Multiple oxidation states

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.

Other 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

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).

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

Fact entries

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.

Date: 2015-12-11; view: 2093