> Is this some sort of chemical reaction?

No, it's dramatically more complex than a chemical reaction, see https://en.wikipedia.org/wiki/White_hole#Big_Bang/Supermassive_White_Hole - "The Einstein–Cartan–Sciama–Kibble theory of gravity extends general relativity by removing a constraint of the symmetry of the affine connection and regarding its antisymmetric part, the torsion tensor, as a dynamical variable.
Torsion naturally accounts for the quantum-mechanical, intrinsic angular momentum (spin) of matter.
According to general relativity, the gravitational collapse of a sufficiently compact mass forms a singular black hole.
In the Einstein–Cartan theory, however, the minimal coupling between torsion and Dirac spinors generates a repulsive spin–spin interaction that is significant in fermionic matter at extremely high densities.
Such an interaction prevents the formation of a gravitational singularity.
Instead, the collapsing matter on the other side of the event horizon reaches an enormous but finite density and rebounds, forming a regular Einstein–Rosen bridge.
The other side of the bridge becomes a new, growing baby universe.
For observers in the baby universe, the parent universe appears as the only white hole.
Accordingly, the observable universe is the Einstein–Rosen interior of a black hole existing as one of possibly many inside a larger universe.
The Big Bang was a nonsingular Big Bounce at which the observable universe had a finite, minimum scale factor."

And that's just a postulate/hypothesis, not something we have any definitive evidence for, or can even test with current technology.

> Secondly do we not know as a species why black holes/ white holes are able to exist.

We know many (but not all) details of how black holes form and how they behave - we predicted them, then found a bunch with our telescopes.

I believe there's some contention about how the Pauli exclusion principle gets squashed when a neutron star gets big enough to transition, but perhaps I'm simply not well-read enough.

We do not know what happens beyond the event horizon, although there are several competing ideas even amongst the highest echelons of cosmological theorists and pure mathematicians.

> Third question, have we ever observed a white hole colliding with a black hole.

We're not convinced that the big bang is a white hole, but if it is, it's the only one we've ever seen - there's no evidence that there are (other) white holes in the universe even though they show up in our math.

If white holes are a thing, and the big bang is an example of a white hole, then I'm not sure how a white hole and a black hole could exist in the same space in order for them to collide?

If all of the matter in the universe came from a white hole, then prior to this there would be no matter to form the density required to create a black hole. And surely if there were any such matter then the mass of the 'white hole' would supercede that of the black hole and as such would have already swallowed it up.

Not claiming to have any great knowledge on this stuff, by the way. Just thinking out loud...

[deleted]