Energy and Enthalpy First Law of ThermoDynamics Hess's Law Kirchoff's Relation

1. Thermochemistry Dr. Ruchi S. Pandey

8.  H for some typical chemical reactions Dr. Ruchi S. Pandey

9. Dr. Ruchi S. Pandey Because there is no way to measure the absolute value of the enthalpy of a substance, must I measure the enthalpy change for every reaction of interest? Establish an arbitrary scale with the standard enthalpy of formation (  H 0 ) as a reference point for all enthalpy expressions. The standard enthalpy of formation of any element in its most stable form is zero. Standard enthalpy of formation (  H 0 ) is the heat change that results when one mole of a compound is formed from its elements at a pressure of 1 atm. f  H 0 (O 2 ) = 0 f  H 0 (O 3 ) = 142 kJ/mol f  H 0 (C, graphite) = 0 f  H 0 (C, diamond) = 1.90 kJ/mol f

10. Dr. Ruchi S. Pandey Enthalpy is a state function. It doesn’t matter how you get there, only where you start and end. The standard enthalpy of reaction (  H 0 ) is the enthalpy of a reaction carried out at 1 atm. rxn a A + b B c C + d D  H 0 rxn d  H 0 (D) f c  H 0 (C) f = [ + ] - b  H 0 (B) f a  H 0 (A) f [ + ]  H 0 rxn n  H 0 (products) f =  m  H 0 (reactants) f  -