Balancing Chemical Equations

A chemical equation describes how molecules (called reactants) combine to produce other molecules (called products).

N₂ + H₂ –> NH₃

For example, the equation above shows that nitrogen gas (N₂) combines with hydrogen gas (H₂) to produce ammonia (NH₃).

Unfortunately, this is an unbalanced chemical equation. Count the atoms on each side of the arrow – there are 2 nitrogens (N) and 2 hydrogens (H) on the left, but one nitrogen and three hydrogens on the right. An unbalanced chemical equation gives us the ingredients, but not in the correct proportions so it doesn't actually work. Here's another example that might help:

egg + milk + flour –> pancake

How many eggs? How much flour? How many pancakes? We can't tell yet. Balancing the chemical equation lets us know how much of each reactant (or ingredient) makes how much of each product (or pancake).

Let's return to our original chemical equation. To make it a balanced chemical equation, we need to adjust the number of molecules used on one or both sides.

Let's start with nitrogen (N). There's two on the left and one on the right, so let's use two molecules on the right by placing a 2 in front of the molecule. (This is called a coefficient.) This gives us two nitrogens on both sides.

N₂ + H₂ –> 2NH₃

Now let's look at hydrogen (H). There are two on the left, and 6 on the right. (That's two molecules with three hydrogens each: 2×3=6.) If we use three molecules on the left, we'll get 3×2=6 hydrogens on the left to match the 6 on the right.

N₂ + 3H₂ –> 2NH₃

Count them up now – both sides have 2 nitrogen and 6 hydrogen. This is a balanced chemical equation.

In another example, consider this equation where methane (CH₄) and oxygen gas (O₂) combine to produce carbon dioxide (CO₂) and water (H₂O):

CH₄ + O₂ –> CO₂ + H₂O

Again, count up the atoms on each side. On the left we have 1 carbon (C), 4 hydrogen (H), and 2 oxygen (O); on the right we have 1 carbon (C), 2 hydrogen (H), and 3 oxygen (O).

So far, the carbon's look good but we need more hydrogens on the right. If we increase the coefficient on the water (H₂O) molecule to 2, we have:

CH₄ + O₂ –> CO₂ + 2H₂O

Now, on the left we have 1 carbon (C), 4 hydrogen (H), and 2 oxygen (O); on the right we have 1 carbon (C), 4 hydrogen (H), and 4 oxygen (O). Carbon and hydrogen are good, but we need more oxygen on the left side. If we increase the coefficient on the oxygen gas (O₂) molecule to 2, we have:

CH₄ + 2O₂ –> CO₂ + 2H₂O

Now, on the left we have 1 carbon (C), 4 hydrogen (H), and 4 oxygen (O); on the right we have 1 carbon (C), 4 hydrogen (H), and 4 oxygen (O). This is a balanced chemical equation.

To review, a balanced chemical equation gives both the ingredients and the proportions for combining the reactants into the products. You can balance a chemical equation by adjusting the number of molecules on either side of the reaction until the number of each type of atom is the same on both sides.

Get More Help!

Click one of these links to get more help from another Cramlet in this same chapter:

• Study Sheet of Molarity and Stoichiometry
• Bottomless Worksheet of Mole Conversions
• Bottomless Worksheet of Molarity
• Bottomless Worksheet of Molar Masses
• Mole Conversions
• Molarity
• Calculating Molar Mass
• Balancing Chemical Equations