Recent progress in elucidating chemical reactions allows to explain chemistry by the potential energy of the involved chemical structures. Nevertheless, from an educational point of view, empirical results indicate that students often do not connect the core idea of energy with other chemical concepts. From a resource-oriented perspective, students do not draw on a coherent concept of chemistry to solve a problem but rather activate diverse cognitive resources, crucially depending on the context. It is thus of interest which resources high school students activate to reflect on energetic aspects of a chemical reaction. In this study, 38 German high school students in 16 focus groups were asked to explain kinetic and thermodynamic aspects of the reaction between hydrogen and chlorine. The unguided focus group phase and the following semistructured qualitative interview were analysed by qualitative content analysis. Results show that students have a diverse network of cognitive resources on energetic aspects. However, this network's structure seems to be dominated by terminology and a few prominent ideas such as activation energy. In contrast, students seldom drew connections between bond-making and energy release. Many students mainly argued on a macroscopic level and relied heavily on technical terms. If they argued on the sub-microscopic scale, however, they often focussed on the whole system rather than on specific molecules and their structure. Hence, students interpreted concepts like activation energy or reaction coordinate diagrams on the system level leading to unproductive reasoning. Overall, it seems that students seldom activate resources on molecular structures to argue about energetic changes in chemical reactions. Also, they rarely refer to the fundamental principle of energy minimisation to reason about the driving force of reactions. These results suggest that chemical reactions should be explained already in high schools on a molecular level providing a more explicit reference to energy as a function of chemical structures.