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Combustion reactions always involve the oxygen molecule, O2 as a reactant. The general form of a combustion reaction is substance X reacts with oxygen to form a new XO compound. Most often, we think of fire and combustion as being the same thing. However, when iron rusts, solid iron reacts with O2 in the atmosphere. Technically, rusting iron is a combustion reaction even though fire is not produced. 
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A hydrocarbon is a molecule that only contains hydrogen and carbon. Methane (CH4), propane (C3H8), and butane (C4H10) are examples of hydrocarbons. When hydrocarbons are burned, they always form carbon dioxide and water. Combustion reactions can be very challenging to balance! You often have to go through the balancing process many times.
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| Write the balanced reaction for the combustion of magnesium metal.
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| Relationships |
In a combustion reaction, the given reactant combines with oxygen to form a product. |
| Solve |
- Write the reactants.
- Magnesium does not form a diatomic molecule: Mg
- Oxygen forms a diatomic molecule: O2
- The reactants are: Mg + O2
- Find the charges of the reactants and criss-cross to form a compound.
- Magnesium = Mg2+ and oxygen = O2+
- Criss-cross charges; in this case the charges cancel: MgO
- Write the reaction: Mg + O2 → MgO
- Balance the reaction: 2Mg + O2 → 2MgO
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| Answer |
The balanced reaction is: 2Mg + O2 → 2MgO |
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| Write the balanced reaction for the combustion of pentane, C5H12.
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| Relationships |
This is the combustion of a hydrocarbon. The hydrocarbon will react with oxygen, and the products will be carbon dioxide and water. |
| Solve |
- When a hydrocarbon undergoes combustion, it reacts with oxygen to produce carbon dioxide and water. No criss-crossing is necessary.
- Balance the reaction:
- Try balancing carbon first, then hydrogen, and balance oxygen last. Keep going back and forth until the reaction is balanced.
- C5H12 + 8O2 → 5CO2 + 6H2O
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| Answer |
The balanced reaction is: C5H12 + 8O2 → 5CO2 + 6H2O |
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