Been via Chicago a couple of times, 1:40 plenty of time but it depends on any delays on your 1st leg. Only airport I would want 2 hours+ is Atlanta, all others I have used I would aim for 1:30mins+ if poss but have accepted less in the past.
Have you checked if the airline you are using has anymore flights the same day for your 2nd leg? if you miss your connection they usually get you on the next, if its the last of the day then it would mean them putting you in a hotel overnight and 1st available flight next day.
Our friends missed an Atlanta connection July 2009, told they were definitely on one 3 hours later but on standby for one 1.5 hours later, they got on the 1.5 hours.