The top five teams from each conference make it to the MLS playoffs and Chicago (12-12-7) moved into the No. 5 spot in the East -- a point in front of Philadelphia.
Chicago won for just the second time in seven games.
Larentowicz slipped the ball just inside the right post from a bad angle in the 24th minute after Juan Luis Anangono had beaten two defenders to hit him with a pass.
Soumare scored on a goalmouth scramble, his first tally in MLS. Quincy Amarikawa added a Chicago goal in the 89th minute.
The winless streak for last-place D.C. United was extended to nine games.