Question: is 50 minutes enough layover in chicago?
Answer: 50 minutes is allowed, and is legal. I'm pretty sure that I had a terminal change when I did United SFO to ORD to AMS. If you miss your flight, know that there are two other UA flights to London later in the day, also two UA codeshares operated by Lufthansa to Frankfurt, which they can put you on to get to Amsterdam.