Is there a chemical I can use to change brass to antique patina?

Answer 1

Most new brass products are sold with a light lacquer finish to keep them bright and shiny. A cleaner that will remove the lacquer is really all you will need. In a few months, especially if you are in a humid area, the brass will start to age naturally.
Fingerprints will turn brown/black, so if possible handle the item with gloves when removing the lacquer. When it gets to the point where you like the patina, you can spray it again with a light coat of satin lacquer to seal it and stop further oxidation or depending on what the item is, you can use a light coat of a good wax, like Johnson's paste wax.

Answer 2

Modern Masters has a line called Metal Effects, I believe.