SME
Grade 7 Science · Problem type
Iron rusting: iron and oxygen make iron oxide
Use this for “How many oxygen molecules react with 4 iron atoms to make rust?”
Diagram
4 Fe + 3 O₂ → 2 Fe₂O₃
Oxygen: 3 × 2 = 6 before, 2 × 3 = 6 after
Iron: 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
Fe
+
b
O₂
→
c
Fe₂O₃
i₁
Iron atoms before
worked out (not typed)
o₁
Oxygen atoms before
worked out (not typed)
i₂
Iron atoms after
worked out (not typed)
o₂
Oxygen atoms after
worked out (not typed)
Formulas
a
=
2
×
c
(iron
atoms
used
=
2
×
iron
oxide
units
made)
4 = 2 × 2
b
=
3
×
c
÷
2
(oxygen
molecules
used
=
3
×
iron
oxide
units
made
÷
2)
3 = 3 × 2 ÷ 2
i₁
=
a
(iron
atoms
before
=
iron
atoms
used)
4 = 4
o₁
=
2
×
b
(oxygen
atoms
before
=
2
×
oxygen
molecules
used)
6 = 2 × 3
i₂
=
2
×
c
(iron
atoms
after
=
2
×
iron
oxide
units
made)
4 = 2 × 2
o₂
=
3
×
c
(oxygen
atoms
after
=
3
×
iron
oxide
units
made)
6 = 3 × 2
Assumptions
•
Iron left in damp air slowly turns into rust, iron oxide: 4 Fe + 3 O₂ → 2 Fe₂O₃.
•
Rust is a new substance: orange, crumbly and not magnetic, unlike grey, strong, magnetic iron.
•
The rust weighs more than the iron did, by exactly the oxygen it took from the air.
•
Two rust units at a time, because 3 O₂ carry 6 oxygen atoms.
Step-by-step
WE KNOW
c = 2 (iron oxide units made)
FIND
iron atoms used (a), oxygen molecules used (b), iron atoms before (i₁), oxygen atoms before (o₁), iron atoms after (i₂), oxygen atoms after (o₂)
Step 1 · Find iron atoms used (a)
Use a = 2 × c (iron atoms used = 2 × iron oxide units made)
Each Fe₂O₃ holds 2 iron atoms, so twice as many iron atoms go in.
a
=
2
×
2
a
=
4
Step 2 · Find oxygen molecules used (b)
Use b = 3 × c ÷ 2 (oxygen molecules used = 3 × iron oxide units made ÷ 2)
Each Fe₂O₃ needs 3 oxygen atoms and each O₂ brings 2: three O₂ for every two Fe₂O₃.
b
=
3
×
2
2
b
=
6
2
b
=
3
Step 3 · Find iron atoms before (i₁)
Use i₁ = a (iron atoms before = iron atoms used)
Each iron atom is one atom.
i₁
=
4
Step 4 · Find oxygen atoms before (o₁)
Use o₁ = 2 × b (oxygen atoms before = 2 × oxygen molecules used)
Each O₂ molecule has 2 oxygen atoms.
o₁
=
2
×
3
o₁
=
6
Step 5 · Find iron atoms after (i₂)
Use i₂ = 2 × c (iron atoms after = 2 × iron oxide units made)
Each Fe₂O₃ has 2 iron atoms.
i₂
=
2
×
2
i₂
=
4
Step 6 · Find oxygen atoms after (o₂)
Use o₂ = 3 × c (oxygen atoms after = 3 × iron oxide units made)
Each Fe₂O₃ has 3 oxygen atoms.
o₂
=
3
×
2
o₂
=
6
Check
Put the numbers back in. Both sides should match.
4 = 2 × 2 ✓
3 = 3 × 2 ÷ 2 ✓
4 = 4 ✓
6 = 2 × 3 ✓
4 = 2 × 2 ✓
6 = 3 × 2 ✓
Topic check
See what you know: 5 questions on this topic
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