After separating crude oil by fractional distillation and converting hydrocarbons through cracking, many of the alkanes produced are ultimately used as fuels. Understanding why these molecules release large amounts of energy when they combust explains one of their most important real-world applications, while also introducing the idea that chemical reactions often involve trade-offs between benefits and consequences. This provides the foundation for exploring the pollutants formed during combustion and the technologies used to reduce their impact.