Balancing Chemical Equations

A chemical equation describes a chemical reaction using chemical symbols that shows the amount of atoms in the reactants and products
Atoms cannot be created or destroyed, therefore the number of atoms on each side must be the same
The atoms are balanced by changing the coefficients (numbers in front of chemical formulae), not the subscripts (numbers that are part of the chemical formulae)
This process can be done by trial and error changing the coefficients
You can follow these simple steps to balance simple chemical equations

1. Write an equation with the accurate chemical formula for reactants and products
2. Balance atoms that appear in the most complex substances. Atoms in compounds have priority than atoms that are pure elements
3. Balance atoms that appear on their own by adjusting their coefficients
4. If any coefficient is a fraction, clear the fraction by multiplying all the coefficients times the denominator of the fraction
5. Check if the atoms are balanced on both sides of the equation

Exam Tip

A balanced chemical equation is mandatory before performing any kind of calculation. Therefore, you must always check if your chemical equation as soon as you start solving a problem during your exam

Worked example

Write a balanced equation for the reaction between aluminum and copper(II) oxide to produce aluminum oxide and copper

Answer:

Step 1: Write an equation with the accurate chemical formula for reactants and products

Al + CuO → Al₂O₃ + Cu

Step 2: Balance atoms that appear in the most complex substances. Atoms in compounds have priority than atoms that are pure elements

Balance O

Al + CuO → Al₂O₃ + Cu

1 O atom → 3 O atoms

To balance O atoms, place a 3 before CuO

Al + 3CuO → Al₂O₃ + Cu

Step 3: Balance atoms that appear on their own by adjusting their coefficients

Balance Cu

Al + 3CuO → Al₂O₃ + Cu

3 Cu atoms → 1 Cu atom

To balance Cu atoms, place a 3 before Cu

Al + 3CuO → Al₂O₃ + 3Cu

Balance Al

Al + 3CuO → Al₂O₃ + 3Cu

1 Al atom → 2 Al atoms

To balance Al atoms, place a 2 before Al

2Al + 3CuO → Al₂O₃ + 3Cu

Step 4: If any coefficient is a fraction, clear the fraction by multiplying all the coefficients times the denominator of the fraction

This step is not necessary for this example

Step 5: Check if the atoms are balanced on both sides of the equation

2Al + 3CuO → Al₂O₃ + 3Cu

	Left	Right
Al	2	2
Cu	3	3
O	3	3

The equation is balanced

Worked example

Write a balanced equation for the reaction between butane (C₄H₁₀) and oxygen gas to produce carbon dioxide and water

Answer:

Step 1: Write an equation with the accurate chemical formula for reactants and products

C₄H₁₀ + O₂ → CO₂ + H₂O

Step 2: Balance atoms that appear in the most complex substances. Atoms in compounds have priority than atoms that are pure elements

Balance C

C₄H₁₀ + O₂ → CO₂ + H₂O

4 C atoms → 1 C atom

To balance C atoms, place a 4 before CO₂

C₄H₁₀ + O₂ → 4CO₂ + H₂O

Balance H

C₄H₁₀ + O₂ → 4CO₂ + H₂O

10 H atoms → 2 H atoms

To balance H atoms, place a 5 before H₂O

C₄H₁₀ + O₂ → 4CO₂ + 5H₂O

Step 3: Balance atoms that appear on their own by adjusting their coefficients

Balance O

C₄H₁₀ + O₂ → 4CO₂ + 5H₂O

2 O atoms → 13 O atoms

To balance O atoms, place a 13/2 before O₂

C₄H₁₀ + $\frac{13}{2}$ O₂ → 4CO₂ + 5H₂O

Step 4: If any coefficient is a fraction, clear the fraction by multiplying all the coefficients times the denominator of the fraction

(1 × 2) C₄H₁₀ + ( $\frac{13}{2}$ ×2) O₂ → (4 × 2) CO₂ + (5 × 2) H₂O

2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O

Step 5: Check if the atoms are balanced on both sides of the equation

2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O

	Left	Right
C	8	8
H	20	20
O	26	26

The equation is balanced

Balancing Chemical Equations (College Board AP Chemistry)

Revision Note