Balancing Chemical Equations (College Board AP® Chemistry)
Study Guide
Written by: Martín
Reviewed by: Stewart Hird
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
Write an equation with the accurate chemical formula for reactants and products
Balance atoms that appear in the most complex substances. Atoms in compounds have priority than atoms that are pure elements
Balance atoms that appear on their own by adjusting their coefficients
If any coefficient is a fraction, clear the fraction by multiplying all the coefficients times the denominator of the fraction
Check if the atoms are balanced on both sides of the equation
Examiner Tips and Tricks
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 exampleStep 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 + 
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 + (×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
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