What is the equation for the dissociation and re-formation of water?

Question

is the re-formation opposite of dissociation

Answer 1

H20 <--> H+ + OH-

OR

2H2O <--> H30+ + OH-

Either one is commonly used.  You may see the second more often in texts as the hydronium ion created in that equilibria is more physically significant, since it has been shown to exist, than simply protons in solution--as is shown in the first one.

And YES, dissociation and "re-formation" are the opposite processes.  Also known as dissociation and association (also heard of recombination).  So you can see that I used double arrows for the two equations above, denoting an equilibrium,  to shown that the process is reversible.

Hope this helps!