Page 104 - Elements of Chemical Reaction Engineering 3rd Edition

76 Rate Laws and Stoichiometry Chap. 3

reaction is sometimes encountered, and it involves the mechanism or molecular
path of the reaction. This definition is discussed in Chapter 7.'
In the study of reaction orders and kinetic mechanisms, reference is
sometimes made to the molecularity of a reaction. The molecularity is the
number of atoms, ions, or molecules involved (colliding) in the rate-limiting
step of the reaction. The terms unimolecular, bimolecular, and termolecular
refer to reactions involving, respectively, one, two, or three atoms ,'(or mole-
cules) interacting or colliding in any one reaction step.
The most common example of a unimolecular reaction is radioactive
decay, such as the spontaneous emission of an alpha particle from uranium 238
to give thorium and helium:

92U238 --+ 90Th234 + ,He4
If the hydrogen-iodine and the nitric oxide oxidation reactions did indeed
result simply from the collision of the molecular species named in the overall
stoichiometric equations, they would be bimolecular and termolecular reactions,
respectively. There is considerable doubt, though, about whether this actually
occurs.
The reaction between methyl bromide and sodium hydroxide is classified
as a nucleophilic aliphatic substitution:
NaOH + CH, Br ____) CH30H + NaBr
This irreversible reaction has an elementary rate law and is carried out in aque-
ous ethanol. Therefore, like almost all liquid-phase reactions, the density
remains almost constant throughout the reaction. It is a general principle that
for most liquid-phase reactions, the volume V for a batch reaction system and
V (batch) and u
(flow) are constant the volumetric flow rate u for a continuous-flow system will not change apprk-
for most liquids ciably during the course of a chemical reaction.
We want to write the rate of disappearance of methyl bromide, - rMB , in
terms of the appropriate concentrations. Because this reaction is elementary
the reaction orders agree with the stoichiometric coefficients.
lNaOH + 1CH3Br --+ 1CH30H + lNaF3r
01 = 1, first-order with respect to sodium hydroxide
p = 1, first-order with respect to methyl bromide (MB)

-rMB = kCNaOHCCHjBr
Overall, this reaction is second-order.

Stnctly SpeaEung + Strictly speaking, elementary reactions involve only single steps such as one iodide
molecule colliding and reacting with one hydrogen molecule. However, most reactions
involve multiple steps and pathways. For many of these reactions, the powers in the
rate laws surprisingly agree with the stoichiometric coefficients. Consequently, to facil-
itate describing this class of reactions, reactions where the rate law powers and sto-
ichiometric coefficients are identical may also be referred to as elementary reactions.
R. T. Momson and R. N. Boyd, Organic Chemistry, 4th ed. (Needham Heights,
Mass.: Allyn and Bacon, 1983).