Absolute Enthalpy & Enthalpy of Formation ● Absolute enthalpy = sum of enthalpy = (energy of chemical bonds) + (sensible enthalpy change) o hi T = h f , i T ref h s ,i T ref Absolute enthalpy at temp T Enthalpy of formation at std. Ref. state (Tref, Po) Sensible enthalpy change going from Tref to T o hs , i T ref ≡ h f , i T ref − hi T Ref. state: Tref = 25oC (298.15 K) Pref = Po = 1 atm o Enthalpy of formation, h f ≡0 for elements in naturally occurring state at ref. state Oxygen Nitrogen Carbon O2 N2 C gas gas solid (graphite) Enthalpy of Combustion Enthalpy of Combustion = enthalpy of reaction, h R Heat of combustion= heating value hc hc =− h R (assuming H2O in HHV = higher heating value products condensed into liquid) LHV = lower heating value (assuming no condensation)