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.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.