A chemical equation is a shorthand method for representing the changes that take place during a chemical reaction. In describing the formation of water from its elements, a chemist could say, for example, that "two molecules of hydrogen gas combine with one molecule of oxygen to form two molecules of water." Or she could write the following chemical equation that contains the same information in a much more compact form:
2 H 2 + O 2 → 2 H 2 O
At the minimum, a chemical equation contains the chemical symbols and formulas for the elements and compounds involved in the reaction and the + and → signs that indicate reactants and products. The term reactants refers to the substances present at the beginning of the reaction, and the term products refers to the substances formed in the reaction.
In the example above, the reactants are represented by the symbols H for hydrogen and O for oxygen. The product is represented by the formula H 2 O for water. The + sign indicates that hydrogen (H) has combined with oxygen (O) in the reaction. The → indicates that the two have reacted with each other to form water.
One of the fundamental laws of chemistry is the law of conservation of matter. That law says that matter can be neither created nor destroyed in an ordinary chemical reaction. In terms of the above reaction, the law means that there must be the same number of hydrogen atoms and oxygen atoms at the beginning of the reaction and at the end of the reaction.
The coefficients in the chemical equation assure that this condition is true. The coefficients are the numbers in front of the chemical symbols or formulas: 2 H 2 and 2 H 2 O. One of the skills that beginning chemistry students need to learn is how to select the correct coefficients in order to make sure the equation obeys the law of conservation of matter. Choosing those coefficients is called balancing the chemical equation.
Balancing an equation: The process of selecting coefficients for symbols and formulas in a chemical equation to make sure that the law of conservation of matter is not violated.
Chemical reaction: Any chemical change in which one new substance is formed.
Chemical symbol: A letter or pair of letters that represent a specific quantity of a chemical element.
Coefficient: A number selected for use in balancing a chemical equation. Coefficients are placed in front of the chemical symbols and formulas in an equation.
Products: The substances formed in a chemical reaction.
Reactants: The substances present at the beginning of a chemical reaction.
Most chemical equations contain other symbols (in addition to chemical symbols) that provide further information about the reaction. The subscript 2 in the symbols for hydrogen and oxygen (H 2 and O 2 ), for example, tells that each molecule of hydrogen and oxygen consists of two atoms of the element.
Other symbols are often used to indicate the physical state of the substances in the reaction. The same reaction for the formation of water can also be represented as:
2 H 2 (g) + O 2 (g) → 2 H 2 O (l)
The symbols g and l tell us that hydrogen and oxygen are gases and water is a liquid. Other symbols used for this purpose include (s) for solids and (aq) for substances dissolved in water (the aq ueous condition). Upward and downward pointing arrows (↑ and ↓) can also be used to indicate the formation of gases and precipitates (solids), respectively, during a chemical reaction.
Finally, symbols can also be used to indicate the gain or loss of heat in a chemical reaction. Nearly all reactions are accompanied by such changes, and they can be represented by means of the symbol ΔH. In the case of the reaction above, for example, the complete reactions could be written as:
2 H 2 (g) + O 2 (g) → 2 H 2 O (l)
ΔH = −571.6 kJ
In this case, the added information, ΔH = −571.6 kJ tells us that 571.6 kilojoules of heat energy were given off during the formation of water from its elements. (A joule is the metric unit of measurement for energy. One kilojoule is 1,000 joules.)
The use of chemical equations is absolutely essential in dealing with any discussion of chemical reactions. It would be completely unreasonable for chemists to describe chemical reactions in English sentences, as indicated in the first paragraph of this entry. Thus, all reports of chemical research, books and articles on chemical topics, and any other written commentaries on chemistry all include chemical equations.
[ See also Reaction, chemical ]