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 bond apart is called the dissociation energy (it’s the negative of the binding energy). In more complicated reactions, of course, we can both break and form bonds, and so there could be a net input or output of energy. In more complicated reactions, of course, we can both break and form bonds, and so there could be a net input or output of energy. In either case, enthalpy would still be the relevant quantity for a reaction taking place at constant pressure, and the change in the enthalpy is simply the heat that is added or removed from a system at constant pressure. Let's consider the following example:

To break apart a single H2, molecule, H2 → H, the dissociation energy is 4.52 ev.

Required:
a. How much energy would be required fin k/) to dissociate 1 mol of hydrogen molecules
b. How many moles of II atoms are in the bubble after we have dissociated all the hydrogen molecules?
c. What are the initial and final volumes of the bubble? Has the bubble expanded or contracted?
d. Does the thermal energy of the system increase, decrease, or stay the same in this process?