Combustion

AP Chemistry · Learn by Concept

Help Questions

AP Chemistry › Combustion

1 - 3

Which of the following conditions would describe a combustion reaction?

Exothermic with a negative $\Delta H$

CORRECT

Endothermic with a positive $\Delta H$

Exothermic with a positive $\Delta H$

Endergonic with a negative $\Delta H$

Exergonic with a positive $\Delta H$

Explanation

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

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

Exergonic reactions have a negative $\Delta G$ 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.

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

The combustion of methane

The combustion of propane

The combustion of butane

CORRECT

The combustion of isobutane

The combustion of isopropane

Explanation

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.

What is the balanced equation when heptane is combusted?

2 C7H14 + 5 O2 → 3 H2O + 6 CO2

C5H12 + 8 O2 → 6 H2O + 5 CO2

C7H16 + O2 → H2O + CO2

C7H16 + 11 O2 → 8 H2O + 7 CO2

CORRECT

C3H8 + 5 O2 → 4 H2O + 3 CO2