This document defines key concepts in energetics including: 1) ∆H (change in enthalpy) represents the change in energy for a reaction, with negative values indicating exothermic (energy-releasing) reactions and positive values indicating endothermic (energy-absorbing) reactions. 2) The standard enthalpy of formation is the enthalpy change when forming 1 mole of a compound from its elements under standard conditions. 3) The standard enthalpy of combustion is the enthalpy change when 1 mole of a substance completely combusts under standard conditions. 4) Hess' Law states that the enthalpy change of a reaction is independent of the