impossible vs unacceptable

impossible is a noun but unacceptable is not a noun.

Word	Noun	Adjective	Verb	Adverb
impossible	Yes	Yes	No	No
unacceptable	No	Yes	No	No

As adjectives, impossible and unacceptable are synonyms defined as:

impossible and unacceptable: (used of persons or their behavior) not acceptable or reasonable

impossible (noun)	unacceptable (noun)
something that cannot be done

impossible (adjective)	unacceptable (adjective)
not capable of occurring or being accomplished or dealt with	not acceptable; not welcome
(used of persons or their behavior) not acceptable or reasonable	(used of persons or their behavior) not acceptable or reasonable
totally unlikely	not conforming to standard usage
	not adequate to give satisfaction