Balancing a Combustion Reaction
Practice balancing a hydrocarbon combustion reaction. This fundamental skill is essential for stoichiometry calculations and understanding energy release in reactions.
Problem Scenario
Propane (C3H8) undergoes complete combustion with oxygen gas to produce carbon dioxide and water. Balance this chemical equation.
Given Data
Requirements
- Write the unbalanced equation
- Balance the carbon atoms first
- Balance the hydrogen atoms
- Balance the oxygen atoms last
- Verify atom counts on both sides
Solution
Step 1:
Write the unbalanced equation: C3H8 + O2 -> CO2 + H2O
Step 2:
Balance carbon: There are 3 C atoms on the left, so we need 3 CO2 on the right. C3H8 + O2 -> 3CO2 + H2O
Step 3:
Balance hydrogen: There are 8 H atoms on the left, so we need 4 H2O on the right (4 x 2 = 8). C3H8 + O2 -> 3CO2 + 4H2O
Step 4:
Balance oxygen: On the right, we have (3 x 2) + (4 x 1) = 10 oxygen atoms. We need 10/2 = 5 O2 molecules. C3H8 + 5O2 -> 3CO2 + 4H2O
Step 5:
Verify: Left side: 3 C, 8 H, 10 O. Right side: 3 C (in 3CO2), 8 H (in 4H2O), 10 O (6 in CO2 + 4 in H2O). Balanced!
Final Answer
C3H8 + 5O2 -> 3CO2 + 4H2O. The balanced equation shows that 1 mole of propane reacts with 5 moles of oxygen to produce 3 moles of carbon dioxide and 4 moles of water.
Key Takeaways
- ✓Always balance C and H before O in combustion reactions
- ✓Oxygen is balanced last because it appears in multiple products
- ✓Use fractional coefficients if needed, then multiply through
- ✓Always verify by counting atoms on both sides
Common Errors to Avoid
- ✗Changing subscripts instead of coefficients
- ✗Forgetting that O2 is diatomic
- ✗Not counting all atoms when compounds have coefficients
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Common questions about this problem type
You can either leave it as a fraction or multiply all coefficients by the denominator to get whole numbers. Both are correct, but whole numbers are preferred for stoichiometry calculations.
In combustion reactions, oxygen appears in both products (CO2 and H2O). Balancing C and H first determines how many of each product you need, then oxygen is adjusted to match.