Balance of Chemical Equations

Suppose you want to write an equation to explain a chemical reaction that has just been performed in the laboratory. How do you proceed? Since the reactants are known, it is possible to write their chemical formulas. It is more difficult to establish the identity of the products. It is often possible to predict the product(s) of simple reactions. In more complicated reactions in which there are three or more products, chemists will need other tests to establish the presence of specific compounds.

Once the reactants and products have been identified, and their correct formulas written, they are arranged according to the conventional sequence: reactants on the left, separated by an arrow from the products, which are placed on the right-hand side. It is very likely that the equation that has been written at this point is not “balanced”, that is to say that the number of each type of atoms is different on both sides of the arrow. In general the balance of a chemical equation is verified by the following steps:

All reactants and products are identified, and their correct formulas are written on the left and right side of the equation, respectively.

The balancing of the equation is started by trying different coefficients to equal the number of atoms of each element on both sides of the equation. You can change the coefficients (the numbers that precede the formulas), but not the subscripts (the numbers that are part of the formulas). If you change the subscripts, you change the identity of the substance. For example, 2NO2 means “two molecules of nitrogen dioxide”, but if you double the subscripts you have N2O4, which is the formula for dinitrogen tetroxide, a totally different compound. First we look for the elements that appear only once on each side of the equation and with the same number of atoms: the formulas containing these elements must have the same coefficient. Therefore it is not necessary to adjust the coefficients of these elements at this point. Next, the elements that appear only once on each side of the equation but with different number of atoms are searched for. The balance of these elements is made. Finally, that of the elements that appear in two or more formulas on the same side of the equation.

The obtained equation is checked to make sure that there is the same total number of each type of atoms on both sides of the equation.
Consider a specific example. In the laboratory, small amounts of gaseous oxygen can be prepared by heating potassium chlorate (KClO3). The products are gaseous oxygen (O2) and potassium chloride (KCl). From this information, it is written:

(For simplicity the physical states of reactants and products are omitted.) The three elements (K, Cl and O) appear only once on each side of the equation, but only K and Cl have equal numbers of atoms on both sides of the equation. Thus, KClO3 and KCl must have the same coefficient. The next step is to make the number of O atoms equal on both sides of the equation. Since there are three O atoms on the left side and two on the right side of the equation, these atoms are equalized by placing a 2 to the left of KCl03 and a 3 to the left of O2.

As a final check, a balance sheet can be made for reactants and products where the numbers in parentheses indicate the number of atoms of each element:

Note that the balance of this equation can also be done with coefficients that are multiples of 2 (for KCIO), 2 (for KCl) and 3 (for 02); for example.

However, the simplest set of integer coefficients is used for balancing an equation. The equation satisfies this convention.

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