When hydrogen burns in oxygen, a great deal of heat is dispersed, far more than is needed to compensate for the fall in entropy caused by creating the steam. This excess or “free” energy can be used to produce mechanical work—such as drive a car engine. But it can also be used to force other chemical reactions to proceed in the reverse or “nonspontaneous” direction, just as heat flow in one house reversed it in the other. In this context, the available energy is often referred to as Gibbs free energy, and it’s the means by which chemical reactions are coupled.
