8.67 Enthalpy of Formation

[Emily Glaser 1F]

In this question, it asks to find the enthalpy of formation of various substances, like H20. Why are the answers different than the enthalpy of formation found in Appendix A2.

The delta H formation of H20 in this problem is -242kj

In Appendix A2, the delta H formation of H20 is -285kj

Both are in liquid phase

[Sara Varadharajulu]

The enthalpy of formation will be different as we are using bond enthalpies to estimate. While the bond enthalpies for diatomic molecules are accurate (since they are measured for these molecules), the other bond enthalpies are simply estimates---they are found by averaging the data gathered from many molecules. this is why when using bond enthalpies to calculate the enthalpy of formation, your answer is not exact.

[Andrea ORiordan 1L]

Also note that the ∆H of H2O is not -242 as a liquid. That is its gaseous state. To obtain the answer, -286 kJ/mol, you simply subtract the ∆H(vap)= 44.0kJ, to get -286kJ/mol.