When an electric current is passed through acidified water, the bonds between the atoms break, forming ions of hydrogen and oxygen. The positively charged hydrogen ions migrate to the negative electrode, where they acquire electrons and are reduced to hydrogen gas-new molecules of H2. Negatively charged ions migrate to the positive electrode, where they release electrons and oxygen gas (02) is liberated. (This process, discovered in 1800 by two English scientists, William Nicholson and Anthony Carlisle, was called electrolysis by English chemist Michael Faraday (1791-1867). If the gases produced in this way are mixed, and then a spark is applied, the mixture explodes and the gases recombine to form water. Both changes, the decomposition and the recombination, are examples of chemical reactions, in contrast to changes in physical state. They illustrate two important principles:
■ Substances will change into entirely different substances if conditions are right. Such changes are called reactions. When a chemical reaction occurs, the molecules at the end of the reaction are different from the molecules at the beginning.
■ Chemical reactions can be reversed. As in the case of water, a compound can be changed into different compounds and these can then be changed back into the original compound. Under the right conditions, nearly all chemical reactions are reversible.
Molecules and energy
When a current of electricity passes through acidified water, electrical energy breaks up the water molecules into ions, which are then either oxidized or reduced to form diatomic molecules of hydrogen and oxygen. Avogadro was the first to propose the diatomic nature of these gases. If a spark—energy in the form of heat and light—is applied to a mixture of these gases, a reaction is triggered, causing the gases to recombine to form water. The reaction releases energy explosively, in the form of light, heat, and sound.
Some chemical reactions occur instantly—or appear to. For example, the reaction time is very short when dynamite explodes or a base neutralizes an acid. Rapid reactions often occur when the products of the reaction are chemically more stable than the reactants— that is, when their molecules contain less energy than the molecules of the reactants. Several factors determine the energy content of a molecule, so an instantaneous reaction does not always give off recognizable energy. Nevertheless, many reactions do give off measurable, usable energy. An example is the combustion of natural gas: when gas burns, its molecules oxidize, releasing energy for heating homes and cooking food.
Energy is released when gas burns, but is the reaction instantaneous? Gas does not begin to burn by itself. The reaction occurs only when the gas is mixed with air and a spark is applied. Why?
When a reaction occurs, chemical bonds break and new ones form. In the electrolysis of water, the bonds between hydrogen and oxygen atoms break, and new bonds form between pairs of hydrogen atoms and pairs of oxygen atoms. Natural gas consists mostly of methane, a molecule containing one atom of carbon bonded to four atoms of hydrogen.
The bonds between the carbon and hydrogen break when heat—the spark—is applied because there is oxygen for the carbon to bond with, forming carbon dioxide. The hydrogen combines with oxygen to form water.
What happens in a chemical reaction is similar to a mathematical equation. Both sides of an equation must balance. Likewise, in a reaction, the reactants and the products must contain the same number and types of atoms, though differently connected to form different molecules. This, in essence, is Lavoisier’s Law of Conservation of Mass. In 1789, the French scientist wrote:”… in all the operations of art and nature, nothing is created; an equal quantity of matter exists both before and after the experiment; the quality and quantity of the elements remain precisely the same; and nothing takes place beyond changes and modifications in the combination of the elements.” Another way of stating this law is: The weight of the products of a chemical reaction is exactly equal to the weight of the reacting substances.
But a chemical reaction is a little more complicated than an equation. First, an intermediate or transition state forms. Materials in this state are so unstable that they break down rapidly—they are more energetic than either the reactants or the products. They form because energy has been supplied to start the reaction. Once it begins, a reaction also releases energy. When a few molecules absorb enough energy to reach the transition state, they then release more energy as they decompose into the products of the reaction. The energy they release is then absorbed to create more transition state molecules, and the process continues. A reaction that releases a lot of energy can be triggered with a small energy input and will proceed explosively, like the reaction in the cylinder of a gasoline engine produced by a spark in the air-fuel mixture.
Applying a spark to a piece of paper is not usually enough to generate a self-sustaining reaction. But a lighted match will do the trick— once the paper catches fire, it burns until it is consumed. Reacting with the oxygen in the air, paper molecules release enough energy to trigger the reaction of a few more molecules, but not so many that the reaction occurs explosively—instantaneously. This shows the importance of temperature as a factor controlling the speed of a reaction. Another such factor is the catalyst A catalyst is a substance that changes the rate of reaction of other substances but is not itself consumed in the process. A catalyst can lower the amount of energy needed to start a reaction, thus allowing it to proceed more readily.
Heat and electricity are two forms of energy that can trigger a chemical reaction. Some reactions can also be triggered by light.
The transition state between reactants and products is the key to why reactions are reversible. Supply enough energy to the reactants and some of the molecules reach the transition state. These transition molecules can then break down into products or revert back to reagents. If product molecules are more stable than reagent molecules, then product molecules will accumulate overtime. If enough energy is lost during a reaction, the products may not have enough energy to reform intermediate molecules, and the reaction will not reverse itself. But if enough energy is added to compensate for the loss, the reaction can reverse itself. For example, a piece of iron gradually rusts, forming iron oxide. But if the rust is heated with carbon in a furnace, it decomposes to metallic iron. Application of heat energy reverses the reaction that turned the iron into rust.
In 1863, French chemist Pierre Berthelot (1827-1907) studied a reversible reaction that demonstrated an interesting process called esterification. Berthelot mixed an alcohol with an acid to produce an ester—ethyl acetate—as follows:
Whenever a reaction occurs that adds oxygen to a compound, it is considered an oxidation reaction, and whenever oxygen is removed from a compound, the reaction is considered reduction. It is the change in electrical charge of an atom that determines whether it has been oxidized or reduced. If it loses valence electrons, thereby changing from neutral to positive or from positive to more positive, its oxidation state or oxidation number is said to have increased. If, by gaining valence electrons, it changes from neutral to negative or from negative to more negative, its oxidation state has decreased. Thus, an oxidation reaction, regardless of whether oxygen is involved, is defined as a reaction resulting in an increase in oxidation state, and a reduction reaction is defined as a reaction resulting in a decrease in oxidation state. An electrically neutral atom has an oxidation state of zero.
Water can be produced by setting a spark to an oxygen-hydrogen mixture. During this explosive reaction, the oxidation state of the hydrogen atoms changes from zero to +1, which means the atoms have been oxidized. At the same time, the oxidation state of the oxygen atoms changes from zero to -2, so they have been reduced. For water to form, reduction and oxidation reactions must occur simultaneously. This is also true of many more complex reactions.
Oxidation-reduction reactions have practical applications—in batteries, for example. Storage batteries, also called wet cells, turn the energy of a chemical reaction into electrical energy, and vice versa. By establishing chemical pathways between certain metallic elements, such as zinc and copper, they trigger simultaneous oxidation-reduction reactions that generate a flow of electrons. In zinc-copper galvanic cells, zinc goes into solution in the electrolyte of one cell, releasing electrons—the oxidation reaction. The electrons flow through a wire to a copper rod immersed in a cell containing a solution of copper ions. The ions take up the electrons and become electrically neutral—the reduction reaction. Electrons flow from one cell to the other with a little more than 1 volt of electromotive force, with the two reactions—or more accurately, half-reactions—proceeding as follows:
Zn -» Zn++ + 2 electrons (oxidation)
2 electrons + Cu++ Cu (reduction)
Or, to produce the following overall reaction:
Zn + Cu++ -» Zn-H- + Cu
If the situation is reversed and electrical energy is put into the system, chemical work is done. The electrolysis of molten salt (NaCI) illustrates this, as well as demonstrating oxidation without oxygen. Salt consists of sodium ions with an oxidation number of +1 and chlorine ions with an oxidation number of-1. Electrolysis separates the ions—breaking the ionic bond and producing electrically neutral sodium atoms, their oxidation number reduced from +1 to zero—a reduction reaction. Simultaneously, the chlorine ions undergo an increase in oxidation number, from -1 to zero—an oxidation reaction without oxygen:
2 Na+ + 2 Cl- 2 Na + Cl2t
This is one of the ways in which chlorine is now produced commercially.
Multiple oxidation states
An element that can exist in a variety of different oxidation states can enter into a wide variety of combinations with other elements. For example, nitrogen forms a whole series of compounds with oxygen and hydrogen-oxides and hydrides—as shown in the accompanying diagram. In these compounds, nitrogen’s oxidation state takes on every value between +5 (in dinitrogen pentoxide, N205) and -3 (in ammonia, NH3), including 0 (in free, elemental nitrogen, N2).
Eight oxides of nitrogen are known; all contain nitrogen and oxygen in various proportions. Nitrous oxide (N20), a colorless, unre-active gas sometimes called “laughing gas,” is used as an anesthetic, especially in dentistry. Both N20 and nitrogen dioxide (N02) are used to make nitric acid, an ingredient in the production of fertilizers, drugs, and explosives. The six other oxides of nitrogen are unstable and of little or no practical use.
Ammonia (NH3), one of the most important compounds of nitrogen, is easily and cheaply produced by combining nitrogen with hydrogen from natural gas, using high pressures and temperatures as well as a catalyst. Ammonia is produced in large quantities for making fertilizers that increase the yield and quality of crops. In some farming areas, the gas is injected directly into the soil. Ammonia is also used to make explosives such as TNT (trinitrotoluene) and nitroglycerin. In the textile industry, it is used to make synthetic fibers such as nylon and rayon, and in dyeing wool, cotton, and other natural fibers. Ammonia absorbs a lot of heat when changing from a liquid to a gas, so it is also used in refrigeration equipment.
Another hydride of nitrogen—hydrazine (N2H4)—is an important compound with many uses. Colorless, unstable, and corrosive, it is a major ingredient in jet and rocket fuels and is also used to make chemicals for agriculture and the textile industry, photography, and explosives. Flydrazine is a raw material in the manufacture of pesticides, herbicides, and drugs, as well as foam rubber and plastics.
Other chemical reactions
Most chemical reactions take place between compounds rather than between elements or elements and compounds. In such cases, it is often possible for atoms to exchange the other atoms or groups of atoms to which they are attached. For example, there are two compounds known as sodium sulfate and barium chloride. Sodium sulfate consists of two atoms of sodium combined with a sulfate. Barium chloride consists of an atom of barium combined with two atoms of chlorine. When these two compounds react with each other, they are said to “decompose” into two new compounds. The sulfate stays intact and attaches itself to the barium. The sodium, in turn, attaches to the chlorine and forms sodium chloride, common salt. Such “double decomposition” reactions, as they are called, depend on the ionic character of the compounds in solution.
An atom in a normal or neutral state has the same number of electrons as it does protons.
If there are more electrons than protons, the atom (called an anion) carries a negative electric charge. If there are less electrons than protons, the atom (called a cation) carries a positive electric charge. An electrically charged atom (or group of atoms) is known as an ion. The attraction of cations to anions and vice versa is very important in chemical reactions.
In the reaction involving sodium sulfate and barium chloride, the ions of each compound split apart under the influence of water. The barium and sulfate ions combine very strongly and precipitate (leave the solution). These two qualities ensure that the reaction goes to completion. This means that the conversion of one compound into another is effectively complete.
A common type of double decomposition occurs when an acid is mixed with a base. In general, acidic compounds (acids) are those that can form hydrogen ions (positively charged hydrogen atoms) in a solution, usually of water. Bases (or alkalis, as the common ones are also called) form hydroxyl ions. These are negatively charged molecules consisting of one oxygen atom and one hydrogen atom. The acid strength of a solution is measured in terms of the concentration of hydrogen ions and is expressed as a pH number. A pH of less than 7 indicates an acid; greater than 7, a base.
When acids mix with bases, they are said to neutralize each other. In such acid-base neutralizations, the products are always a salt (the common name for a compound composed of ions) and water.
A reaction in which a single reactant breaks down into more than one product is known as a decomposition reaction. For example, heating the compound calcium carbonate produces carbon dioxide gas and quicklime.
If two reactants break down into other products, the reaction is known as a double decomposition reaction. An example of such a reaction is one involving an acid and a base. These compounds react to form a salt and water.
The opposite of decomposition is an addition reaction. The reactants join together instead of breaking down. There is no removal of any part of the reacting molecule. Such a reaction is common in organic chemistry.
If a reaction requires a continuous source of energy (usually heat) to keep it going, it is termed endothermic. In other cases, once a reaction starts, it produces enough energy to sustain itself. There is no need for continued external supplies of energy. Such reactions are termed exothermic.
In some cases, a molecule may break down only partly. It then reforms chemical bonds in a different manner. Such reactions are called rearrangements. They are often significant in synthetic chemistry (organic chemistry) and in the chemistry of living systems (biochemistry).