A chemical reaction has reactants on the left side of the arrow and products, the new materials formed, on the right of the arrow.

Reactants → products

You can think of a chemical reaction as a rearrangement of atoms. In the reactants, bonds are broken, and new bonds are formed in the products. The Law of Mass Conservation states: Matter is neither created nor is it destroyed. The chemical reaction must have the same number and same types of atoms on both sides of the equation.

To balance a chemical reaction

1. Write the unbalanced equation using the correct chemical formulas
2. Add stoichiometric coefficients to balance atoms– never change subscripts
3. Once balanced, check that coefficients are reported to their smallest whole number value

A chemical reaction we are all familiar with is the combustion of methane, natural gas, in oxygen. Assume complete combustion.

CH₄ (g) + O₂ (g) → CO₂ (g) + H₂O (g)

Methane gas burns in oxygen to produce carbon dioxide gas and water vapor. Before we move on, there is one problem with the reaction. There are different numbers of each type of atom on both sides of the equation. We need to balance the chemical reaction. In order to balance a chemical reaction, you can change the coefficients. For our combustion reaction we can balance the carbon atoms, hydrogen atoms, and the oxygen atoms. It is always best to leave the oxygen atoms for last. We see from the equation, there is one carbon on both sides of the equation, therefore, the equation is balanced for carbon. There are 4 hydrogens on the reactant side, and only 2 hydrogens on the product side. We can put a 2 in front of the water vapor on the product side.

CH₄ (g) + O₂ (g) → CO₂ (g) + 2 H₂O (g)

Now, we have 4 hydrogen atoms on each side of the chemical equation. Next we balance for oxygen. We have two O atoms on the reactant side and 4 O atoms on the product side–2 O atoms in both CO₂ and 2 H₂O. We can place a 2 in front of O₂ on the reactant side.

CH₄ (g) + 2 O₂ (g) → CO₂ (g) + 2 H₂O (g)

We now have 1 C atom, 4 H atoms, and 4 O atoms on both sides of the equation. The chemical equation is now balanced.

Worksheet: Balancing Chemical Equations

Please see our tutorial on Balancing Chemical Equations

Before working on the exercises, the following video might be of interest.
Balancing Chemical Equations
 

Exercises

Exercise 1. Balance the following chemical equations.

 

Exercise 2. Ammonia gas and molecular oxygen gas react to produce nitrogen monoxide gas and liquid water. Write a balanced chemical equation.

Exercise 3. Consider the following chemical equation and interpret it in words (Make sure you balance the equation first).
Fe₂O₃ (s) + C (s) → Fe (s) + CO₂ (g)

Exercise 4. Element A (red spheres) reacts with Element B (blue spheres). Write a balanced chemical equation for this process.

View Solutions to Exercises
 
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