Let us look at some molecules in isolation. Nitrogen can form compounds with oxygen as well as hydrogen. Ammonia consists of molecules in which a single nitrogen atom is attached to three hydrogen atoms. Nitrogen is said to have an oxidation number of —3 in ammonia. In another compound known as dinitrogen pentoxide, two nitrogen atoms are attached to five oxygen atoms. In this case, the nitrogen's oxidation number is +5. If an element can exist in many different oxidation states, it increases the number of possible combinations it can form with other atoms.
Atoms, elements, and molecules: Key chemical reactions 19
Thus, nitrogen can form a whole range of compounds with oxygen and hydrogen. In these compounds, nitrogen's oxidation state takes on all possible values between +5 (in dinitrogen pentoxide) and —3 (in ammonia), including 0 (in free nitrogen gas).
To demonstrate the point made earlier that an oxidation reaction need not involve oxygen, consider the behavior of molten sodium chloride (common table salt). While bound to each other in this compound, sodium has an oxidation number of +1. Chlorine has an oxidation number of —1. Passing an electric current through the sodium chloride breaks up the compound into its separate elements—sodium and chlorine. The sodium atom goes through a reduction reaction. Its oxidation number decreases from +1 to the neutral state, 0. However, the chlorine atom goes through an oxidation reaction. Its oxidation number increases from —1 also to the neutral state, 0. An
oxidation-reduction reaction has occurred without involving oxygen.
A practical application of oxidation-reduction reactions is in the area of electric cells or batteries. Combining certain known elements or some stable compounds produces a predictable oxidation-reduction reaction. This reaction, in turn, generates a voltage, that is, electrical power. Such electricity is the basis of electric cells or batteries.
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
Combustiontakes many forms, all of which are examples of oxidation. Burning (above/ is a comparatively slow reaction in . chemical terms, although often extremely destructive. Even the explosively fast reactions involved in firing a pistol (below! take place in stages. The purple percussion cap at the base of the yellow cartridge (A) is detonated by a hammer or firing pin (B). This ignites the pro-pellant charge, which "burns" to produce a hot gas (CI. The gas then drives the bullet out of the barrel (D).
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20 Atoms, elements, and molecules: Key chemical reactions
Smelting of metals(above) involves chemical reduction. This often requires large amounts of energy to "drive" the reactions. An example is the conversion of iron oxide to metallic iron. The corresponding oxidation reaction, as in the rusting of iron(above right), requires less energy. It proceeds spontaneously in moist air.
of sodium combined with a sulfate (a sulfur atom with four oxygen atoms). 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. See the first article in this section for a fuller discussion of atoms. 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, one of 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
Chemical reactionFor a
reaction to occur between two different molecules, they must come very close together. If they have enough energy to reach a transition state, they may then react. When molecules are heated, they move around more freely. If they are also in an enclosed space, the chance of collisions with other molecules increases. All these factors encourage reactions to happen.