Complete combustion of a hydrocarbon in sufficient oxygen forms carbon dioxide and water and transfers energy to the surroundings. Balance carbon first, hydrogen second and oxygen last.
Change coefficients to balance an equation. Changing a subscript changes the substance. If balancing oxygen gives a half coefficient, multiply every coefficient by two to obtain whole numbers.
With insufficient oxygen, incomplete combustion can form carbon monoxide and carbon as well as water. The products can be a mixture rather than a single universal equation. Soot is particulate carbon; carbon monoxide is a toxic gas that reduces oxygen transport by blood.
Neither an apparently clean flame nor a lack of smell proves that an appliance is safe. This is an equation and explanation activity, not an appliance-testing procedure.
Worked example
Propane: C3H8 + 5O2 → 3CO2 + 4H2O. Three carbons require three CO2; eight hydrogens require four waters.
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Explain why an incomplete-combustion mixture cannot be predicted from carbon count alone.
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Identify the oxygen supply.
Name possible carbon-containing products.
Separate atom conservation from selecting the actual mixture.
Example answer
Carbon atoms must be conserved, but the oxygen supply and combustion conditions influence their distribution between carbon dioxide, carbon monoxide and carbon. Carbon count alone cannot determine the proportions of those products.
An answer to improve
The formula tells you that every carbon becomes CO2.
Improve your answer
State the named substances, coefficients and conditions. Replace a general rule with the evidence that actually supports your conclusion.
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