Ask A Scientist© Chemistry Archive

 
name        jirow
age         20s

Question - Is it possible to break bonds between for example carbon atom.
Using very high frequency longitudinal or sound waves, applied at
the appropriate fundamental frequency.
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The short answer is a definite YES.

1. Uncontrolled process. When a liquid is exposed to an expansion force 
such that the pressure exceeds the vapor pressure, bubbles of vapor form. 
When the force is removed, these bubbles collapse extremely rapidly, 
compressing the vapor adiabatically [i.e. the energy doesn't have time to 
dissipate into the surrounding liquid]. As a result the temperature of the 
explosively collapsing bubble increases by hundreds of degrees. This 
process is called cavitation. There are ultrasonic devices available to do 
this in the laboratory. They are used to clean parts, and to rip cells 
apart, disperse solids in liquids, and many other application where high 
intensity energy is needed. It also occurs at the tip of a motor boat 
propeller if the motor is revved up too quickly and the boat just spins 
its blades.

2. Controlled process. There are devices that generate ultrasonic waves 
using crystals that vibrate when exposed to an alternating electric field. 
It is a process similar to the quartz crystals used in watches and clocks, 
but with a much more intense alternating electric field intensity. The 
frequency can be varied by selecting the crystal size and the frequency of 
the electric field. The signal is usually pulsed, the pulse can be 
detected by a similar crystal doing the opposite -- the ultrasonic pulse 
vibrates the crystal producing an oscillating electric field that is 
amplified and displayed. This receiver is basically a microphone listening 
to the ultrasonic pulse. If you are interested in details, the transmitter 
and receiver are coupled to lock them in phase, but that's a design detail.
The intensity of the pulse can be measured as a function of the 
ultrasonic frequency. When the ultrasonic sound frequency becomes the 
same as some process in the medium, an absorption of the sonic energy is 
absorbed, and there is a decrease in the sound intensity reaching the 
receiving microphone.
These absorbing processes can be internal rotations of the molecules 
comprising the medium, molecular vibrations, and even the breaking of 
chemical bonds, again depending upon the intensity and frequency of the 
sonic pulses. Sometimes it is even possible to observe light pulses 
generated by chemical reactions induced by high intensity ultrasonic 
waves in some media.

Vince Calder
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