Questions: Combustion Reactions

Question 1

What are the typical products when a hydrocarbon fuel undergoes complete combustion?

• A. Carbon and water
• B. Carbon dioxide and hydrogen
• C. Carbon dioxide and water
• D. Carbon monoxide and oxygen

Answer: (C)

Complete combustion of a hydrocarbon (a compound of carbon and hydrogen) in sufficient oxygen produces carbon dioxide (CO2) and water (H2O). This is the standard combustion pattern for fuels like methane, propane, gasoline, and candle wax.

Question 2

Combustion reactions are endothermic because fire needs heat to start.

• T. True
• F. False

Answer: False

While it is true that combustion needs a spark or initial heat to get started (activation energy), the reaction itself releases far more energy than it absorbs, making it exothermic. The initial spark overcomes the activation energy barrier, and then the reaction sustains itself by producing heat.

Question 3

Why does a fire go out when you smother it with a blanket?

Model answer: Smothering a fire cuts off its oxygen supply. Without oxygen, the combustion reaction cannot continue.

Combustion requires three things: fuel, oxygen, and sufficient heat. Removing any one of these stops the reaction. A blanket blocks oxygen from reaching the fire, preventing the fuel from reacting. This is the principle behind many fire extinguishing methods — they remove oxygen, fuel, or heat.