Page 201 - Introduction to chemical reaction engineering and kinetics

8.2 Molecular Catalysis 183

Chlorine atoms catalyze the destruction of ozone in the following two-step cycle:

Cl. + 0, + oc1+ 0,
oc1+ 0s + Cl. + 20,

with the overall result:

203 + 30,

In this cycle, Cl’ is regenerated, and each Cl atom can destroy a large number of 0,
molecules in chain-like fashion.

8.2.2 Acid-Base Catalysis
In aqueous solution, the rates of many reactions depend on the hydrogen-ion (H+ or
HsO+) concentration and/or on the hydroxyl-ion (OH-) concentration. Such reactions
are examples of acid-base catalysis. An important example of this type of reaction is
esterification and its reverse, the hydrolysis of an ester.
If we use the Brijnsted concept of an acid as a proton donor and a base as a proton
acceptor, consideration of acid-base catalysis may be extended to solvents other than
water (e.g., NH,, CH,COOH, and SO,). An acid, on donating its proton, becomes its
conjugate base, and a base, on accepting a proton, becomes its conjugate acid:
acid + base = conjugate base + conjugate acid

For proton transfer between a monoprotic acid HA and a base B,

HA+B = A- +BH+ (8.2-1)

and for a diprotic acid,
H,A+B = HA-+BH+
HA- + B = A2- + BH+ (8.2-2)

In this connection, water, an amphoteric solvent, can act as an acid (monoprotic, with,
say, NH, as a base):
H,O + NH, = OH- + NH,’

or as a base (with, say, CHsCOOH as an acid):

CH,COOH + H,O = CH,COO- + H,O+

Acid-base catalysis can be considered in two categories: (1) specific acid-base catal-
ysis, and (2) general acid-base catalysis. We illustrate each of these in turn in the next
two sections, using aqueous systems as examples.

8.2.2.1 Specific Acid-Base Catalysis
In specific acid-base catalysis in aqueous systems, the observed rate constant, kobs, de-
pends on cu+ and/or on cOH-, but not on the concentrations of other acids or bases
present:

k obs = k, + kH+cH+ + koH-cOH- (8.2-3)