Chemical Reaction, process by which atoms or groups of atoms are redistributed, resulting in a change in the molecular composition of substances. An example of a chemical reaction is formation of rust (iron oxide), which is produced when oxygen in the air reacts with iron.
The products obtained from a given set of reactants, or starting materials, depend on the conditions under which a chemical reaction occurs. Careful study, however, shows that although products may vary with changing conditions, some quantities remain constant during any chemical reaction. These constant quantities, called the conserved quantities, include the number of each kind of atom present, the electrical charge, and the total mass.
Chemical symbols and formulae are used to describe chemical reactions; they denote substances having one set of formulae changing into substances having another set of formulae. Consider the chemical reaction in which methane, or natural gas (formula CH4), burns in oxygen (O2) to form carbon dioxide (CO2) and water (H2O). If we assume that only these four substances are involved, the formulae (used mainly as abbreviations for names) would be stated:
Because atoms are conserved in chemical reactions, however, the same numbers of atoms must appear on both sides of the equation. Therefore, the reaction might be expressed as:
Chemists substitute an arrow for “gives” and delete all the “1s” to get the balanced chemical equation:
Electrical charges and numbers of each kind of atom are conserved. Chemists often also show the physical state of each reacting substance by adding an abbreviation after its symbol: "(s)" for a solid; "(l)" for a liquid; "(g)" for a gas; or "(aq)" for an aqueous solution.
Balanced chemical equations are balanced not only with respect to charge and numbers of each kind of atom but also with respect to weight, or, more correctly, to mass. The periodic table lists these atomic masses: C = 12.01, H = 1.01, O = 16.00.
Thus, 16.05 atomic mass units (amu) of CH4 react with 64.00 amu of O2 to produce 44.01 amu of CO2 plus 36.04 amu of H2O. The total mass on each side of the equation is conserved:
Thus charge, atoms, and mass are all conserved.
Types of Chemical Reactions
An understanding of reaction mechanisms can be gained from a study of ionic and covalent bonds. One kind of reaction, ion association, is easy to understand as owing to the pairing of ions to form neutral ionic substances, as in Ag+ + Cl - ÂAgCl, or 3 Ca2++ 2 PO43-Â Ca3(PO4)2. Here the double arrow expresses the fact that it is possible for the reaction to go in two possible directions, with molecules dissociating into ions at the same time as ions are associating to form molecules. Covalent single bond changes in which both electrons come from (or go to) one reactant—that is, an electron pair is donated or accepted by one reactant—are called acid-base reactions, as in
A pair of electrons from the base enters an empty orbital (a quantum state that can potentially be occupied by electrons) of the acid to form the covalent bond. Covalent single bond changes in which one bonding electron comes from (or goes to) each reactant are called radical reactions, as in H· + ·H ÂHËH.
Sometimes reactants gain and lose electrons, as in oxidation-reduction, or redox, reactions: 2 Fe2+ + Br2Â 2 Fe3+ + 2 Br -. Thus, in an oxidation-reduction reaction, one reactant is oxidized (loses one or more electrons) and the other reactant is reduced (gains one or more electrons). Common examples of redox reactions involving oxygen are the tarnishing or corrosion of metals such as iron (in which case the metals are oxidized by atmospheric oxygen), combustion, and the metabolic reactions associated with respiration. An example of a redox reaction that does not involve atmospheric oxygen is the reaction that produces electricity in the lead storage battery: Pb + PbO2 + 4H+ + 2SO42- ± 2PbSO4 + 2H2O.
The joining of two groups is also called addition; their separation is called decomposition. Multiple addition involving many identical molecules is called polymerization.
Energy is conserved in chemical reactions. Most reactions can be generalized into two distinct steps. Firstly, the bonds of initial reactants are broken and secondly the resulting constituents rearrange themselves, forming new bonds. Breaking a bond, pulling a molecule apart, requires a certain amount of energy, that will later be released if that same bond reforms. “Strong” bonds take more energy to break them apart. If stronger bonds form in the products than are broken in the reactants, energy is released to the surroundings as heat, and the reaction is termed exothermic. If stronger bonds break than are formed, energy must be absorbed from the surroundings, and the reaction is endothermic. Because strong bonds are more apt to form than weak bonds, spontaneous exothermic reactions are common. For example, the combustion of carbon-containing fuels—that is, the combination of the carbon with oxygen from the air—gives carbon dioxide and water, both of which possess strong bonds. Spontaneous endothermic reactions, however, are also well known; the dissolving of salt in water is one example.
Endothermic reactions are always associated with the spreading of energy. This can be measured as an increase in the entropy of the system. The net effect of the tendency for strong bonds to form and the tendency of energy to spread out can be measured as the change in free energy of the system. All spontaneous changes at constant pressure and temperature involve a decrease in free energy.
Chemical Rates and Mechanisms
Some reactions, such as explosions, occur rapidly. Other reactions, such as rusting, take place slowly. Chemical kinetics, the study of reaction rates, shows that three conditions must be met at the molecular level if a reaction is to occur. The molecules must collide; they must be positioned so that the reacting groups are together in a transition state between reactants and products; and the collision must have enough energy to form the transition state and convert it into products. Not all collisions have this energy, but more do so at higher temperatures.
Fast reactions occur when these three criteria are easy to meet. If even one is difficult, however, the reaction is typically slow, even though the change in free energy permits a spontaneous reaction.
Rates of reaction increase in the presence of catalysts, substances that provide a new, faster reaction mechanism but are themselves unchanged or regenerated so that they can continue the process. Mixtures of hydrogen and oxygen gases at room temperature do not explode. But the introduction of powdered platinum leads to an explosion as the platinum surface becomes covered with adsorbed oxygen. The bonds of the adsorbed oxygen atoms are stretched, weakening them. This lowers the activation energy for the reaction (the energy required to permit the reaction to occur). Individual oxygen atoms then react rapidly with hydrogen molecules as they collide with them, forming water and regenerating the catalyst. The steps by which a reaction occurs are called the reaction mechanism.
Rates of reaction can be changed not only by catalysts but also by changes in temperature and by changes in concentrations. Raising the temperature increases the rate by increasing the kinetic energy of the molecules of the reactants, and therefore the probability that any given molecule will have more than the activation energy. Increasing the concentration or temperature can also increase the reaction rate by increasing the rate of molecular collisions.
Rates of reaction of solid materials can also be increased by finely dividing the solid. This increases the surface area so that more molecules can collide.
As a reaction proceeds, the concentration of the reactants decreases as they are used up. The rate of reaction will, therefore, decrease as well. Simultaneously, the concentrations of the products increase, so it becomes more likely that they will collide with one another to reform the initial reactants. Eventually, the decreasing rate of the forward reaction becomes equal to the increasing rate of the reverse reaction, and net change ceases. At this point the system is said to be at chemical equilibrium. Forward and reverse reactions occur at equal rates.
Changes in systems at chemical equilibrium are described by Le Châtelier's principle, named after the French scientist Henri Louis Le Châtelier: Any attempt to change a system at equilibrium causes it to react so as to minimize the change. Raising the temperature causes the equilibrium to shift in the endothermic direction; lowering the temperature causes the equilibrium to shift in the exothermic direction. Raising the pressure favours reactions that lower the volume, and vice versa.
The principal goals of synthetic chemistry are to create new chemical substances and to develop better, less-expensive methods for the synthesis of known substances. Sometimes simply purifying naturally occurring substances is sufficient either to obtain an important chemical or to increase use of that chemical as a starting material for other syntheses. For instance, the pharmaceutical industry often uses the complicated organic chemicals found in crude oil as sources of the starting materials in the synthesis of important medicines. More commonly, especially for rare or expensive naturally occurring substances, it is necessary to synthesize the substance from less expensive or more abundant raw materials.
One task of synthetic chemistry, then, is to produce additional amounts of substances already found in nature. Examples are the recovery of copper metal from its ores and the syntheses of certain naturally occurring medicines (such as aspirin) and vitamins (such as ascorbic acid—vitamin C). A second task is to synthesize materials not found in nature, such as steel, plastics, ceramics (space shuttle tiles, for example), and adhesives.
Over 15 million chemical compounds are now catalogued with the Chemical Abstracts Service in Columbus, Ohio; thousands of new ones are synthesized every day. Hundreds of new compounds come into commercial production each year. Each such compound is tested not only for its benefits and intended use, but also for any potentially harmful effects on humans and the environment before it is allowed to go on to the market. Determining toxicity is made difficult and expensive by the wide variation in toxic dose levels among humans, plants, and animals and by the difficulty of measuring the effects of long-term exposure.
Synthetic chemistry was not developed as a sophisticated and highly rigorous science until well into the 20th century. Until then, the synthesis of a substance was often first accomplished by accident, and the uses of these new materials were limited. The sketchy theoretical ideas prior to the turn of the century also limited chemists' ability to develop systematic approaches to synthesis. In contrast, it is now possible to design new chemical substances that fill specific needs (for example, medicines, structural materials, or fuels), to synthesize in the laboratory almost any substance found in nature, to invent and prepare new compounds, and even to predict, based on sophisticated computer modelling, the properties of a “target” molecule and its long-term effects in medicine or in the environment.
Much of the recent progress in synthesis rests on the ability of scientists to determine the detailed structure of a range of substances and to understand the correlations between a molecule's structure and its properties, or structure-activity relationships. In fact, the likely structures and properties of a series of target molecules can now be modelled ahead of their synthesis, giving scientists a better understanding of the types of substances most needed for a given purpose. Modern penicillin drugs are synthetic modifications of the substance first observed in nature by the British bacteriologist Alexander Fleming. More than 1,000 human diseases have been identified as stemming from molecular deficiencies, and many can be treated by remedying that deficiency using synthetic pharmaceuticals. Much of the search for new fuels and for methods of using solar energy is based on the study of the molecular properties of synthetic materials. One of the most recent accomplishments of this type is the fabrication of superconductors based on the structure of complicated inorganic ceramic materials, such as YBa2Cu3O7 and other structurally similar materials.
It is now possible to synthesize hormones, enzymes, and genetic material identical to that found in living systems, thereby increasing the possibility of treating the root causes of human illness by genetic engineering. This has been made easier in recent years by computer-assisted design of syntheses and by the powerful modelling capabilities of modern computers.
One of the most successful recent developments in synthetic biochemistry has been the routine use of simple living systems, such as yeasts, bacteria, and moulds, to produce important substances. The biochemical synthesis of biological materials is now possible. Escherichia coli bacteria, for example, are used to produce human insulin. Yeasts are also used to produce alcohol, and moulds are used to produce penicillin.