Chemical Energy

Chemical energy is the energy of chemical substances that is released when they undergo a chemical reaction and transform into other substances. Some examples of storage media of chemical energy include batteries, food, gasoline, and oxygen gas. Breaking and re-making of chemical bonds involves energy, which may be either absorbed by or evolved from a chemical system.

Energy that can be released or absorbed because of a reaction between chemical substances is equal to the difference between the energy content of the product and the reactants, if the initial and final temperature is the same. This change in energy can be estimated from bond energies of the reactants and products. The internal energy change of a chemical process is equal to the heat exchanged if it is measured under conditions of constant volume and equal initial and final temperature, as in a closed container such as bomb calorimeter. However, under conditions of constant pressure, as in reactions in constant vessels open to the atmosphere, the measured heat change is not always equal to the internal energy change because pressure-volume work also absorbs or releases energy. (The heat change at constant pressure is called the enthalpy change; in this case the enthalpy of reaction, if initial and final temperatures are equal).

A related term is the heat of combustion, which is the energy mostly of the weak double bonds of molecular oxygen released due to a combustion reaction and often applied in the study of fuels. Food is similar to hydrocarbon and carbohydrate fuels, and when it is oxidized to carbon dioxide and water, the energy released is analogous to the heat of combustion (though assessed differently than for the hydrocarbon fuel).

Chemical potential energy is a form of potential energy related to structural arrangement of atoms or molecules. This arrangement may be the result of chemical bonds within a molecule or the interactions within them. Chemical energy of a substance can be transformed into other forms of energy by chemical reaction. For example, when fuel is burned, the chemical energy of molecular oxygen is converted to heat. Green plants transform solar energy to chemical energy (mostly of oxygen) through the process of photosynthesis, and electrical energy can be converted to chemical energy and vice versa to electrochemical reactions.

The similar term chemical potential is used to indicate the potential of a substance to undergo a change of configuration, be it in the form of chemical reaction, spatial transport, particle exchange through a reservoir, etc. It is not a form of potential energy itself, but it is more closely related to free energy. The confusion in terminology arises from the fact that in other areas of physics not dominated by entropy, all potential energy is available to do useful work and drives the system to spontaneously undergo changes of configuration, and thus there is no distinction between "free" and "non-free" potential energy (hence the term "potential"). However, in systems of large entropy such as chemical systems, the total amount of energy present (and conserved according to the first law of thermodynamics) of which this potential energy is part of, is separated from the amount of that energy--thermodynamic free energy (from which chemical potential was derived)--which (appears to) drive the system forward spontaneously as the global entropy increases (in accordance to the second law of thermodynamics).

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