Absolute Enthalpy &
Enthalpy of Formation
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Absolute enthalpy = sum of enthalpy
= (energy of chemical bonds) + (sensible
enthalpy change)
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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
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 hs , i T ref ≡ h f , i T ref − hi T 
Ref. state:
Tref = 25oC (298.15 K)
Pref = Po = 1 atm
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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)