Undergraduate kinetics experiment demonstrating unusual behavior

John M. Malln University of Missouri Columbia. 65201

An Undergraduate Kinetics Experiment Demonstrating Unusual Behavior of kOb,

Henrique E. Toma and Ernesto Giesbrecht lnstituto de Quimica Universidade de Sao Paulo Sao Paulo, Brazil

I

I n determining t h e rate law of a chemical reaction, a comm o n anoroach is to studv t h e behavior of k,~,,, a measured pseudo first-order rate cbnstant. For example, consider t h e mechanism h~ A+B+C (1)

..

k,

When IAl is -10-fold IB1 o r lareer. B is converted into C h v a process following pseudo first-order kinetics. T h e measured rate constant varies with IAl accordina to the relationshin . k,h. .~~= &[A] k,.' T h e spec&; rates of steo ~~~~~

.

~~~~~~~

~

region of rare muration and rends to decrmsr nr 1t.w values of Ihll'/'].'l'hicsicnifirsthc.incrmsed importa~~ccofk..at ihu IXIP,'] in determining the final position 01 equtlilrrium and nlsc,m rmernino the rate at which equilibrium is achieved. As an additional exercise. students may be enconmeed to verifv the limits to eqn. (2); i.e., that'k,b. = k l a t high [ ~ ~ z + r a k,h, nd; h - ~ a t law [MPz+].This can be done by investigating the dependence of k,e on [dmso] at high and low [MPzt]. Volume 54, Number 6, June 1977 / 385

Appendix

Synthesis of KMethylpyrazinium Iodide

Given the mechanism

Ten grams of pyrnnme is dissolved in -25 ml methyl iodide in a glass-stoppered tlnsk. After standing 3dn m the dark at roum teniperaturr. 23.': g tR5% y d d ,