 This change in energy between the reactants and products of a reaction is a key feature of all chemical reactions And we have a special term to describe it. It's called a change in enthalpy Enthalpy is the energy stored in the bonds of a molecule Different bonds have different enthalpies. So when a chemical reaction occurs and old bonds are broken and new bonds are formed A change in enthalpy occurs Now the actual enthalpy of a molecule is difficult to measure but changes in enthalpy are not We do that just by looking at how the temperature of the surroundings changes when a reaction occurs The size of the change in enthalpy tells you about how much energy is involved. That seems obvious But the sign of an enthalpy value is also important. It tells you whether the reaction is exothermic or endothermic Now notice that the symbol for change in enthalpy is delta H H is the symbol for enthalpy and the capital Greek letter delta the triangle means change in or difference between So a negative delta H value means that energy was lost during the reaction So this would be an exothermic reaction So you've got the reactant enthalpy minus the energy that gets released equals the product enthalpy On the other hand a positive delta H means that energy was gained during the reaction So this would be an endothermic reaction So you would have the reactant enthalpy plus the energy that gets absorbed equals the product enthalpy Okay, so far so good We're beginning to develop a graph that shows us how the energy changes during a reaction But there's another important feature that we haven't yet included