Chemical Energy (Non-Fossil Fuels)

FIELDS OF STUDY: Inorganic Chemistry; Geochemistry; Metallurgy

ABSTRACT

The principles underlying bond energy are explained, as well as how bond energy can be used to supply the activation energy required for a chemical reaction. The energy contained in chemical bonds is necessary for chemical reactions to take place.

The Nature of Chemical Energy

Chemical compounds are formed by the interaction of electrons in the outermost valence shells of atoms. In molecules, the atomic orbitals combine and hybridize to form molecular orbitals. According to the modern theory of atomic structure, the electrons in atomic and molecular orbitals must correspond to very specific energies. The geometrical structures of chemical compounds have ideal values, representing the minimum energy configuration of a particular molecule; for example, the ideal angle between the four bonds of a carbon atom in a tetrahedral geometry is 109.5 degrees. However, chemistry is a real process rather than an ideal one. A number of factors, including electron-electron repulsion and orbital interactions, force bond angles to deviate from their ideal values, contributing to bond-angle strain that forces the bonds out of their minimum energy configuration and into a higher bond energy configuration. Other factors, such as pi-bond formation, add to the stability of the interaction between two chemically bonded atoms, increasing the potential energy contained in the bonds of a molecule.

Atoms in a molecule do not exist in isolation from each other, and conditions that affect one bond in a molecule affect all of the bonds in the molecule. During chemical reactions, some bonds are broken and others are formed. When a bond is broken as part of a reaction, energy is released; when a bond is formed, energy is captured and held within the bond. The enthalpy change of a reaction is the difference between the energy released by bonds being broken and the energy captured by bonds being formed.

Activation Energy in Reactions

Spontaneous reactions begin as soon as the reactants come into contact with each other, although the rate of a spontaneous reaction can be so slow as to be unnoticeable. All other reactions have energy requirements that must be met before a reaction can occur. The activation energy of a reaction is the minimum energy that must be supplied to the reactants so the reaction can proceed. This energy may be needed to break an initial bond, or it may be needed to bring the reactants into the proper conformation for reaction. In an exothermic reaction, in which the reactants contain more energy than the resulting products, the energy that is released by the reaction is typically more than sufficient to provide the activation energy of subsequent reactions.

Governing all chemical reactions are the laws of conservation of energy and conservation of mass. Accordingly, all individual energy transactions that take place during a chemical reaction must combine to produce the overall energy change in the reaction. Just as there can be no extra or missing mass at the end of a reaction, there can be no extra or missing energy.

Chemical Energy Transactions

Breaking and making bonds is the currency of chemical transformations, especially in biological systems. In respiration—and all other biochemical processes involving ATP, NAD, and similar compounds—energy is recovered and stored by the enzyme-mediated formation of a triphosphate group. In this process, an inorganic phosphate group, PO₄³⁻, is joined to an existing diphosphate group. The new bond is considered a high-energy bond; when a process requires an input of energy, an enzyme causes this bond to be cleaved, and the energy it releases is used to drive the subsequent reaction.

Great amounts of solar energy are stored in the formation of glucose during photosynthesis. This glucose is used both to fuel cellular processes and to form structural materials such as cellulose. The energy is released through chemical reactions with molecular oxygen, via either respiration within the living cell or combustion. In all cases, the energy stored in the various bonds of the glucose molecules is made available to other processes and eventually released into the environment.

PRINCIPAL TERMS

• bond energy: the amount of energy necessary to break the chemical bonds in a given molecule, measured in kilojoules per mole.
• combustion: a reaction between a fuel and an oxidizing agent that results in new chemical compounds and the release of heat; most often takes place between organic material and molecular oxygen, in which case the products include carbon dioxide and water.
• enthalpy: the total heat content within a thermodynamic system, defined as internal energy plus the product of pressure and volume; also, the change in heat content associated with a chemical process.
• potential energy: the energy contained in an object due to its position, composition, or arrangement that is capable of being translated into kinetic energy or the performance of work; for example, the energy contained in the bonds between atoms, which can be used to fuel a chemical reaction.

Bibliography

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