Chemical Reaction

I

Introduction

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.

II

Chemical Equations

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.

III

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 ^- ⇋AgCl, or 3 Ca²⁺+ 2 PO₄^3-⇋ 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 (see Acids and Bases). Covalent single bond changes in which one bonding electron comes from (or goes to) each reactant are called radical reactions, as in H· + ·H ⇋H8H.

Sometimes reactants gain and lose electrons, as in oxidation-reduction, or redox, reactions: 2 Fe²⁺ + Br₂⇋ 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⁺ + 2SO₄ ^2- → 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. See Polymer.

IV

Chemical Energetics

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. See Chemistry, Physical; Thermodynamics.