The slumping Illini lost their third straight game and all of the setbacks have been by at least 14 points.
Hearn scored 23 points, making 12-of-14 from the foul line. Northwestern was 26-of-31 overall from the charity stripe. The Wildcats built a 15-point halftime lead and stayed comfortably in front throughout the final 20 minutes.
Jared Swopshire and Alex Marcotullio both scored 12 points for Northwestern (11-7, 2-3 Big Ten Conference), which had lost three of its previous four games.
Illinois (14-5, 1-4) was led by Brandon Paul with 21 points.