of the reacting species A and B. The term ( k ) is the rate constant which is independent of concentration and varies only with temperature. For the permanganate reduction, we write Rate = k [MnOa-I '"[NOa-I" [ P ~ , l p (3) where the concentration of hydrogen is expressed as its pressure. It is normal to assume that the pressure of hydrogen in an open system is 1atm; therefore this term can be neglected to give the equation Rate = k [MnO4-] "'[NOC]" (4) This exoeriment will determine the order. n. for the nitrate ion: the reaction is known to he first-order with respect to the concentration . ~ will use the decolorization of of permanganate and h y d r ~ g e n We permanganate ion as an endpoint indicator. Equation (4) can be rewritten as Table 1. Sample Composltlon
h o p c M KNOS
Drops H 2 0
1 2
2 4
3
8
4 5 6
10 12
10 8 6 4 2 0
Tube # -.~
8
Table 2. Data
-1ogT Tube # Test-
Drop NOs-
NO3-
T(s)
(Rate)
log[NO.-]
-1.5
-2
-I
IO~[NO
