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Catalysts & Enzymes
Catalysts are substances that speed up the rate of reactions, and yet are not altered by the reaction
(i.e. they are not affected). They are very useful because only a small amount of catalyst is needed
for a reaction and it can be re-used again and again. Enzymes are a kind of catalyst that work inside
Catalysts work in one of two ways to increase the chance of molecules colliding (which is how new
substances are formed). The first is called `adsorption'. This is where molecules from the chemicals
in the reaction stick to the catalyst. This means it's more likely for the molecules to collide with each
other because they're so close. Once the molecules have collided, the new chemical formed no
longer sticks to the catalyst.
For example: The reaction of ethene and hydrogen with a nickel catalyst
CH=CH + H -- CHCH
In this example the ethene molecules are adsorbed onto the catalyst, nickel, and the double bond
between the hydrocarbons breaks (instead the electrons bind the molecules to the catalyst). The
hydrogen molecules are also bound to the surface of the catalyst and break down into single atoms.
One of the bonds that held the ethene to the nickel instead holds a hydrogen atom. At some point the
same will happen to the other bond and the final chemical will no longer be attached to the catalyst.
The second is called `intermediate compounds'. This is where the catalyst reacts with the original
chemicals and forms an intermediate compound. These compounds are either very unstable and
break down, producing the catalyst and the new chemicals, or they are reactive and react with the
Catalysts are divided into two kinds heterogeneous and homogeneous. Homogeneous reactions
are where both the reactants and the catalyst are in the same `phase' i.e. both are liquids or solids
or gases and you cannot see the boundary between the two. Heterogeneous reactions have
reactants and catalysts that are in different distinct phases e.g. one is solid and one liquid, and are
not a solution, or both are liquid but one is oily and the other not, meaning they separate. Most
heterogeneous catalysts work by adsorption.
Enzymes are effectively protein biological catalysts they speed up the rate of reactions in the body
so are very important in the digestive system. Enzymes are similar to other catalysts but are also
more easily `denatured' as they need the correct temperature, pH etc. to work properly. Another
difference between enzymes and normal catalysts is that enzymes can only be used for one reaction
i.e. each kind of enzyme has its own chemical reaction.
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There are two kinds of enzymes; intra-cellular (found inside cells) and extra-cellular (found outside
cells, for example in the stomach). There are enzymes in every single chemical reaction in living
organisms and most of them are made of proteins.
Enzymes also have a unique shape which goes with the shape of the substrate(s) they target,
meaning they need exact conditions to work (if the shape changes, for example from heavy metal
ions, they cannot fit their chemical).…read more