When we calculate how much energy is needed to break a chemical bond, it makes a difference if we are talking about a single molecule in a vacuum, or lots of molecules contained in a fluid. The latter situation is more realistic and less abstract, but also requires that we consider not just the energy to break the bond, but the energy required to "deal with" the environment. If, for example, as is common in chemistry (or biology), a reaction takes place at a constant pressure and temperature, then the energy we put into the reaction is used not only to break apart bonds but also to do work – the volume must be increased to make room for the additional particles that are created so the pressure remains constant. This is why we might use enthalpy as our energy needed to cause a reaction rather than just the energy needed to break the bond itself: enthalpy also includes the amount of energy required to change the volume, i. e., the work. The energy needed to simply pull the