unacceptable vs impossible

unacceptable is not a noun while impossible is a noun.

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

As adjectives, unacceptable and impossible are synonyms defined as:

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

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

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