AP Chemistry : Combustion
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Example Questions
Example Question #1 : Reactions And Equilibrium
What is the balanced equation when heptane is combusted?
C3H8 + 5 O2 → 4 H2O + 3 CO2
C7H16 + 11 O2 → 8 H2O + 7 CO2
C7H16 + O2 → H2O + CO2
2 C7H14 + 5 O2 → 3 H2O + 6 CO2
C5H12 + 8 O2 → 6 H2O + 5 CO2
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 #1 : Reaction Types
Which of the following reactions has the most exothermic heat of reaction?
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 #2 : Reactions And Equilibrium
Which of the following conditions would describe a combustion reaction?
Exothermic with a positive
Endothermic with a positive
Exergonic with a positive
Endergonic with a negative
Exothermic with a negative
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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