ganisms to smaller ones, with the release of energy. Respiration is an example of a catabolic series of
reactions. See metabolism. Compare
anabolism.
catalysis The process of changing
the rate of a chemical reaction by use
of a *catalyst.
catalyst A substance that increases
the rate of a chemical reaction without itself undergoing any permanent
chemical change. Catalysts that have
the same phase as the reactants are
homogeneous catalysts (e.g. *enzymes in biochemical reactions or
transition-metal complexes used in
the liquid phase for catalysing organic reactions). Those that have a
different phase are hetereogeneous
catalysts (e.g. metals or oxides used
in many industrial gas reactions). The
catalyst provides an alternative pathway by which the reaction can proceed, in which the activation energy
is lower. It thus increases the rate at
which the reaction comes to equilibrium, although it does not alter the
position of the equilibrium. The catalyst itself takes part in the reaction
and consequently may undergo physical change (e.g. conversion into powder). In certain circumstances, very
small quantities of catalyst can speed
up reactions. Most catalysts are also
highly speciÜc in the type of reaction
they catalyse, particularly enzymes
in biochemical reactions. Generally,
the term is used for a substance that
increases reaction rate (a positive catalyst). Some reactions can be slowed
down by negative catalysts (see inhibition).
catalytic converter A device used
in the exhaust systems of motor vehicles to reduce atmospheric pollution.
The three main pollutants produced
by petrol engines are: unburnt hydrocarbons, carbon monoxide produced
by incomplete combustion of hydrocarbons, and nitrogen oxides produced by nitrogen in the air reacting
with oxygen at high engine temperatures. Hydrocarbons and carbon
monoxide can be controlled by a
higher combustion temperature and
a weaker mixture. However, the
higher temperature and greater availability of oxygen arising from these
measures encourage formation of
nitrogen oxides. The use of threeway catalytic converters solves this
problem by using platinum and
palladium catalysts to oxidize the
hydrocarbons and the CO and
rhodium catalysts to reduce the nitrogen oxides back to nitrogen. These
three-way catalysts require that the
air–fuel ratio is strictly stochiometric.
Some catalytic converters promote
oxidation reactions only, leaving the
nitrogen oxides unchanged. Threeway converters can reduce hydrocarbons and CO emissions by some 85%,
at the same time reducing nitrogen
oxides by 62%.
catalytic cracking See cracking.
catalytic rich gas process See crg
process.
cataphoresis See electrophoresis.
catechol See 1,2-dihydroxybenzene.
catecholamine Any of a class of
amines that possess a catechol
(C 6 H 4 (OH) 2 ) ring. Including
*dopamine, *adrenaline, and *noradrenaline, they function as neurotransmitters and/or hormones.
catenane A type of compound consisting of two or more large rings
that are interlocked like the links of
a chain. In a catenane, there is no
chemical bonding between the rings;
the rings are held together by *mechanical bonding.
catenation 1. The formation of
107
catenation
c
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reactions. See metabolism. Compare
anabolism.
catalysis The process of changing
the rate of a chemical reaction by use
of a *catalyst.
catalyst A substance that increases
the rate of a chemical reaction without itself undergoing any permanent
chemical change. Catalysts that have
the same phase as the reactants are
homogeneous catalysts (e.g. *enzymes in biochemical reactions or
transition-metal complexes used in
the liquid phase for catalysing organic reactions). Those that have a
different phase are hetereogeneous
catalysts (e.g. metals or oxides used
in many industrial gas reactions). The
catalyst provides an alternative pathway by which the reaction can proceed, in which the activation energy
is lower. It thus increases the rate at
which the reaction comes to equilibrium, although it does not alter the
position of the equilibrium. The catalyst itself takes part in the reaction
and consequently may undergo physical change (e.g. conversion into powder). In certain circumstances, very
small quantities of catalyst can speed
up reactions. Most catalysts are also
highly speciÜc in the type of reaction
they catalyse, particularly enzymes
in biochemical reactions. Generally,
the term is used for a substance that
increases reaction rate (a positive catalyst). Some reactions can be slowed
down by negative catalysts (see inhibition).
catalytic converter A device used
in the exhaust systems of motor vehicles to reduce atmospheric pollution.
The three main pollutants produced
by petrol engines are: unburnt hydrocarbons, carbon monoxide produced
by incomplete combustion of hydrocarbons, and nitrogen oxides produced by nitrogen in the air reacting
with oxygen at high engine temperatures. Hydrocarbons and carbon
monoxide can be controlled by a
higher combustion temperature and
a weaker mixture. However, the
higher temperature and greater availability of oxygen arising from these
measures encourage formation of
nitrogen oxides. The use of threeway catalytic converters solves this
problem by using platinum and
palladium catalysts to oxidize the
hydrocarbons and the CO and
rhodium catalysts to reduce the nitrogen oxides back to nitrogen. These
three-way catalysts require that the
air–fuel ratio is strictly stochiometric.
Some catalytic converters promote
oxidation reactions only, leaving the
nitrogen oxides unchanged. Threeway converters can reduce hydrocarbons and CO emissions by some 85%,
at the same time reducing nitrogen
oxides by 62%.
catalytic cracking See cracking.
catalytic rich gas process See crg
process.
cataphoresis See electrophoresis.
catechol See 1,2-dihydroxybenzene.
catecholamine Any of a class of
amines that possess a catechol
(C 6 H 4 (OH) 2 ) ring. Including
*dopamine, *adrenaline, and *noradrenaline, they function as neurotransmitters and/or hormones.
catenane A type of compound consisting of two or more large rings
that are interlocked like the links of
a chain. In a catenane, there is no
chemical bonding between the rings;
the rings are held together by *mechanical bonding.
catenation 1. The formation of
107
catenation
c
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