Hello,

The first topic/chapter that comes to mind from 11th standard that's widely used in class 12th is Chemical Bonding. Chemical Bonding concepts are used heavily in Organic Chemistry. Inorganic chemistry is also a very important chapter, used in Organic chemistry. Thermodynamics, S-block, P-block, Hydrocarbons are used in Chemical Kinetics, Coordination compounds etc.

Basic concepts of chemistry and Atomic structure are used as well in bigger concepts.

Hope this helped. Good luck.