SME
Grade 10 Science · Problem type
Balancing a synthesis: aluminum oxide
Use this for “Balance Al + O₂ → Al₂O₃.”
Diagram
4 Al + 3 O₂ → 2 Al₂O₃
Oxygen: 3 × 2 = 6 before, 2 × 3 = 6 after
Aluminum: 4 × 1 = 4 before, 2 × 2 = 4 after
Balanced: the same atoms on both sides, only rearranged.
This is an example. Choose your own values to start a new problem.
a
Al
+
b
O₂
→
c
Al₂O₃
Formulas
c
=
a
2
2
=
4
2
b
=
3
×
c
2
3
=
3
×
2
2
Assumptions
•
Aluminum burns in oxygen to make aluminum oxide, one product from two reactants.
•
Oxygen comes in pairs and Al₂O₃ holds 3, so the oxygen atoms must be a multiple of 6: 4 aluminum atoms at a time.
•
The balanced equation uses the smallest whole numbers, 4, 3 and 2; larger inputs are the same reaction run more times.
•
Aluminum oxide is ionic: each formula unit is two Al³⁺ and three O²⁻, drawn as ions with no bonds.
Step-by-step
WE KNOW
a = 4
FIND
aluminum oxide formula units (c), oxygen molecules (b)
Step 1 · Find aluminum oxide formula units (c)
Use c = a/2
Aluminum: each Al₂O₃ holds 2 aluminum atoms, so half as many formula units as atoms.
c
=
4
2
c
=
2
Step 2 · Find oxygen molecules (b)
Use b = 3 × c/2
Oxygen: each Al₂O₃ holds 3 oxygen atoms and each O₂ brings 2.
b
=
3
×
2
2
b
=
3
Check
Put the numbers back in. Both sides should match.
2
=
4
2
✓
3
=
3
×
2
2
✓
← Previous lesson
Balancing the combustion of an alkene
Next lesson →
Balancing a single replacement: zinc in acid
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
Refresh
Related lessons
Chemical equations: balancing, types of reactions and predicting products
Main lesson
Balancing the combustion of an alkane
Use this to balance any alkane burning, such as “C₃H₈ + O₂ → CO₂ + H₂O”: carbon first, hydrogen next, oxygen last.
Balancing the combustion of an alkene
Use this to balance an alkene burning, such as “C₅H₁₀ + O₂ → CO₂ + H₂O”: with an odd number of carbons the fuel’s coefficient is 2.
Balancing a single replacement: zinc in acid
Use this for “Balance Zn + HCl → ZnCl₂ + H₂.”
Predicting a single replacement
Use this for “Does zinc react with copper(II) sulfate, and what does it make?”