All about catalysts

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All about catalysts
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Catalysts are substances that increase the rate
of a chemical reaction by reducing the activation
energy, but which is left unchanged by the
reaction. (MODIFIED from URL) This definition
allows for the possibility that small amounts of
the catalyst are lost in the reaction or that the
catalytic activity is slowly lost. However, the
catalyst affects only the rate of the reaction,
it changes neither the thermodynamics of the
reaction nor the equilibrium composition
http//www.lboro.ac.uk/departments/cg/Projects/200
2/dhonsi/Theory.html
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Why are catalysts important?
The principal purpose of catalysis is to increase
the rate of the chemical reactions. It is
important to understand the mechanism by which
the reaction takes place. Catalysis is of
crucial importance for the chemical industry the
number of catalysts applied in industry is very
large and catalysts come in many different forms,
from heterogeneous catalysts in the form of
porous solids and homogeneous catalysts dissolved
in the liquid reaction mixture to biological
catalysts in the form of enzymes. A reaction
catalysed by a heterogeneous catalyst can be
represented by a flow chart
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Making and breaking bonds
absorb adsorb?
Reactant molecules are adsorbed at active sites
onto the surface of the catalyst. This involves
the formation of weak bonds between reactant
molecules and the catalyst which causes other
bonds in the reactant molecule to be stretched
and weakened. The weakened structure is
converted to another complex that is essentially
the product attached to the catalyst. Finally,
this complex breaks down to release the product
molecule which moves away to leave the catalyst
surface ready to interact with another reactant
molecule
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Mechanisms Langmuir-Hinshelwood-Hougen-Watson
(LHHW) mechanism 2 ltreference.htmlgt
(A) Two molecules adsorb onto the surface. (B)
They diffuse across the surface and interact when
they are close. (C) A molecule is formed which
desorbs.
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Eley-Rideal (E-R) mechanism
A) Molecules adsorbs onto the surface. (B)
Another atom passes by which interacts with the
one on the surface. (C) A molecule is formed
which desorbs.
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http//www.docbrown.info/page03/ASA2rates.htm
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http//www.saskschools.ca/curr_content/chem30_05/g
raphics/2_graphics/catalyst1.gif
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http//www.agen.ufl.edu/chyn/age2062/lect/lect_03
/3_43.gif
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Intermediate Compound theory
Catalyst C
A B -------------gt D slow reaction
C
A B -------------gt D ?
A C -------------gt AC intermediate produced
quickly
AC B-------------gt D C produced quickly
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The characteristic double humped profile for an
intermediate compound theory. Notice again the
lower EA
AB
AC
D
modified image!
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http//waynesword.palomar.edu/images/enzyme5.gif
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The active site on the enzyme attaches to a
substrate molecule (such as a disaccharide)
forming an enzyme-substrate complex. While
attached to the substrate, the enzyme causes a
weakening of certain chemical bonds in the
substrate molecule, resulting in a breakdown
(hydrolysis) of the substrate into two smaller
product molecules (such as two monosaccharides).
The enzyme is unaltered during the reaction and
is free to catalyze the breakdown of another
substrate molecule. If the active site on the
enzyme is blocked by a poison molecule, this
vital hydrolysis reaction cannot occur. See the
next section
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Active Site Of Enzyme Blocked By Poison Molecule
In this illustration a poison molecule has bonded
to the active site of an enzyme. Now the enzyme
is unable to recognize the substrate molecule
that it normally conjugates with because its
active site has been blocked. In locoweed
poisoning of livestock, the alkaloid swainsonine
blocks a key enzyme called mannosidase. When the
active site of mannosidase is blocked, it cannot
catalyze the breakdown of the sugar mannose,
resulting in a lethal accumulation of mannose in
vacuoles of cells of the central nervous system
(brain and spinal cord). The actual vacuoles are
swollen organelles called lysosomes where the
enzymatic breakdown process normally occurs. The
afflicted animal becomes paralyzed and eventually
dies. A similar scenario occurs in infants
suffering from the storage disease mannosidosis.
In this disease, the vital enzyme mannosidase is
lacking due to a mutant recessive gene. At the
present time there is no cure for this disease.
It is passed on via heterozygous carriers, and
shows up with a 25 percent probability when two
heterozygous individuals have a child. One of the
better known storage diseases is Tay Sachs
Disease, in which nerve cells fill up with a
lipid called ganglioside or GM2 because they lack
the vital enzyme HEX A needed to break down GM2
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http//fajerpc.magnet.fsu.edu/Education/2010/Lectu
res/17_Enzymes_files/image012.jpg