In this problem you will use tabulated gas phase enthalpies of formation to esti
ID: 475159 • Letter: I
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In this problem you will use tabulated gas phase enthalpies of formation to estimate bond dissociation enthalpies, often called 'bond strengths'. Consult the table below for enthalpies of formation. I have indicated all bonds as single lines although they may be "single bonds", "double bonds", etc. Compare your answers with the values at the website below (note their units are kcal and we are using kJ). c-H bond using methane, O-H bond using water C-C bond using octane, C-C bond using benzene, C-C bond using ethylene, C-C bond using acetylene. For these calculations use your C-H result from methane, above. C-O bond using methanol, C-O using carbon dioxide. For these, use your C-H, and o-H bond results from above. Now try working backwards to use your bond dissociation enthalpy values along with the enthalpies of formation of the atomic species (C, H, O, etc.) to calculate the heats of formation of butane (C_4H_10), propene (C_3H_6), and ethanol. How well do these estimates work? Enter your bond dissociation enthalpies for parts a-f in the table, below.Explanation / Answer
Heat of formation of CH4 (four C-H bonds) = -74.8 kJ/mol
heat of formation of one C-H bond = -74.8 kJ/mol/4 = -18.7 kJ/mol
bond dissociation enthalpy = + 18.7 kJ/mol
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Heat of formation of H2O (two O-H bonds) = -241.8 kJ/mol
heat of formation of one O-H bond = -241.8 kJ/mol/2 = -120.9 kJ/mol
bond dissociation enthalpy of O-H bond = + 120.9 kJ/mol
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Heat of formation of octane (7 C-C bonds + 18 C-H bond) = -208.5 kJ/mol
heat of formationof one C-C bond = [-208.5 - (18 *-18.7 kJ/mol)]/7 = 18.3 kJ/mol
bond dissociation enthalpy = -18.3 kJ/mol
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Heat of formation of ethylene (one C=C + 4 C-H) = 52.3 kJ/mol
Heat of formation of C=C = 52.3 kJ/mol - 4*-18.7 kJ/ mol = 127.1 kJ/mol
Bond dissociation enthalpy = 127.1 kJ/mol
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heat of formation of acetylene (two C-H bond + one C=C bond) = 226.7 kJ/mol
heat of formation of one C=C bond = 226.7 kJ/mol - 2*-18.7kJ/mol = 264.1 kJ/mol
Bond dissociation enthalpy = +264.1 kJ/mol
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Heat of formation of Benzene (six C-C bond + six C-H bond) = 82.9 kJ/mol
heat of formation of one C-C bond = [82.9 kJ/mol- 6 *-18.7kJ/mol]/6 = 32.52 kJ/mol
Bond dissociation enthalpy = -32.52 kJ/mol
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methanol contains three C-H bond, one C-O bond and one O-H bond.
heat of formation of methanol =-201.0 kJ/mol
heat of formation of C-O = -201.0 kJ/mol - [3* -18.7 kJ/mol + 1*-120.9kJ/mol] = -24kJ/mol
Bond dissociation enthalpy = + 24kJ/mol
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Heat of formation of CO2 (two C=O) = -393.5 kJ/mol
heat of formation of one C=O = -393.5/2 = -196.6 kJ/mol
Bond dissociation enthalpy = +196.6 kJ/mol
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