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A catalyst is a substance
which alters the rate of a chemical reaction but is chemically unchanged at the end of the
reaction.
Why not memorise this definition?
"..... but is chemically unchanged at the end of the
reaction." This means that there is just as much catalyst at the end of a
reaction as there was at the beginning. The catalyst is used over and over again. Because
catalysts work so rapidly and are used again and again, it is only necessary to have very
small quantities of catalyst present to make a chemical reaction go faster.
You might have a "catalytic converter" in the exhaust
pipe of your car. If the catalyst was Platinum you might expect silly people to start
stealing car exhaust pipes; but there is so little catalyst there that it would not be
worthwhile for them. A little bit of catalyst goes a long way! What does the catalytic
converter do? Well without it the fumes from your car would cause too much pollution and
the car might fail its MOT.
Perhaps you don't think that catalysts
are very important.
"..... alters the rate of a chemical reaction
....." This means that catalysts make chemical reactions go faster. I am
still looking for one which will make you do your homework faster, and another which will
make me mark it faster. What about chemical reactions. Some of them go very slowly, your
chemistry experiment might take hours, days, weeks, or ever years. Imagine if your
chemistry teacher asked you to find out what gas is released from Hydrogen Peroxide: you
might have to sit there watching your test tube for weeks; your chemistry teacher would
keep on asking why you had not finished your work. Eventually you would have enough gas to
test; so weeks later you would say "Oh, it is Oxygen Miss." If you had put a
little pinch of Manganese Dioxide into the test tube, the gas would be produced in a few
minutes. So, you would be able to go long before the end of the lesson. Even better, you
would still have the Manganese Dioxide catalyst which you would be able to sell back to
your teacher to use with another class.
How about the chemical industry. Well they will make much more
money if they can make their products quickly. The manufacturers of Nitric Acid use
Platinum as a catalyst. Even though this is a very expensive metal, it does not cost too
much to use it because they are only using small amounts of it.
"A catalyst is a substance ....." This
means that it is some kind of chemical substance! It could be a pure element; e.g.
Platinum, Nickel; or it could be a pure compound, e.g. Manganese Dioxide, Silica, Vanadium
V Oxide, Iron III Oxide; it coulb be dissolved ions, e.g. Copper ions, Cobalt II ions; or
it could be a mixture, e.g. Iron-Molybdenum, or it could be a much more complicated
compound such as protein (all enzymes are proteins; you learn about them in your biology;
they are special cases.)
Enzymes are biological catalysts. They are slightly different in
that they are easily denatured by heat. If you want to know more about enzymes, jump to
the enzyme page (look at the Biology Index).
Most catalysts make chemical reactions go faster.
Chemists call such catalysts "positive catalysts" or "promoters".
However, sometimes we want a chemical reaction to go more slowly. So we choose a
"negative catalyst"; we could call this an "inhibitor". My wife put a
negative catalyst in our central heating system. She did this to stop the iron bits from
rusting. We did not have a problem with the Copper pipes (Copper does not rust), but we
might have had a problem with the old Iron radiators: we wanted to stop them from rusting
so we used an inhibitor. I think that we also have an inhibitor in the water cooling
system of our car so that the car radiator does not rust. This is cheaper than buying a
new car every year when the old one has got too rusty.
My baker puts an inhibitor into the bread he makes. This slows
down the chemical reactions which make bread go stale. This is important since we only go
shopping once a week. We used to put Lead in our petrol; this stops the engine from
"knocking". Now we have a better car which uses lead free petrol but the engine
can burn it without knocking.
You might wonder how catalysts work.
There are two ways in which catalysts work. You already know that
when two different molecules bump into each other, they might react to make new chemicals.
We usually talk about "collisions" between molecules, it would be much simpler
to say that the molecules bumped into each other. How fast a chemical reaction is depends
upon how frequently the molecules collide. You have probably been told about the
"kinetic theory" which is all about heat and how fast molecules move around.
What catalysts are doing when they make a chemical reaction go faster is to increase the
chance of molecules colliding. The first method is by "adsorption", the second
method is by the formation of intermediate compounds.
Adsorption This occurs when a molecule sticks
onto the surface of a catalyst. Make sure that you spell this word correctly; it is not
the same as absorption. Here is an example: it is possible to use Platinum as a catalyst
to make sulphur Trioxide from Sulphur Dioxide and Oxygen. Sulphur Trioxide is very
important because it is used to make Sulphuric acid which is needed for car batteries. The
molecules of the two gases (Sulphur Dioxide and Oxygen) get adsorbed (stuck onto) the
surface of a Platinum catalyst. Because the two molecules are held so close together, it
is more likely that they will collide and therefore react with each other. The Sulphur
Trioxide easily falls off the catalyst leaving space for more Sulphur Trioxide and Oxygen.
Intermediate Compounds Many catalysts, including
all enzymes" work by forming intermediate compounds. What happens is very simple: the
chemicals involved in the reaction combine with the catalyst making an intermediate
compound, but this new compound is very unstable. When the intermediate compound breaks
down it releases the new compounds and the original catalyst.
Well: if you have understood all this, it should be easy to
memorise the definition of a catalyst given at the top of the page.
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