How to balance a modern chemical equation

The Law of Conservation of Mass

Chemical equations give information in two major areas.

First, they tell us what substances are reacting (those being used up) and what substances are products (those being made).

Second, the coefficients of a balanced equation tell us in what ratio the substances react or are produced.

This last point has practical consequences whenever chemicals react. For example, the large middle tank of the space shuttle actually has two smaller tanks in it - one holding liquid oxygen and the other holding liquid hydrogen. The tank with the hydrogen holds twice as much as the oxygen-holding tank. Why?

Answer - hydrogen and oxygen react in a 2:1 ratio. For any of oxygen used by the shuttle, twice as much hydrogen is needed. If the two tanks were equal sizes, the hydrogen tank would run dry when the oxygen tank was still half-filled.

The reactants are on the left side of a chemical equation and the products are on the right side.

However, you might ask, "On the left and right side of what?"

Answer - the arrow.

2H_{2}+ O_{2}---> 2H_{2}O

On the left side are the reactants - hydrogen and oxygen. We will ignore the two in front of the hydrogen for a moment.

On the right side is the product - water. We will also ignore the two in front of the water, but we will soon return to it.

Please be aware that there can be one, two, three, or more substances on either side of the arrow, as in this more complex (and unbalanced) equation:

Ca(H_{2}PO_{4})_{2}+ CaSO_{4}+ HF ---> Ca_{10}F_{2}(PO_{4})_{6}+ H_{2}SO_{4}

Typically the arrow is replaced with "produces" or "yields" when the equation is said out loud.

Coefficients are the numbers in front of the formulas.

Here is the example equation again:

2H_{2}+ O_{2}---> 2H_{2}O

Note the presence of a two in front of the hydrogen and also the water. These are called the coefficients. These numbers give two very important pieces of information about the equation. You must understand both in order to read and to use chemical equations successfully.

First: the coefficients give the number of molecules (or atoms) involved in the reaction. In the example reaction, two molecules of hydrogen react with one molecule of oxygen and produce two molecules of water.

Second: the coefficients give the number of moles of each substance involved in the reaction. In the example reaction, two moles of hydrogen react with one mole of oxygen and produce two moles of water.

The point just made is **CRITICAL**.

Let's repeat the point:

The coefficients of an equation tell us how many **moles** of each reactant are involved as well as how many **moles** of each product get produced.

By the way, Avogadro's Number is the factor between the two points above.

Also, what about the oxygen? Why isn't there any coefficient in front of it? Answer - when no coefficient is present, assume a one to be present.

**Quick Review**

There are three important things to remember about reading an equation.

One: Reactants are on the left and products are on the right. Of what? The arrow.

Two: Coefficients are the numbers in front of each formula. If no number is shown, a one is understood.

Three: The coefficients tell us how many molecules (moles) of each reactant used and how many molecules (moles) of each product made.

**Practice Problems**

Identify the reactants, the products and the coefficients of these equations:

1) Zn + 2HCl ---> ZnCl_{2} + H_{2}

2) 2KClO_{3} ---> 2KCl + 3O_{2}

3) S_{8} + 24F_{2} ---> 8SF_{6}

4) 4Fe + 3O_{2} ---> 2Fe_{2}O_{3}

5) 2C_{2}H_{6} + 7O_{2} ---> 4CO_{2} + 6H_{2}O

How to balance a modern chemical equation