The Foundations of Chemistry

reaction is controlled only by the availability of reaction sites on the Pt surface, and not

by the total number of NO 2 molecules available.

Some other important reactions that are catalyzed by transition metals and their oxides

follow.

1.The Haber process for the production of ammonia (Section 17-7).

Fe, Fe oxides

N 2 
3H 2 8888888888n2NH 3

high T, P

2.The contact process for the production of sulfur trioxide in the manufacture of

sulfuric acid (Section 24-11).

V 2 O 5

2SO 2 
O 2 88888n 2SO 3

400°C

3.The chlorination of benzene (Section 27-16).

FeBr 3

C 6 H 6 
Cl 2 88888n C 6 H 5 Cl 
HCl

benzene chlorobenzene

4.The hydrogenation of unsaturated hydrocarbons (Section 27-17).

Pt

RCHUCH 2 
H 2 888nRCH 2 CH 3 Rorganic groups

Enzymes as Biological Catalysts

Enzymesare proteins that act as catalysts for specific biochemical reactions in living

systems. The reactants in enzyme-catalyzed reactions are called substrates.Thousands

of vital processes in our bodies are catalyzed by many distinct enzymes. For instance, the

enzyme carbonic anhydrase catalyzes the combination of CO 2 and water (the substrates),

facilitating most of the transport of carbon dioxide in the blood. This combination reac-

tion, ordinarily uselessly slow, proceeds rapidly in the presence of carbonic anhydrase; a

single molecule of this enzyme can promote the conversion of more than 1 million mole-

cules of carbon dioxide each second. Each enzyme is extremely specific, catalyzing only a

few closely related reactions—or, in many cases, only one particular reaction—for only

certain substrates. Modern theories of enzyme action attribute this to the requirement of

very specific matching of shapes (molecular geometries) for a particular substrate to bind

to a particular enzyme (Figure 16-19).

When heated, a sugar cube (sucrose,
melting point 185°C) melts but does
not burn. A sugar cube rubbed in
cigarette ash burns before it melts.
The cigarette ash contains trace
amounts of metal compounds that
catalyze the combustion of sugar.

696 CHAPTER 16: Chemical Kinetics

A space-filling model of the enzyme lysozyme. This enzyme catalyzes the hydrolysis of

polysaccharides (complex carbohydrates) found in bacterial cell walls, that is, it breaks the

link between two adjacent sugar units in the polysaccharide. The intact polysaccharide,

shown here in green, fits into the active site, which is a cleft in the surface of the lysozyme

molecule. The arrangement of hydrogen-bonding groups in the active site of the enzyme

surface matches that on the polysaccharide, but other molecules with different hydrogen-

bonding groups do not fit as well, so the enzyme exhibits very specific bonding for

polysaccharides. After the hydrolysis reaction has taken place, the product molecules do

not fit the active site as well, so they are released, making way for another polysaccharide to

be bound to the enzyme active site.