Chem 120A is listed as a prerequisite for Chem 120B, and we recommend that Chem 120A and 120B be taken in sequence. Students with scheduling conflicts or other concerns that might require taking Chem 120 A and B concurrently or out of sequence should consult the instructors and the undergraduate advising office.
The modern understanding of physical chemistry builds from microscopically detailed properties of molecules and their interactions, up to the collective behaviors we observe at macroscopic scales. The former require a quantum description and there are therefore many benefits of studying quantum mechanics (the subject of Chem 120A) in depth before learning statistical mechanics (the subject of Chem 120B). Model quantum systems (discussed in Chem 120A), such as harmonic oscillators and rigid rotors, provide simple contexts for developing basic tools and concepts of statistical mechanics (Chem 120B); familiarity with their energy levels and wavefunctions makes this development clearer and more approachable. In addition, understanding of molecular energy levels is useful for development of molecular statistical mechanics and kinetic theories. Further, the practice gained in Chem 120A with the mathematics of probability distributions is called upon in Chem 120B. Students who take these courses concurrently or out of order may be at a disadvantage. However, provided students are willing and able to take the results of the quantum mechanics of harmonic oscillators and rigid rotors, etc, at face value, they should be able to succeed in Chem 120B before having completed Chem 120A.