2H2 O2 2H2O
Fundamentals of Chemistry
Na CO + Ca(OH) NaOH + CaCO
There are two Na atoms on the left so start with the Na by writing
Na CO + Ca(OH) 2NaOH + CaCO (guideline b).
By adding the 2, the equation is now completely balanced. This equation illustrates that
not all equations are that hard to balance.
Most chemical equations do not indicate a number of important facts about the chemical
reactions they represent. Chemical equations do not necessarily describe the path by which the
substances reacting are converted to products.
The equation would seem to imply that two molecules of hydrogen collide with one molecule
of oxygen, and two molecules of water are produced. The actual mechanism by which this
reaction takes place is much more complicated and involves a series of processes. Chemical
equations do not indicate the rate at which the reaction proceeds, or even whether the reaction
will occur in a finite time. In many cases, reactions will occur only under a particular set of
circumstances and then only at a definite rate. Chemical equations do not show whether the
reaction proceeds to completion or, if incomplete, the extent of reaction. In most cases, the
substances that react never completely disappear; however, their concentration may be
exceedingly small. Reactions that do not go to completion are usually represented in chemical
equations by using double horizontal arrows . In general, a reaction will go to completion only
if one or more of the products is removed from the field of the reaction. This is often
accomplished if one of the products is a gas or is insoluble in the reaction mixture.
In the discussion of chemical equations, emphasis is normally placed on the number of atoms or
molecules involved in the reaction. However, chemical equations are very effective in
representing chemical reactions on a macroscopic scale. Practical chemical calculations involve
very large numbers of atoms and molecules.