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Standard Enthalpies of Formation Chapter 8.5 Standard Enthalpies of Formation “Heat of Formation” The standard enthalpy of formation of a substance, denoted DHfo, is the enthalpy change for the formation of one mole of a substance in its standard state from its component elements in their standard state. – Note that the standard enthalpy of formation for a pure element in its standard state is zero. NH3 = -46.1 You have a similar table on page 209 of your text Using DHfo to Calculate DHo for a Rxn The law of summation of heats of formation states that the enthalpy of a reaction is equal to the total formation energy of the products minus that of the reactants. DH o nDH of (products ) mDH of (reactants ) S is the mathematical symbol meaning “the sum of”, and m and n are the coefficients of the substances in the chemical equation. Why use DHfo to Calculate DHo when we can use Hess’s Law? Simply put it is easier Finding combinations of reactions to fit your particular reaction gets tedious We have very accurate data tables that give us the DHfo values Pure elements are 0! Let’s do a Practice Problem!!! 4NH 3 (g ) 5O 2 (g ) 4NO(g ) 6H 2O(g ) 4( 45.9) 5(0) 4(90.3) 6( 241.8) -183.6 + 0 361.2 + (-1450.8) -183.6 for reactants -1089.6 for products Prod. – React. = -1089.6 – (-183.6) = -906 kJ *Hint: First record the values of DHfo under the formulas in the equation then multiplying them by the coefficients in the equation. You can then determine DHo by subtracting the values for the reactants from the values for the products. Homework for 8.5 Problems #38 (don’t forget DH in equations!), 39, 43, and 45 Chem Dollar problem #52 ($2)