Chemical Equation

chemical equation, group of symbols representing a chemical reaction.

Basic Notation Used in Equations

The chemical equation 2H₂+O₂→2H₂O represents the reaction of hydrogen and oxygen to form water. The arrow points in the direction of the reaction—from the reactants (substances that react) toward the product or products. In this case the reactants are hydrogen (written H₂ because each molecule consists of two atoms of hydrogen) and oxygen (written O₂ because each molecule consists of two atoms of oxygen) and the product is water. The coefficient 2 before the H₂ indicates that two molecules of hydrogen take part in the reaction, and the 2 before the H₂O indicates that two molecules of water are produced. When no number is written, as in front of the O₂, a one is assumed; one molecule of oxygen takes part in the reaction. The equation shows that two molecules of hydrogen react with one molecule of oxygen to form two molecules of water. Because of the relationship between molecules and the mole, the equation also shows that two moles of hydrogen react with one mole of oxygen to form two moles of water. The same sort of relationship holds with the gram-formula weight.

Methodology for Writing an Equation

There are three steps involved in writing a chemical equation. The first step is to decide which substances are the reactants and which are the products. For example, natural gas (cooking gas) burns in air, providing heat and producing no visible products. The natural gas is principally methane, and the portion of the air that reacts (supports combustion) is oxygen. These are the reactants. Products of the reaction are heat and two invisible gases, carbon dioxide and water vapor. We can now write the word equation methane + oxygen → carbon dioxide + water vapor + heat. The next step is to determine the correct formula for each substance and substitute it for the name. The equation now becomes CH₄+O₂→CO₂+H₂O. (A notation for heat is often omitted.)

The final step is to balance this equation. As the equation is now written, three oxygen atoms are produced from two, and four hydrogen atoms become only two. This cannot occur, since atoms are not created or destroyed in chemical reactions. The equation is already balanced for carbon, since there is one carbon atom on the reactant side and one carbon atom on the product side. There are four hydrogen atoms in the methane molecule on the reactant side, so there must be four hydrogen atoms in water molecules on the product side (since water is the only product containing hydrogen); thus there must be two water molecules, each containing two hydrogen atoms. The equation can now be written CH₄+O₂→CO₂+2H₂O. It is not yet balanced, since there are only two oxygen atoms shown as reactants and four as products. The equation is completely balanced by showing two oxygen molecules (four atoms) as reactants: CH₄+2O₂→CO₂+2H₂O.

Additional Symbols Used in Chemical Equations

There are a number of other symbols used in chemical equations. A symbol written above or below the reaction arrow indicates special reaction conditions. For example, when mercuric oxide is heated it decomposes into mercury metal and oxygen gas; this reaction is shown by the equation 2HgO [right arrow over Delta] 2Hg + O₂↑. The Greek letter delta under the arrow represents the heating. The upward-pointing arrow after the O₂ indicates that this product is gaseous and escapes. When a precipitate is formed by a reaction, the substance that precipitates is often followed by a downward-pointing arrow, e.g., AgNO₃ + NaCl [arrow with H₂O over it] AgCl↓ + NaNO₃. The H₂O above the arrow shows that the reaction takes place in the presence of water—in this case, in water solution. The formulas AgNO₃, NaCl, and NaNO₃ do not represent molecules, since these substances are almost completely ionized in water solution (see ion).

When chemical equilibrium occurs in a reaction, the double arrow is used instead of the single arrow. For example, liquid water dissociates to form hydronium ions (H₃O⁺) and hydroxide ions (OH⁻). These ions exist in equilibrium with water molecules. The equation is 2H₂O [double arrow with H₂O over it] H₃O⁺ + OH⁻. The sign = is sometimes used in place of the double arrow.

Bibliography

See J. B. Dence, Mathematical Techniques in Chemistry (1975).

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chemical equation

Method of writing the essential features of a chemical reaction using chemical symbols (or other agreed-upon abbreviations). By convention, reactants (present at the start) are on the left, products (present at the end) on the right. A single arrow between them denotes an irreversible reaction, a double arrow a reversible reaction. The law of conservation of matter (see conservation law) requires that every atom on the left appear on the right (the equation must balance); only their arrangements and combinations change. For example, one oxygen molecule combining with two hydrogen molecules to form two water molecules is written 2H₂ + O₂ → 2H₂O. The dissociation of salt into sodium and chloride ions is written NaCl → Na⁺ + Cl⁻. See also stoichiometry.

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Chemical Equation 

a statement of a chemical reaction in the form of chemical signs, chemical formulas, numbers, and mathematical signs. Although the possibility of such statements on the basis of the law of conservation of mass was suggested by A. Lavoisier in 1789, chemical equations came into general use only in the first half of the 19th century.

A chemical equation consists of a left side and a right side joined together by an equal-sign. Sometimes, the symbol → is used in place of an equal-sign in order to indicate the direction of a reaction, with double arrows (⇄) used in the case of reversible reactions. The formulas for the reactants are written on the left side, and those for the products are written on the right; + signs are placed between the formulas on each side of the equation.

In writing chemical equations, the mass of the products must be equal to that of the reactants, and the number of atoms of any given element must be the same on both sides of the equation. Coefficients, which must be whole numbers, are placed in front of the formulas for the reactants and products. For example, knowing that water and carbon dioxide are formed from the combustion of methane in oxygen, the chemical equation for this reaction may be written immediately:

(1) CH₄ + 2O₂ = 2H₂O + CO₂

In more complicated cases, other methods are used, such as those for redox reactions (seeOXIDATION-REDUCTION REACTION) and those based on the solution of systems of indeterminate equations. For example, assume that the coefficients in the chemical equation for the roasting of pyrite (FeS₂) in oxygen are sought:

(2) xFeS₂ + yO₂= zFe₂O₃ + tSO₂

It is obvious that x = 2z, t = 2x, and 2y = 3z + 2t. Letting z = 1, we have x = 2, t = 4, and y = 5.5. Multiplying these numbers by 2, we obtain

4FeS₂ + 11O₂ = 2Fe₂O₃ + 8SO₂

Chemical equations serve as a basis for calculations required in the laboratory and in industry.

REFERENCE

Nekrasov, B. V. Osnovy obshchei khimii, 3rd ed., vol. 1. Moscow, 1973.

S. A. POGODIN