Energy can be converted from one form into another, but all of the energy present before a change occurs always exists in some form after the change is completed. This observation is expressed in the law of conservation of energy: during a chemical or physical change, energy can be neither created nor destroyed, although it can be changed in form.
According to the law of conservation of matter, there is no detectable change in the total amount of matter during a chemical change. When chemical reactions occur, the energy changes are relatively modest, and the mass changes are too small to measure. Thus, the laws of conservation of matter and energy hold well. However, in nuclear reactions, the energy changes are much larger (by factors of a million or so), the mass changes are measurable, and matter-energy conversions are significant.
Substances act as reservoirs of energy, meaning that energy can be added to them or removed from them. Energy is stored in a substance when the kinetic energy of its atoms or molecules is raised. The greater kinetic energy may be in the form of increased translations (travel or straight-line motions), vibrations, or rotations of the atoms or molecules. When thermal energy is lost, the intensities of these motions decrease, and the kinetic energy falls.
The total of all possible kinds of energy present in a substance is called the internal energy (U), sometimes symbolized as E.
As a system undergoes a change, its internal energy can change, and energy can be transferred from the system to the surroundings, or from the surroundings to the system. Thus, the surrounding also experiences an equal and opposite change in its energy.
Internal energy is an example of a state function (or state variable), whereas heat and work are not state functions. The value of a state function depends only on the state that a system is in, and not on how that state is reached. If a quantity is not a state function, then its value does depend on how the state is reached. An example of a state function is altitude or elevation. Standing on the summit of Mt. Kilimanjaro at an altitude of 5895 m, it does not matter how it was reached, whether someone hiked there or parachuted there. The distance traveled to the top of Kilimanjaro, however, is not a state function. One could climb to the summit by a direct route or by a more roundabout, circuitous path. Thus, the distances traveled would differ (distance is not a state function); however, the elevation reached would be the same (altitude is a state function).
This text is adapted from OpenStax Chemistry 2e, Section 5.1: Energy Basics and OpenStax Chemistry 2e, Section 5.3: Enthalpy.
