REACTIONS: THERMODYNAMIC ASPECTS, KINETICS, AND CATALYSIS - Biomass as a Sustainable Energy Source for the Future - page 151

can therefore readily move between oxidation states. Life itself gives good examples
of catalysis through the proteins called enzymes. Their three-dimensional
shape enables them to stabilize a temporary association between substrates, the
molecules that will undergo the reaction. By bringing two substrates together in
the correct orientation, an enzyme lowers the activation energy required for new
bonds to form.
Some examples of homogeneous catalysis are:

1. The oxidation of sulfur dioxide to sulfur trioxide in the gas phase, using NO
as a catalyst.

Without catalyst : 2SO 2 (g) + O 2 (g) 2SO 3 (g)

With NO, initially consumed and then re-formed :

!

2NO g ðÞ +O 2 g ðÞ! 2NO 2 g ðÞ

ðÞ
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2SO 2 g

2SO 2 g ðÞ + 2NO 2 g ðÞ! 2SO 3 g ðÞ + 2NO g

ðÞ +O 2 g ðÞ! 2SO 3 g ðÞ

2. Thermal decomposition in the gas phase of acetaldehyde catalyzed by iodine I 2
vapors.

Without catalyst :CH 3 CHO(g) CH 4 (g) + CO(g)

With I 2, initially consumed and then re-formed :

!

CH 3 CHO g ðÞ +I 2 g ðÞ! CH 3 Ig ðÞ +HI g ðÞ +CO g ð Þ

CH 3 Ig ðÞ

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CH 3 CHO g

ðÞ +HI g ðÞ! CH 4 g ðÞ +I 2 g

ðÞ! CH 4 g ðÞ +CO g ðÞ

3. If the catalytic process takes place in water, the catalytic effect is often due to
hydronium H 3 O + (acid catalysis) or to hydroxyl OH − (basic catalysis).
4. An example of how an enzyme works is the role played by carbonic anhydrase
that converts dissolved carbon dioxide (CO 2 ) into carbonic acid, which dissoci-
ates into bicarbonate and hydrogen ions:

HCO 3 − +H +

CO 2 +H 2 O ! H 2 CO 3 !

The carbonic anhydrase catalyzes this reaction through the deep cleft of its
active site. Deep within the cleft are three histidines all pointed at the same place
in the center of the cleft. Together, they hold a zinc ion (Zn ++ ) that will be the
cutting blade of the catalytic process. Immediately adjacent to the position of
the zinc atom in the cleft are a group of amino acids that recognize and bind
CO 2 . When the CO 2 binds to this site, it interacts with the Zn ++ in the cleft,
orienting in the plane of the cleft. Meanwhile, water bound to the zinc is rapidly
converted to hydroxide ion that is now positioned to attack the CO 2 . When it
does so, HCO 3 − is formed and the enzyme is unchanged (Figure 5.5).