AP Chemistry
6.8 Enthalpy of Formation
6.7 Bond Enthalpies
A student determines the standard enthalpy change, \(\Delta H^\circ_{\text{rxn}}\), for the synthesis of liquid ethanol represented by the following equation:
\[ \text{C}_2\text{H}_4(g) + \text{H}_2\text{O}(g) \rightarrow \text{C}_2\text{H}_5\text{OH}(l) \]
The student calculates \(\Delta H^\circ_{\text{rxn}}\) using standard enthalpies of formation, \(\Delta H_f^\circ\), obtaining a value of \(-88 \text{ kJ/mol}_{\text{rxn}}\). When estimating \(\Delta H^\circ_{\text{rxn}}\) using average bond enthalpies, the student obtains a value of \(-42 \text{ kJ/mol}_{\text{rxn}}\). Which of the following statements best accounts for why the bond enthalpy method yields a significantly less exothermic value?
\[ \text{C}_2\text{H}_4(g) + \text{H}_2\text{O}(g) \rightarrow \text{C}_2\text{H}_5\text{OH}(l) \]
The student calculates \(\Delta H^\circ_{\text{rxn}}\) using standard enthalpies of formation, \(\Delta H_f^\circ\), obtaining a value of \(-88 \text{ kJ/mol}_{\text{rxn}}\). When estimating \(\Delta H^\circ_{\text{rxn}}\) using average bond enthalpies, the student obtains a value of \(-42 \text{ kJ/mol}_{\text{rxn}}\). Which of the following statements best accounts for why the bond enthalpy method yields a significantly less exothermic value?
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