The placement testing system confused me, and just about everybody else, back when I was entering NU a few years ago too.
The ALEKS test isn’t very difficult. Its mostly pre-calc and is used to make sure you have basic mathematical proficiency, the kind you’d need to enter the very easiest calculus class at NU that most people are able to skip over (so it seemed to me at least).
I didn’t have to take the math placement test because I didn’t start off in McCormick, so I can’t comment on it.
The Chem test…its hard. Very hard. Even people who took AP Chem and took that test had a bad time. I had never taken AP Chem, just a rigorous regular Chem class during sophomore year, and after studying intensely for that test it still was incredibly difficult. But remember, everyone is facing the same thing, so there’s no need to worry. And it should help place you in the fitting Chem course at NU, which is a good thing.