How to Make a Tortoise Go Like "Lightning" and Vice Versa
by L. Vlasov and D. Trifonov
A hundred odd years ago a chemist carefully introduced a platinum wire into a vessel containing a mixture of hydrogen and oxygen. The result was extraordinary. The container filled up with fog, that is, with water vapour. The temperature remained unchanged and so did the pressure, but the reaction between hydrogen and oxygen, "calculated" to take thousands of years, occurred in a matter of seconds.
Nor was this all. The platinum wire that had caused the two gases to combine instantly, had undergone absolutely no change. Its appearance, chemical composition, and weight, were exactly the same after the experiment as before it.
Now the scientist was by no means a magician, one of those who invents all kinds of clever tricks to amuse a curious public. This was a serious investigator, the German chemist Dobereiner.
The phenomenon he observed is now called
catalysis. Substances capable of "making tortoises go like lightning"
are called catalysts. Catalysts are literally legion. They may be metals, solid
or powdered, oxides of a great variety of elements, salts, or bases. They may
be used in the pure form or as mixtures.
Without a catalyst the efficiency of ammonia synthesis is very low, no matter how we vary the pressure and temperature.
But the presence of a catalyst makes things entirely different. Ordinary metallic iron with an admixture of aluminum and potassium oxides accelerates the reaction considerably.
Twentieth-century chemistry owes its unprecedented progress to the use of catalysts. Nor is this all.
Various vital processes occur in animal and plant organisms owing to the presence of special catalysts called enzymes.
The chemistry of all animate and inanimate nature, such is the range of these wonderful accelerators!
But what if we take a copper, aluminum or iron wire instead of a platinum one? Will the vessel walls again become foggy?
Alas! Hydrogen and oxygen display no
inclination to react as they did when urged on by the magic platinum wand...
Not every substance can accelerate any particular process.
Therefore chemists say that catalysts are selective in their action: they may influence one reaction vigorously without paying attention at all to another.
Of course, there are exceptions to this rule. For example, aluminum oxide is capable of catalyzing several dozen different synthesis reactions of both organic and inorganic compounds.
Finally, different catalysts may make a mixture of the same substances react differently to form different products.
There are substances with no less surprising properties, called promoters. Taken by themselves, they do not influence the course of the reaction, neither accelerating nor decelerating it.
If added to a catalyst, promoters accelerate the reaction to a much greater degree than the catalyst itself. A platinum wire with "impurities" of iron, aluminum, or silicon dioxide, would cause a still more impressive effect in a mixture of hydrogen and oxygen.
There is also another kind of catalysis,
inside out catalysis. There are anticatalysis and anticatalysts. Scientists
called them inhibitors. Their purpose is to slow down rapid chemical reactions.
Suppose we have a mixture of two gases, chlorine and hydrogen, in a glass flask. Under ordinary conditions, they react very slowly.
But try lighting a strip of magnesium near the flask. An explosion occurs immediately (if anybody wants to try this experiment, be sure to shield the flask with a hood made of thick wire).
Now why does the mixture of chlorine and hydrogen explode under the action of bright light?
The answer is that a chain reaction is involved. If we heated the flask to about 700 degrees, it would also explode: the chlorine and the hydrogen would combine instantly, in a split second.
This would not surprise us, because we know that heat increases the activation energy of the molecule manifold. But in the experiment just mentioned the temperature did not change. This reaction was caused by light.
Quanta, these tiniest portions of light, carry a large amount of energy. Much more than that needed to activate molecules.
Now when a chlorine molecule happens to get into the path of a light quantum, the quantum tears it apart into atoms and passes its energy over to them.
The chlorine atoms are now in an excited, energy-rich state.
These atoms, in their turn, bear down upon the hydrogen molecules and tear them apart into atoms too. One of the latter combines with a chlorine atom and the other remains free. But it is excited. It craves to give away part of its energy. To whom? Why, to a chlorine molecule. And when it collides with one, that is the end of the phlegmatic chlorine molecule.
And now again there is an active chlorine ion at large, but it does not take long for this atom to find an outlet for its energy.
Thus we get a long consecutive chain of reactions.
As soon as the reaction starts, more and more molecules are activated by the energy liberated as a result of the reaction. The rate of the reaction increases like an avalanche of snow rolling down a mountain.
When the avalanche reaches the valley it dies down. The chain reaction dies out when all the molecules have been caught up by it, when all the hydrogen and chlorine molecules have reacted.
Chemists know multitudes of chain reactions. Our prominent scientist Nikolai Semyonov has studied how these reactions occur in great detail.
Chain reactions are known to physicists too. The fission of uranium atoms is an example of a physical chain reaction.