Section 7: Enthalpy: Exothermic and Endothermic Reactions Crucial Chemistry In the field of thermodynamics, scientists study how heat and other forms of energy are involved in chemical reactions. o When a reaction is endothermic overall, energy moves from the surroundings into the system. To start a reaction, chemical bonds must be broken. The process of breaking chemical bonds is endothermic. o When a reaction is exothermic overall, energy moves from the system into the surroundings. When chemical bonds are formed, energy is released. The process of forming chemical bonds is exothermic. In the field of kinetics, scientists study the rates of reactions. Many variables can increase the rate of a reaction: o increasing the concentration of the reactants o decreasing the particle size/increasing the surface area of the reactants o increasing the temperature o adding a catalyst A catalyst speeds up a reaction by lowering the activation energy of a reaction. The activation energy is the energy that must be gained to break the initial bonds and start a reaction. A reaction is considered to be spontaneous if it continues without the input of energy once it begins: o Two factors determine if a reaction is spontaneous: Enthaply (∆H) or the change in heat energy, and Entropy (∆S) or the change in disorder. Scientists often draw energy diagrams to represent the change in enthalpy during a reaction. o If the energy of the reactants is higher than the energy of the products, then the reaction is exothermic overall, and ∆H<0. o If the energy of the reactants is lower than the energy of the products, then the reaction is endothermic overall, and ∆H>0.