Gasoline stores chemical energy in the bonds of hydrocarbon molecules, which are made mostly of carbon and hydrogen atoms. This energy originally comes from ancient organic matter that was transformed over millions of years into crude oil, then refined into fuels. In a car engine, gasoline is valuable because a small mass of liquid fuel can release a large amount of energy quickly.
That energy can be converted into heat, expanding gases, and mechanical work.
Key Facts
- A useful model for gasoline is octane, C8H18.
- Complete combustion of octane: 2 C8H18 + 25 O2 -> 16 CO2 + 18 H2O + energy.
- Energy is released because forming C=O and O-H bonds releases more energy than is needed to break C-H, C-C, and O=O bonds.
- Gasoline energy density is about 44 MJ/kg, though the exact value depends on the blend.
- Enthalpy change for combustion is negative: Delta H < 0 for an exothermic reaction.
- Engine efficiency = useful work output / chemical energy input.
Vocabulary
- Hydrocarbon
- A compound made only of carbon and hydrogen atoms, such as many molecules found in gasoline.
- Combustion
- A chemical reaction in which a fuel reacts with oxygen and releases energy, usually as heat and light.
- Chemical potential energy
- Energy stored in the arrangement of atoms and electrons within chemical bonds.
- Exothermic reaction
- A reaction that releases energy to the surroundings because the products have lower chemical energy than the reactants.
- Activation energy
- The minimum energy needed to start a chemical reaction, such as the spark needed to ignite gasoline vapor.
Common Mistakes to Avoid
- Saying energy is stored inside a single bond, rather than in the whole molecular arrangement. Energy changes depend on both bonds broken in reactants and bonds formed in products.
- Thinking combustion destroys atoms. Combustion rearranges atoms into new molecules, mainly carbon dioxide and water during complete combustion.
- Forgetting that oxygen is a reactant. Gasoline alone does not release its stored energy unless it reacts with O2 and is ignited.
- Assuming all gasoline energy becomes motion. Real engines lose much of the released energy as heat, sound, exhaust energy, and friction.
Practice Questions
- 1 A car burns 0.50 kg of gasoline with an energy density of 44 MJ/kg. How much chemical energy is released?
- 2 Using 2 C8H18 + 25 O2 -> 16 CO2 + 18 H2O, how many moles of CO2 form when 4.0 moles of C8H18 burn completely?
- 3 Explain why gasoline vapor burns much more easily than a pool of liquid gasoline, using the ideas of oxygen contact and activation energy.