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Catalysts
A catalyst is a substance that speeds up a chemical reaction without being _______________.
Catalysts appear to work in two different ways. They either stabilize the
________________________ state and as a result lower the activation energy of an
elementary step, but otherwise leave the reaction mechanism unchanged. Alternatively they
catalyst can participate in the formation of a new reaction ________________________,
thereby providing an alternate pathway that has a ________________ activation energy. In
both cases the lower activation energy results in a _______________ reaction. Which of the
two catalyzed reactions below is providing an alternate mechanism?
Catalysts are often divided into two general categories: __________________________ and
__________________________.
a) Homogeneous Catalysts
A homogeneous catalyst is in the ______________ phase as the reactants. These
catalysts tend to interact with reactants in multistep sequences to provide an alternate,
___________________ activation energy mechanism. Examples include:
1) Acid and base catalysis in solution. In acid-base catalysis, a reactant either
_____________ or ______________a proton which increases the rate of the reaction.
For example in the preparation of an ester.
Many digestive enzymes (biological catalysts) which catalyze the hydrolysis of proteins,
fats, and carbohydrates in foods also use acid/base catalysis where the acids and bases
come from specific _______________ _____________ side chains.
2) NO catalyzing the production of _______________ in the lower atmosphere. The NO
that escapes from ___________________ reacts with oxygen in the lower atmosphere
to produce NO2. The NO2 then absorbs _______________________, decomposes, and
forms _________________. This O then combines with ______ to form O3
(____________). Ozone is a powerful oxidizing agent that reacts with other substances
to form chemicals that irritate eyes and lungs.
3) NO catalyzing the _____________________ of the ozone in the upper atmosphere.
This is environmentally detrimental because ozone is needed in the upper atmosphere to
absorb _______________.
b) Heterogeneous Catalysts
A heterogeneous catalyst is in a _____________________ phase than the
___________________. Usually the catalyst is a ________________ and the reactants
are ______________ or _______________. Heterogeneous catalysts are used widely in
industrial processes and are often ______________________ ________________ or
their _______________________. These catalysts lower the activation energy by
providing a ______________ for the reaction and a better ___________________ of
one reactant so that the other reactant can collide with ______________________
____________________.
For example:
1) Haber Process –uses finely divided ____________ to catalyze the synthesis of
________________
2) Hydrogenation of unsaturated fats in oils to form saturated fats in shortening-uses
________, ________, or ________
3) Catalytic Converters-convert exhaust gases like _________ and __________ to
_________ and _________ using transition metal oxides, _________ and
__________.
Enzymes have features of both homogeneous and heterogeneous catalysis. Like heterogeneous
catalysis the enzyme provides a _______________________ on which a reactant is
temporarily immobilized in a favorable position for reaction. Like homogeneous catalysis, the
amino acid R groups in the enzyme interact with the reactant (substrate) in multistep sequences
to provide an alternative lower _________________________ __________________
pathway.
Silberberg: Rd p720-722 do Q p732 #16.76, 16.77, 16.78, 16.79