Expert: Henry Boyter - 5/13/2004

Question
when a reaction happens all of the bonds between the molecules are broken then the atoms rearrange themselves so that they are closer to entropy or are at a lower state of energy, right? so why does it always go to one of the two that is opposite it? why cant a molecule break all of its bonds and then form back into the same molecule? what gives the molecule the momentum to keep going through the reaction so that it changes what it is?

thanks

shawn H.S. 11

Answer
It happens all the time.  It is called an equilibrium.

A + B = C + D

There is nothing that says that A and B break up and can't go back to A and B.  The most famous would be

H2O = H+ + OH-

If nothing else is present, the units just go back and forth.  Only when entropy is increased or the energy is lowered does the reaction move forward to form something new.If the reaction is not an equilibrium:

A + B > C + D

there is nothing that says that A = A* + a is not happening at the same time.  In many cases that may be a necessary step.  It is also possible that it happens so fast that the A* and a can never be measured.