Accuracy of Enthalpy Calculation

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Accuracy of Enthalpy Calculation

Postby Tiffany_Cacy_3D » Sun Jan 27, 2019 12:22 pm

In my notes from lecture on January 25th I wrote that bond enthalpies of diatomic molecules are accurate but other bond enthalpies are not as accurate and I was wondering why? I didn't write an explanation so I was struggling to make sense of what this means.

Samantha Hoegl Roy 2C
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Re: Accuracy of Enthalpy Calculation

Postby Samantha Hoegl Roy 2C » Sun Jan 27, 2019 12:32 pm

the bond enthalpy of a diatomic molecule is the exact enthalpy needed to break the bond. For other bonds, the bond enthalpy is the average enthalpy of breaking this bond in several different molecules. This makes the number rather incorrect since it is just the average and not the exact enthalpy for the molecule you are concerned with.

Alyssa Bryan 3F
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Re: Accuracy of Enthalpy Calculation

Postby Alyssa Bryan 3F » Sun Jan 27, 2019 12:38 pm

The actual bond enthalpy depends on the neighboring molecules as well, so that is why there are varying values for the other molecules and the average is given.

David S
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Re: Accuracy of Enthalpy Calculation

Postby David S » Sun Jan 27, 2019 12:54 pm

This is because in reality, the strength and energy of a bond between individual atoms is influenced by other electrostatic forces in more complex molecules, and so therefore there is no single enthalpy value to easily describe most bonds.

Take for example a bond enthalpy for O-H. Not all O-H bonds in more complex molecules are the same, which means that their enthalpy values will likely differ as well. For instance, O-H bond is weaker in HClO4 (due to the electron withdrawing power of the other 3 O atoms) compared to the O-H bond in HClO.

To get around the differences in bond enthalpies for a given bond in different molecules, scientists averaged the enthalpies of that bond found in different molecular contexts.

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