Why is it when using bond enthalpies to calculate enthalpies of reaction, they are not always accurate and enthalpies of formation are preferred?

Answer 1
Well, those are defined to be averages, so for example, a #"C"-"C"# bond enthalpy is considered to be an estimate, and you know that estimates are inherently less accurate.

The effects of nearby atoms can strengthen or weaken any bond.

As an illustration:

#"H"_3stackrel(color(red)(2))"C"-stackrel(color(red)(1))"C""H"_2-"Br"#
Bromine is an electron-withdrawing group, so electron density is drawn towards it, making carbon-1 more electropositive (more partially-positively-charged) than without #"Br"#:
#"H"_3"C"-stackrel(color(red)(delta^(+)))"C""H"_2-stackrel(color(red)(delta^(-)))"Br"#
To stabilize itself, a more electropositive atom draws electron density from a less electropositive atom, like carbon-2 --- which, though it has become electropositive, is less so because it is farther away from #"Br"#.
#"H"_3stackrel(color(red)(delta^(+)))"C"-stackrel(color(red)(delta^(+)))stackrel(color(red)(delta^(+)))"C""H"_2-stackrel(color(red)(delta^(-)))stackrel(color(red)(delta^(-)))"Br"#
That makes the electron sharing in the #"C"-"C"# bond uneven, weakening the bond.
Thus, the bond enthalpy for this particular, weaker #"C"-"C"# bond is lower than average because less energy is required to break it.
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Answer 2

Bond enthalpies are not always accurate for calculating enthalpies of reaction because they represent average values and do not account for factors such as bond angle variations, molecular geometry, and electronic effects. Enthalpies of formation are preferred because they directly measure the energy change associated with forming one mole of a compound from its constituent elements in their standard states, providing more precise and reliable values for calculating enthalpies of reaction.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

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