Page 105 - Elements of Chemical Reaction Engineering 3rd Edition

Sec. 3.1 Basic Definitions 77

Example 3-2 Describing a Reaction

Another nucleophilic aliphatic substitution is the reaction petween sodium hydrox-
ide and tert-butyl bromide (TBB):
CH3 CH3
I I
NaOH + CH,-C-CH, CH,-C-CH, + NaBr
I I
Br OH
State the reaction order with respect to each species as well as the overall reaction
1-
ordm-and generally describe this reaction.
Solution
Just because this reaction is similar to the previous nucleophilic aliphatic substitu-
tion, one should not jump to the conclusion that the rate law and kinetics will be
similar. The mte law is determined from experimental observation. It relates the
rate of reaction at a particular point to the species concentrations at that same
point. In this case if one consults an organic chemistry text,7 one will find that the
rate law is
/
-~TBB = ~CTBB (E3-2.1)

Using the definitions above, the reaction of sodium hydroxide with tert-butyl
bromide (TBB) can be described as an irreversible, homogeneous, liquid-phase
reaction which is first-order with respect to tert-butyl bromide, zero-order with
respect to sodium hydroxide, overall first-order, and nonelementary.

3.1.4 Reversible Reactions

.All rate laws for reversible reactions must reduce to the thermodynamic
relationship relating the reacting species concentrations at equilibrium. At lequi-
librium, the rate of reaction is identically zero for all species (i.e., -rA E 0).
That is, for the general reaction
aA+bB e (2- 1 )
cC+dD
the concentrations at equilibrium are related by the thermodynamic relation-
ship (see Appentdix C).

Thermodynamic
Equilibrium (3- 1 0)
Relationship
The units of K, are (m~l/drn~)~+~-~-~.
To illustrate how to write rate laws for reversible reactions we will use
the combination of two benzene molecules to form one molecule of hydrogen

Ibid.