Chemistry and Energy

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Chemistry and Energy

Energy is central to our understanding of chemistry, for atoms adopt arrangements that correspond to the lowest possible energy and electrons in atoms adopt the lowest possible energy distribution. The adoption of lowest energy arrangements of atoms is responsible for the characteristic shapes of molecules. For instance, the tetrahedral shape of the methane molecule (CH₄) corresponds to the lowest energy arrangement possible for one carbon atom and four hydrogen atoms. The characteristic bond lengths in a molecule are the distances between centers of atoms corresponding to the atoms' lowest energy arrangement. The distribution of electrons in molecules—whether they are present as forming single, double, or triple bonds, or whether they do not participate in bonding at all—corresponds to the arrangement of lowest energy for those electrons.

Energy considerations are central to determinations of whether a reaction will run in one direction or another, but great care must be exercised in this regard for although most reactions run in the direction of decreasing energy (e.g., the combustion of a fuel), some reactions run in the direction of increasing energy (e.g., the decomposition of pure ammonia maintained at a high temperature). To determine the spontaneous direction of a reaction, it is necessary to consider, not the quantity of energy absorbed or released but its quality as measured by the change in entropy. This is the domain of the second law of thermodynamics.

Energy changes are also used to identify materials and to explore their detailed structures. This is the domain of spectroscopy , in which molecules are excited to higher energy states by the absorption of energy in the form of electromagnetic radiation. In some materials, the excess energy of an excited state is emitted as radiation: this is the origin of fluorescence and phosphorescence, in which high frequency radiation (such as ultraviolet radiation ) is absorbed and emitted as lower energy visible radiation.

When most people think of energy in connection with chemistry, however, they have in mind the production of energy for manufacturing, transportation, and the functioning of living organisms. The energy released by chemical reactions may be extracted as heat or as work. The ultimate source of the energy is the change in energy that accompanies the rearrangements of electrons and nuclei as atoms exchange partners. For instance, when a hydrocarbon fuel burns, the carbon–hydrogen and carbon–carbon bonds are replaced by the stronger carbon–oxygen and hydrogen–oxygen bonds of carbon dioxide and water, and the excess energy is released as heat. That is, at the high temperature of the reaction, the atoms are "loosened" from each other and allowed to adopt arrangements of lower energy.

The energy produced in a chemical reaction may also be extracted electrically. The device needed is called an electrochemical cell, which is a reaction vessel equipped with two electrodes. Oxidation (the loss of electrons) takes place at one electrode (the anode), and the electrons lost from the reactant are transferred to the electrode. They then travel through an external circuit, which might contain an electric motor, and re-enter the reaction vessel at the other electrode (the cathode), where they bring about reduction (the gain in electrons). In a primary cell, the reactants are sealed in at time of manufacture and the production of electricity continues until the chemical reaction has reached equilibrium . In a secondary cell, an electric current is driven through the cell and reactants are formed at the electrodes. Once the cell is "charged," the newly formed reactants can be allowed to form products in the same way as in a primary cell. In a fuel cell, reactants are supplied continuously from the outside, and electricity is produced for as long as they are supplied.

The production of energy by any chemical reaction has potential consequences for the environment. Of greatest concern is the generation of carbon dioxide (CO₂) by the combustion of hydrocarbon fuels (the so-called fossil fuels—natural gas, coal, and petroleum). Carbon dioxide is a potent greenhouse gas and its accumulation in the atmosphere appears to be furthering global warming, with dire consequences for humanity. Fuel cells that use hydrocarbons as feedstock are also sources of pollution, as they form carbon dioxide, but they are more efficient, and less carbon dioxide is produced for a given supply of energy. Fuel cells may also operate using hydrogen as feedstock, in which case they produce only water, which has no impact on the already wet environment. However, the most economical supply of hydrogen is the burning of hydrocarbons, which produces the unwanted carbon dioxide. There is hope that photochemical sources of hydrogen will become sufficiently economical and eliminate the covert pollution step, but it is unlikely that sufficient energy can be achieved diurnally: fossil fuels represent the accumulation of years of solar energy.

Nuclear energy, which is obtained when nucleons (protons and neutrons) are allowed to adopt lower energy arrangements and to release the excess energy as heat, does not contribute to the carbon dioxide load of the atmosphere, but it does present pollution problems of a different kind: radioactive waste. Optimists presume that this waste can be contained, in contrast to the burden of carbon dioxide, which spreads globally. Pessimists doubt that the waste can be contained—for thousands of years. Nuclear power depends directly on the discipline of chemistry in so far as chemical processes are used to extract and prepare the uranium fuel, to process spent fuel, and to encapsulate waste material in stable glass blocks prior to burial. Nuclear fusion, in contrast to nuclear fission , does not present such serious disposal-related problems, but it has not yet been carried out in an economic, controlled manner.

Chemistry contributes in many ways to the more efficient and cleaner utilization of fossil fuels. Chemists find ways of extracting fuels from ever scarcer supplies; of promoting cleaner, more efficient combustion; and of producing cleaner exhausts through the development of catalysts. They are involved in the development of electrode materials for more efficient fuel cells, and in the development of photovoltaic systems to be used for more effective solar power generation.