talk vs chat up

talk is a noun but chat up is not a noun.

Word	Noun	Adjective	Verb	Adverb
talk	Yes	No	Yes	No
chat up	No	No	Yes	No

As verbs, chat up is a hyponym of talk; that is, chat up is a word with a more specific, narrower meaning than talk:

talk (noun)	chat up (noun)
the act of giving a talk to an audience
an exchange of ideas via conversation
idle gossip or rumor
a speech that is open to the public
discussion; (`talk about' is a less formal alternative for `discussion of')

talk (verb)	chat up (verb)
express in speech	talk or behave amorously, without serious intentions
exchange thoughts; talk with	talk to someone with the aim of persuading him
reveal information
use language
deliver a lecture or talk
divulge confidential information or secrets