The ability to build mechanistic explanations of natural phenomena is highly valued in all scientific disciplines, including chemistry. In this paper, we summarize the results of a qualitative study designed to characterize the nature of the mechanistic explanations built by college General Chemistry students when analyzing a physical process involving energy transfer between parts of a system at different temperatures, and a chemical process involving energy transformation and transfer due to chemical bonding. Prior studies have mostly focused on eliciting "what" students think happens in terms of energy transfer when chemical bonds break or form (e.g., energy is absorbed or released) and in this investigation we provide insights into their reasoning about "how" and "why" it happens. Our findings reveal that many students struggle to build sound mechanistic explanations for energy transfer between objects at different temperatures and for energy transformation and transfer during the formation of chemical bonds.