Balancing Equations
All matter, for practical purposes, is composed of atoms. They are discrete entities and indivisible under normal circumstances. This has a profound effect on all of chemistry in that any compound must be composed of a whole number of atoms. That is, oxygen may occur as a compound containing 2 oxygen atoms (which is commonly referred to a molecular oxygen) or a compound containing 3 oxygen atoms ( called "ozone") but never as a compound containing 2.5 oxygen atoms. Fractional coefficients can be used in chemical formulas but they have very specific meanings. Discrete molecular entities must always contain a whole number of atoms.
Another important property of atoms is that they can not change into one another. That is, under normal circumstances, an atom of oxygen will always be an atom of oxygen (see nuclear chemistry for exceptions). Certainly within the context of ordinary chemical reactions this must be the case.
The result of these two properties is that balance must always be maintained in chemistry between the reacting species and the products of the reaction. If three oxygen molecules go into a reaction then six oxygen atoms must be found somewhere in the products. Matter is neither created nor destroyed, only transformed.
An example of a balanced equation is the combustion of hydrogen to produce water: 2H2 + O2 = 2H2 O. By convention, this equation is written with whole numbers as the coefficients for the species. This reaction is very simple but it illustrates "balance." Each side of the reaction contains four hydrogen atoms and two oxygen atoms. The same atoms appear on both sides and the reaction balances.
The process of balancing equations is referred to as stoichiometry and is an important aspect of all chemical reactions. The number of atoms in the reactants must always correspond to the number of atoms in the products. However, sometimes balancing reactions can be confusing if there are polyatomic species present. For example, the phosphate ion is a discrete ionic entity, a polyatomic ion, in which the atoms are covalently bound. It is balanced as a single unit much in the same way as the other atoms are treated.
In this case, the number of atoms on each side is the same (i.e. three sodium ions are on each side) but the phosphate ion can be treated as a single species for the purposes of keeping track of the atoms. Thus, each side of the reaction has only a single phosphate group.
A slightly more complicated example is the following reaction:
Again, it is fairly straightforward to verify that there are the same number of each type of atom on each side of the reaction. There are 3Cu, 8N, 8H, and 24O. But the discrete grouping of atoms in the nitrate ion (NO3-) has undergone a reaction to give the nitric oxide (NO) and water. Still, balance is maintained throughout the reaction. Matter is neither created nor destroyed.
Balance, in chemistry, is also found in such things as the total mass of the elements involved in a reaction. This is a simple extension of the principle that all of the atoms must be conserved. It was, in fact, the historical observation that preceded the notion of atoms and balanced equations. By careful measurement of the mass of reactants before a reaction and the mass of the products after, chemists were able to devise the law of definite proportions and the notion that matter is conserved.
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