Man's physical universe

xanabras

UNIT IX

SECTION 4

AIR AND WATER ARE TRANSFORMED INTO MANY

USEFUL MATERIALS BY SYNTHESIS IN THE PRESENCE

OF CATALYSTS

When man tries to prepare compounds found in nature, he often

finds it

necessary to utiHze very high temperatures or pressures; but

it is typical of nature that its reactions take place relatively rapidly

at ordinary temperatures and pressures. The secret of chemical reactions

in nature seems to lie in catalysis. A catalyst is a substance which

needs to be present in only very small amounts either to activate molecules

in such a way that reactions which would otherwise take place

so slowly as to give no appreciable results take place quite rapidly or,

less frequently, to produce a marked decrease in the rate of a chemical

reaction. It is also characteristic of catalysis that the catalyst does

not become a part of the products of the reaction.

The study of many processes taking place in nature led scientists to

the conclusion that nearly every chemical change can be catalyzed.

Some of the greatest developments of modern chemistry are the results

of attempts to find the proper catalysts for desired reactions.

Synthesis is the building of more complex substances from simpler

ones. The preparation of water from oxygen and hydrogen is an example

of synthesis.

In many cases desired syntheses will take place only in the presence

of the proper catalysts. The preparation of high explosives and fertilizers

from air and water are typical examples of processes which involve

the use of catalysts.

Catalysts Are Effective in Very Small Concentrations.

It is customary to prepare oxygen in the laboratory by heating

potassium chlorate, KCIO3. The addition of a small amount of manganese

dioxide, Mn02, will cause this reaction to take place much more

rapidly and at a lower temperature; in this case the manganese dioxide

is the catalyst. The more of the catalyst that is present, the more

rapid will be a given chemical reaction, but many reactions are noticeably

accelerated by astonishingly small quantities of catalysts.

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