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# chm9 - Because there is no way to measure the absolute...

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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. f 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 (atmospheric pressure). f The standard enthalpy of formation of any element in its most stable form is zero. 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

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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 = Σ m H 0 (reactants) f Σ - Hess’s Law: When reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps.
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chm9 - Because there is no way to measure the absolute...

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