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Under method two- additional comments from the notes it says “ 2) bond enthalpies of diatomic molecules are accurate (measured for those molecules)” what does this mean? And enthalpies of diatomic molecules are more accurate as opposed to what, exactly?
Bond enthalpies of diatomic molecules are able to be measured because they are only between two types of atoms, whereas enthalpies of other bonds are averages from different molecules. The bond enthalpy of the diatomic molecule H2 is +436 kJ. This enthalpy is able to be measured consistently because it is between the same atoms and the bond will most likely be the same each time. The bond enthalpy between O-H however, differs based on what molecule it is in. The O-H bond enthalpy in water is slightly different than that of methanol, so the O-H bond enthalpy is the mean bond enthalpy taken from different molecules.
Because diatomic molecules are not bonded to anything else, the bond enthalpies measured for those bonds are the most accurate. If that same bond occurs in a larger molecule, the bond enthalpy for the same bond in the larger molecule may be slightly different.
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