SME
Grade 7 Science · Problem type
Burning methane
Use this for “Balance CH₄ + O₂ → CO₂ + H₂O” and “How many oxygen molecules does one methane molecule burn with?”
Diagram
CH₄ + 2 O₂ → CO₂ + 2 H₂O
Carbon: 1 × 1 = 1 before, 1 × 1 = 1 after
Hydrogen: 1 × 4 = 4 before, 2 × 2 = 4 after
Oxygen: 2 × 2 = 4 before, 1 × 2 + 2 × 1 = 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
CH₄
+
b
O₂
→
c
CO₂
+
d
H₂O
A coefficient of 1 is not written: 1 O₂ is written O₂.
Formulas
c
=
a
(carbon
dioxide
molecules
=
methane
molecules)
1 = 1
d
=
2
×
a
(water
molecules
=
2
×
methane
molecules)
2 = 2 × 1
2
×
b
=
2
×
c
+
d
(2
×
oxygen
molecules
=
2
×
carbon
dioxide
molecules
+
water
molecules)
2 × 2 = 2 × 1 + 2
Assumptions
•
Methane (natural gas) burns in oxygen to make carbon dioxide and water: CH₄ + 2 O₂ → CO₂ + 2 H₂O.
•
Carbon first: one CO₂ per CH₄. Hydrogen next: 4 H make 2 H₂O. Oxygen last: 4 O need 2 O₂.
•
Forming the new bonds gives out more energy than breaking the old ones took, so the flame is hot.
Step-by-step
WE KNOW
a = 1 (methane molecules)
FIND
carbon dioxide molecules (c), water molecules (d), oxygen molecules (b)
Step 1 · Find carbon dioxide molecules (c)
Use c = a (carbon dioxide molecules = methane molecules)
Carbon first: one CO₂ for each CH₄.
c
=
1
Step 2 · Find water molecules (d)
Use d = 2 × a (water molecules = 2 × methane molecules)
Hydrogen next: each CH₄ has 4 hydrogen atoms and each H₂O takes 2, so two H₂O per CH₄.
d
=
2
×
1
d
=
2
Step 3 · Find oxygen molecules (b)
Use 2 × b = 2 × c + d (2 × oxygen molecules = 2 × carbon dioxide molecules + water molecules)
Oxygen last: count the oxygen atoms after the arrow, 2 in each CO₂ and 1 in each H₂O, then halve for O₂.
2
b
=
2
×
1
+
2
b
=
2 × 1 + 2
2
b
=
2 + 2
2
b
=
4
2
b
=
2
Check
Put the numbers back in. Both sides should match.
1 = 1 ✓
2 = 2 × 1 ✓
2 × 2 = 2 × 1 + 2 ✓
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Mass before and after a reaction
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