107 Stories About Chemistry |
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.
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