Lec 23-24 March 4-6 - 3.7 B Standard Enthalpies of...

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B. Standard Enthalpies of Formation (sect. 7.8) Change in enthalpy that accompanies the formation of 1 mole of a compound from its elements in their standard state. - Elements in their standard state: gases at P = 1 bar (1 bar is about 1 atm) liquids/solids are pure (choose the most stable form for solids) solution (1.0 M) and at some defined temperature ; tables in book use 298 K - For standard state pure elements, the standard enthalpy of formation ( Δ H f °) is zero. (Symbol ° = standard state) Δ H f ° = 0 kJ/mol (by definition; this is our reference point) - O 2 (g), N 2 (g), C (s), Na (s), 1.0 M H + (aq) all have Δ H f ° = 0 kJ/mol (all solutions are referenced to 1.0 M H + (aq) ) 3.7 e.g. Write the reaction for which Δ H = Δ H f ° for CH 3 OH (l) at 298 K: C (s) + 2 H 2 (g) + 1/2 O 2 (g) CH 3 OH (l) Δ H f ° = -238.7 kJ/mol (from text) Note: reactants have to be pure elements in standard state to get Δ H f ° for the products Now use Δ H f ° to calculate the standard enthalpies of reaction ( Δ H rsn ° ) : ) ( ) ( r H n p H n H o f r o f p o rxn ! " # ! " = ! Need these references because we cannot measure absolute enthalpy, H ; only changes in enthalpy, Δ H , are measured. e.g. Find Δ H rxn ° for : CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (l)
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3.8 Δ H rxn ° = {1 mol × Δ H f ° (CO 2 ) + 2 mol × Δ H f ° (H 2 O) } - {1 mol × Δ H f ° (CH 4 ) + 2 mol × Δ H f ° (O 2 ) } Look up Δ H f ° values in data sheet : Δ H rxn ° = {-393.5 kJ + 2 mol × (-285.8 kJ/mol)} - {-74.81 kJ + 2 mol × (0) } = - 890.3 kJ (combustion of methane is exothermic!) C. Bond Dissociation Energies (sect. 10.9) Bond dissociation energy (D) is the energy required to break 1 mole of covalent bonds to form gaseous species Tables in book (pg 409) and in data sheet are average bond dissociation energies under standard conditions (1 atm) and 298 K Compare C-H bond energies in different molecules: D (kJ/mol) H-CBr 3 380 H-CCl 3 380 H-CF 3 430 H-CH 3 414 Bond is very sensitive to molecular environment. But you have only ONE value for C-H bond energy : this is the average bond energy .
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