Why do chemical reactions happen? Why does a certain reaction happen and not numerous others instead? What is going on at the atomic and molecular level when a reaction happens? These questions are usually not answered in high school chemistry education. The goal of this module is to provide answer to them on a general level. To this end, two different theoretical frameworks are used: On the one hand, molecular orbital theory is introduced that enables students to interpret the electronic structure of small molecules and the mechanism of simple (organic) reactions. On the other hand, potential energy diagrams and basic thermodynamics will be used to offer an insight into the energetics of simple reaction mechanisms, more from a quantitative point of view. Equipped with the tools these theories offer, the students will be able to qualitatively and quantitavely understand generally relevant chemical concepts, such as entropy, enthalpy and free energy, reaction kinetics, acid-base and electrochemical equilibria, simple organic reaction mechanisms, catalysis and out-of equilibrium processes. The module avoids the traditional divisions of physical, inorganic and organic chemistry, and consequently serves as a solid theoretical foundation for students who are interested either in chemistry, engineering or life sciences.