AP Chemistry : Combustion

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

Example Question #1 : Oxidation Reduction Reactions

What is the balanced equation when heptane is combusted?

Possible Answers:

C7H16 + 11 O2 → 8 H2O + 7 CO2

2 C7H14 + 5 O2 → 3 H2O + 6 CO2

C3H8 + 5 O2 → 4 H2O + 3 CO2

C7H16 + O2 → H2O + CO2

C5H12 + 8 O2 → 6 H2O + 5 CO2

Correct answer:

C7H16 + 11 O2 → 8 H2O + 7 CO2


Heptane: C7H16. Combustion is when a molecule reacts with O2 and the products are CO2 and H2O. Balancing gives 7 CO2, 8 H2O, 1 heptane, and 11 O2

Example Question #2 : Oxidation Reduction Reactions

Which of the following reactions has the most exothermic heat of reaction?

Possible Answers:
The combustion of methane
The combustion of isopropane
The combustion of isobutane
The combustion of propane
The combustion of butane
Correct answer: The combustion of butane

The longer the hydrocarbon chain, the greater the amount of combustion products (CO2 and H20) generated. Branched molecules such as isopropane and isobutane are more difficult to combust than their straight-chain counterparts.

Example Question #3 : Oxidation Reduction Reactions

Which of the following conditions would describe a combustion reaction?

Possible Answers:

Exothermic with a negative 

Endergonic with a negative 

Exergonic with a positive 

Exothermic with a positive 

Endothermic with a positive 

Correct answer:

Exothermic with a negative 


An exothermic reaction will have a negative  value, indicating that it releases heat. Conversely, an endothermic reaction will have a positive  value, indicating a consumption of heat.

A combustion reaction releases heat; thus it must have a negative  value and be exothermic.

Exergonic reactions have a negative  value, indicating spontaneity, while endergonic reactions are non-spontaneous. While most combustion reactions will be non-spontaneous, it is impossible to draw this conclusion for certain without knowing more about the reaction. The only thing we know for certain is that heat is released, and the reaction is exothermic.

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