The concept of energy is of central significance, as it is a core disciplinary idea in chemistry and a cross-cutting concept in the sciences. This study sought to investigate student understanding of energetics and associated thermodynamic functions in chemical reactions and processes across a sample of introductory-level undergraduate chemistry students, upper-level undergraduate chemistry students, and physical chemistry graduate students. Participants were interviewed using an interview-about-events protocol, interacting with reagents that underwent chemical reactions or dissolutions. Analysis of the interview data revealed specific student difficulties with content, as well as common reasoning patterns used by students when considering energy changes in observed reactions and processes. Implications for curriculum and instruction are discussed.