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# Lecture 26 - e.g Find"H for CH4(g 2 O2(g CO2(g 2 H2O(l A...

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3/18/10 1 24 = [1 mol ! " H f ° (CO 2 ) + 2 mol ! " H f ° (H 2 O(l))] # [1 mol ! H f ° (CH 4 ) + 2 mol ! " H f ° (O 2 )] = [ # 393.5 kJ+2 mol ! ( # 285.6 kJ/mol)] # [ # 74.81kJ+2 mol ! (0)] e.g. Find " H° for : CH 4 (g) + 2 O 2 (g) ! CO 2 (g) + 2 H 2 O (l) A) + 889.9 kJ B) # 890.3 kJ C) + 890.3 kJ D) # 889.9 kJ E) # 604.3 kJ ! H o = n p ! H f o ( p ) " # n r ! H f o ( r ) # Products # Reactants = # 889.9 kJ combustion of methane is exothermic! Careful about phase Obtained using textbook data Use data sheet 25 4.2.C. Bond Dissociation Enthalpies (Section 9.4) E.g. C-H bonds: D (kJ/mol) Only ONE value Bond dissociation energy (D) is the energy required to break 1 mole of covalent bonds to form gaseous species (at 1 bar and the given reference temperature). i) Different types of bonds have different bond energies: See Table 9.2 ii) Same bonds in different molecules can also have different bond energies C # C : 347 kJ/mol C=C : 611 kJ/mol H-CBr 3 380 H-CCl 3 380 H-CF 3 430 H-CH 3 413 Bond sensitive to molecular environment average bond energy C # H : 414 kJ/mol Now use D to calculate bond dissociation enthalpies : ! H o = " n r D ( r ) # " n p D ( p ) " H rxn = D (bonds broken) - D (bonds formed)

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