As we venture into the topic of thermochemistry, we first need to lay the groundwork of some basic terminology. As we've previously discussed, energy is the capacity to do work, and work is a force acting over a distance. We can categorize energy as kinetic or potential energy. Kinetic energy is the energy of motion and includes thermal energy, which is associated with temperature or the average kinetic energy of particles in a substance. Heat is the transfer of thermal energy. Potential energy is due to position or composition. Chemical energy is a subcategory of potential energy and involves the positions of subatomic particles. So in thermochemistry, we'll study the intersection of kinetic and potential energies. We should also keep in mind the law of conservation of energy, which states that energy can neither be created nor destroyed but only transferred from one object to another or from one form to another. For example, if we have a bowling ball placed on a spring and we push it down to compress the spring, we're transferring energy to the spring in the form of elastic potential energy, which is a form of mechanical potential energy. If we let go of the bowling ball and spring, the elastic potential energy will be converted into kinetic energy and transferred to the bowling ball. As the bowling ball rises in the air, the kinetic energy is converted to gravitational potential energy. This process goes in reverse as the bowling ball falls back down to the spring and compresses it again. Some energy is also transferred to the surroundings as heat and possibly sound. Eventually, all of the energy that was initially added to the system by compressing the spring will be transferred to the surroundings, and the bowling ball will come to a stop. When we discuss energy transfer, the system is what we're interested in studying; everything else is considered the surroundings. In our previous example, we could consider the bowling ball to be a system. Any energy transfer to the system must come from the surroundings, and any energy loss by the system must be transferred to the surroundings so that the total energy is conserved.