Calculate the H rxn for the combustion propane using the bond ethalpies Explain

Question

Calculate the H rxn for the combustion propane using the bond ethalpies

Explain why the bond value if different from H°f

Because there are Hbonib methane, ihe he sum of the bond etthal pies of the kui H bodd enthalpy for The bund enthalpy for a given pair of atoms, say C 11, depends another komeally whiali sapports he kika that bonding tran twin H bond enthalples pound, we litud he averape bond rnthalpy is we calculated from CH4. Give It some Thought use the enthalpy at mir tian at the hythocarbon ethare, to the iber of atom pairs ne bel T Table s 4 average bond Table a Mweage Bond Enthalp ass

Explanation / Answer

Our reaction will be

C3H8(g) + 5O2 = 3CO2(g) + 4H2O(g)

H = B.E Reactants – Products

C3H8: 2 C-C * 348 = 696 kJ 8 C-H * 413 = 3304 kJ Total = 4000kJ 5 O2: 5 O=O * 495 = 2475kJ 3 CO2: 6 C=O * 799 = 4794 kJ 4 H2O: 8 H-O * 463 = 3704 kJ H = B.E Reactants - Products H = (4000 & 2475) - (4794 & 3704) H = 6475 - 8498 H = -2023 kJ

Bond enthalpies are average, while formation enthalpies are precise. For example, propane formation enthalpy is calculated using propane only, while C-C bond enthalpy is calculated from different molecules and averaged to get the value you got in that table.

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