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THINK ABOUT IT

The law of conservation of mass states that mass is not lost or gained in a chemical reaction. When you write a chemical equation to describe change, it is important that the equation follows this law. So any equation describing a chemical change must account for every atom involved.

How do you balance atoms in a chemical equation?

To answer this question, you will explore

EXPLORING THE TOPIC

Balancing Chemical Equations

Imagine you’re in business to make ammonia, NH₃, for farmers to fertilize their crops. It would be helpful to know how much nitrogen, N₂(g), and hydrogen, H₂(g), to combine so that nothing is wasted. How do you make sure you have the correct amount of nitrogen and hydrogen so that you have enough of each with no extra?

A chemical equation represents the exact ratio in which reactants combine to form products. A chemical equation that accounts for all the atoms involved and shows them combining in the correct ratio is called a balanced chemical equation. Learning how to balance chemical equations is a necessary part of working with chemical reactions.

FORMATION OF AMMONIA

The unbalanced equation below describes how nitrogen gas and hydrogen gas react to make ammonia gas.

To balance this equation, first take an inventory of the atoms on each side.

Next, balance the atoms on each side by adding units of N₂, H₂, or NH₃. The product side needs one more N atom. The only way to accomplish this is to add a whole ammonia molecule, NH₃, to the product side.

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Take a new inventory. The N atoms are now balanced. Now the reactant side needs four more H atoms. The only way to accomplish this is to add two more molecules of H₂ to the reactant side.

The equation is now balanced. The balanced equation shows that mass is conserved. There are the same numbers of N and H atoms on each side of the arrow.

In the balanced equation above, the 3 in front of the H₂ and the 2 in front of the NH₃ are called coefficients. Coefficients indicate how many units of each substance take part in the reaction. If there is no number in front of a chemical formula in an equation, the coefficient is understood to be a 1.

FORMATION OF RUST

Iron reacts with oxygen in the air to form iron (III) oxide, or rust. Iron (III) oxide is not a molecule. It is a highly organized collection of Fe and O atoms that are bonded to each other ionically. It is an ionic solid. There are a huge number of atoms in any piece of iron (III) oxide. But each piece will have two Fe atoms for every three O atoms. So Fe₂O₃ is the formula unit of iron oxide.

The unbalanced equation below describes how iron reacts with the oxygen in the air to form iron (III) oxide.

Balance the atoms on each side by adding molecules of O₂, atoms of Fe, or formula units of Fe₂O₃. If you add an Fe atom to the reactant side, the Fe atoms are balanced. However, the O atoms are still not equal.

If you add two O₂ molecules to the reactant side and one Fe₂O₃ formula unit to the product side, the oxygen atoms are balanced.

But now the iron atoms are unbalanced again. Add two more iron atoms on the reactant side, increasing the total to four. The balanced chemical reaction is shown here.

It took a few steps, but the equation is finally balanced. Four iron atoms combine with three oxygen molecules to form two formula units of iron oxide.

Coefficients Are Counting Units

Once you have a balanced equation, multiplying all the coefficients by any counting unit will also give a balanced equation. You can multiply the coefficients by a dozen, or a thousand, or a million.

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The coefficients in chemical equations can be multiples of any counting unit. The counting unit that chemists use most often is the mole. We can use moles to count the number of atoms, molecules, or formula units involved in a reaction.

LESSON SUMMARY

How do you balance atoms in a chemical equation?

To conserve mass, the number of atoms on both sides of a chemical equation must be equal. When an equation is balanced, it shows how many molecules, atoms, or formula units of an ionic compound take part in a reaction and how many are produced. You can balance a chemical equation by working with the coefficients in front of the chemical formulas. The coefficients can be any counting unit, including moles. Units of mass or volume cannot be used as coefficients in chemical equations.