second-year vs sophomore

second-year is not a noun while sophomore is a noun.

Word	Noun	Adjective	Verb	Adverb
second-year	No	Yes	No	No
sophomore	Yes	Yes	No	No

As adjectives, second-year and sophomore are synonyms defined as:

second-year and sophomore: used of the second year in United States high school or college

second-year (noun)	sophomore (noun)
	a second-year undergraduate

second-year (adjective)	sophomore (adjective)
used of the second year in United States high school or college	used of the second year in United States high school or college