First, I'll make a grammar correction: Since "Chemical reactions" is plural, you'd have to say "are done." To answer your question though, yes you need an article. Whether you write "a" or "the," depends on context. If you use "a" then you would be referring to any and all laboratories anywhere around the world, but if you want to only refer to a specific one, then use "the."

To add to Lee, when you are talking about countable nouns in general, you use either the plural (like you did with "chemical reactions") or a singular with a/an (e.g. "a chemical reaction"). In your example, if you were talking generally, you would say "a laboratory" or "laboratories" but not "the laboratory".