Page 88 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:36 9in x 6in b985-ch07 Elementary Physical Chemistry

Chemical Kinetics 73

“equilibrium constant”, defined as

‡
K = [activated complex]/[reactants] (7.27)

Moreover, it is assumed that the rate of formation of the product is
proportional to the concentration of the activated complex. Hence,

‡
‡
Rate = αK [reactants] = αK [A][B] (7.28)
where α is the proportionality symbol and A and B are reactants.
Comparing this rate of formation of the product with the actual
formation rate = k[A][B]shows that

‡
‡
‡
k = αK → αe −∆G /RT = αe ∆S /RT −∆H /RT (7.29)
‡
e
This has the form of the Arrhenius equation and will be equal to it if
we set
˜
‡
‡ ∆S /R = P a A.
∆H = E a and αe
More precisely (not derived here),
‡
‡
e
k =(RT/Nh)e ∆S /R −∆H /RT (7.30)
in which,
‡
‡ ˜ ∆S /R
∆H = E a and P A A = RT/Nhe
where N is the number of molecules and h is Planck’s constant.

7.11. Rate Law Mechanisms
This section deals with explanations of reaction kinetics in terms of reaction
mechanisms. Before discussing the reasons, it is important to distinguish
between the concept of order and the concept of molecularity.
• Order is an empirical quantity obtained experimentally.
• Molecularity indicates how many molecules come together to react in
an elementary reaction.