CHEMICAL EQUATIONS
DOE-HDBK-1015/1-93
Fundamentals of Chemistry
CH-01
Rev. 0
Page 40
Chemical Equations
The chemical equation is simply a representation of a chemical reaction in the chemist's
shorthand. In a chemical equation, the substances originally present are called the reactants and
the new substances being formed are called the products. In the chemical reaction, the reactants
are on the left of the arrow and the products are on the right of the arrow. For example, when
looking at the combination of hydrogen and oxygen into water, the water molecule would be
the product on the right.
2H + O 2H O
2
2
2
If looking at the decomposition of water, the hydrogen and oxygen would be the products and
the water is the reactant. The arrow means yields and is used to separate the reactants from the
products. It also indicates the direction of the reaction as illustrated below.
2H O 2H + O
2
2
2
The single arrow in the above equation implies that the reaction will proceed in only one
direction until it subsides or stops. If the reaction is reversible, that is, it can proceed left to
right and right to left until it reaches an equilibrium, then a double arrow is used.
2H O ? 2H + O
2
2
2
When writing an equation, always place the reactant on the left and the products on the right
even in the case of a reversible reaction.
A chemical equation represents not only the reaction, but also shows the number of atoms or
molecules entering into and produced by the reaction. The formulas must be balanced correctly
based on the valences of the constituent elements.
Balancing Chemical Equations
The number of atoms or molecules of each substance is shown by the coefficients in the
equation. Because atoms cannot be created or destroyed in a chemical reaction, a chemical
equation must be balanced so that there are exactly the same number of atoms of each element
on each side of the equation.
