How much does the gravitational and magnetic fields of a planet effect the chemistry in it?
How would the chemical reactions seen everyday on earth be different if the strengths of these fields were different?

Depends. If you're just talking about higher $g$ (i.e $g&gt;9.8\ \rm{m/s}^2$) but the same atmospheric pressure, than not much will change on the surface of the earth.
If the atmospheric pressure changes, there could be changes to chemical reactions. There are several factors controlling chemical reactions. One being, obviously, the identity of the reactants. The other one is temperature. But also important is pressure. Because pressure on the surface of the earth is always 1 atm, no one really thinks or cares about pressure. No one, until it changes. That said, for any substantial change in chemistry, pressure has to change quite a lot, which would make earth uninhabitable for humans, so there would be no one to see that chemistry changed.