Balancing Chemical Equations (Complete)

Chemical equations are a compact way to describe what happens during a chemical reaction. The substances you start with are called reactants, and the substances formed are called products. Balancing an equation makes sure the same number of each type of atom appears on both sides. This matters because matter is not created or destroyed in an ordinary chemical reaction.

The law of conservation of mass explains why balancing works. Atoms can rearrange into new combinations, but the total number of each element must stay the same before and after the reaction. Coefficients placed in front of formulas show how many particles or moles take part in the reaction. By adjusting coefficients without changing subscripts, chemists represent real reactions accurately and can calculate how much product will form.

Key Facts

Reactants are written on the left side of the arrow, and products are written on the right side.
A balanced equation has the same number of atoms of each element on both sides.
The arrow means yields or produces in a chemical reaction.
Coefficients multiply the entire formula, so 2H2O means 4 H atoms and 2 O atoms.
Do not change subscripts when balancing, because that changes the identity of the substance.
Example: 2H2 + O2 = 2H2O

Vocabulary

Reactant: A reactant is a starting substance that is used up or changed during a chemical reaction.
Product: A product is a substance formed by a chemical reaction.
Coefficient: A coefficient is a number placed in front of a chemical formula to show how many units of that substance are involved.
Subscript: A subscript is the small number in a chemical formula that shows how many atoms of an element are in one unit of the substance.
Conservation of mass: Conservation of mass means the total mass and the total number of each type of atom stay the same during a chemical reaction.

Common Mistakes to Avoid

Changing subscripts to force the equation to balance, which is wrong because it changes one substance into a different substance. Only coefficients should be adjusted.
Balancing one element and then forgetting to recheck earlier elements, which is wrong because changing one coefficient affects atom counts for the whole formula. Always do a final count of every element.
Ignoring polyatomic ions that stay together on both sides, which is wrong because they can often be balanced as a single unit. This can make balancing much faster and more accurate.
Assuming a balanced equation means equal numbers of molecules on both sides, which is wrong because balancing requires equal numbers of atoms of each element, not equal total particles.

Practice Questions

1 Balance this equation: H2 + Cl2 = HCl
2 Balance this equation: Fe + O2 = Fe2O3
3 A student changes H2 + O2 = H2O into H2 + O2 = H2O2 to make oxygen atoms match. Explain why this is not a valid way to balance a chemical equation.