To understand why and how reactions take place and why and how bonds form in all their variety, chemists think about the energy changes that take place when processes occur. ‘Its principles: energy and entropy’ introduces thermodynamics, the role energy plays, and how the concepts of entropy and free energy constitute the driving force of chemical change. Energy is both the carrot and the cart of chemical reactions. Its dispersal in disorder is the carrot, the driving power of chemical reactions. The need to overcome the barriers between reactants and products, is also the cart, in the sense of holding back free unrestrained flight towards the carrot.