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catalyst (thing)
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(
thing
)
by
SourKey
Sun Apr 21 2002 at 2:34:03
A catalyst is a substance which is used in
chemical reactions
to accelerate the rate of said reaction. The catalyst accomplishes this by changing the
Activation Energy
(E
A
) of the reaction. Contrary to
popular belief
, though, the catalyst does not do so without reacting itself. If you think about it
logically
, how could a substance which is supposedly completely
inert
and
unreactive
possibly affect a reaction at all? I said substance, so let us disregard
volume
,
pressure
, and
temperature
. That being said, logic dictates that only a
reactive substance
(or
reactant
) could affect a reaction. Since a catalyst affects the reaction, it is, in essence, a reactant. The catalyst does not change the E
A
of a reaction, it in fact changes one reaction into several reactions (by reacting itself with the
original reactants
), each of which has a lower E
A
than the original reaction. The largest of the E
A
s of the lower reactions is taken to represent the E
A
of the
overall reaction
. In the
intermediate reaction
s, the catalyst substance is reproduced completely.
Take for example the following reaction:
I
-
+ OCl
-
--> IO
-
+ Cl
-
Left to its own devices
, this reaction will take an
exceedingly long time
to occur due to its great E
A
. When the catalyst of water (H
2
O) is added, the reaction proceeds as follows:
1. OCl
-
+ H
2
O --> ClOH + OH
-
2. I
-
+ ClOH --> IOH + Cl
-
3. IOH + OH
-
--> IO
-
+ H
2
O
Now cross out all the intermediate substances (being those substances which are produced in one step and consumed in one of the succeeding steps) and you're left with the overall reaction:
I
-
+ OCl
-
+ H
2
O --> IO
-
+ Cl
-
+ H
2
O
The catalyst is unchanged, and the overall reaction is the same as that of the
original
, but with a lower E
A
due to the less energy
demanding
intermediate reactions.
Since the catalyst is, in fact, consumed and
regenerated
fully (or nearly enough) during the course of the reaction, it is plain to see why the common
misconception
of catalysts is that they remain unreacted and unconsumed, but this simply isn't so.
"A catalyst is not a substance which 'takes no part in a reaction' (as you may have heard previously); rather, the catalyst is an active participant which is regenerated in a later step of the
reaction mechanism
."
-- James A. Hebden, Ph. D.
Catalyst 5000
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