Topic overview

Start with the big picture

Energy is the capacity to do work or transfer heat, and it is conserved in an isolated system. Thermodynamic accounting begins by defining the system and its surroundings. Internal energy is a state function, while heat and work depend on the path taken. The First Law relates these quantities; pressure–volume work and enthalpy help describe processes, particularly at constant pressure. Heat capacity links heat transfer to temperature change, and process conditions distinguish adiabatic, isothermal, isochoric, and isobaric changes. Reaction enthalpies can be considered through standard enthalpy changes, Hess’s Law, and bond enthalpies. The Second Law introduces entropy and spontaneity, while Gibbs and Helmholtz free energies provide criteria under specified conditions. The Third Law establishes a reference for absolute entropy.