Electronegativity and acid strength

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Kevin Wright
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Electronegativity and acid strength

Postby Kevin Wright » Fri Nov 27, 2015 8:43 pm

How does the electronegativity of an atom in a molecule effect the strength of it being an acid?

Sandeep Gurram 2E
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Re: Electronegativity and acid strength

Postby Sandeep Gurram 2E » Fri Nov 27, 2015 8:57 pm

The more electronegative a certain atom is in a molecule, the more it draws on the electron density of the overall molecule. This pulling actually results in a higher stabilization state for the molecule. Therefore, the more electronegative a particular element is, the harder it pulls, and the more stable this molecule is. The more stable the molecule is, the stronger the acid. So to summarize, as electronegativty of this atom increases, so does the strength of the acid.

Adriana Juarez 1
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Re: Electronegativity and acid strength

Postby Adriana Juarez 1 » Fri Nov 27, 2015 9:34 pm

Is the above explanation only true for oxoacids?

Because when you compare molecules such as HCl, HF, HBr, and HI, acidity increases as electronegativity (and thus electron affinity) decreases. A strong acid easily loses the H+ ion, so the stronger acids have to have a lower electron affinity...

Leena Tran 2K
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Re: Electronegativity and acid strength

Postby Leena Tran 2K » Sat Nov 28, 2015 12:59 am

If A is more electronegative, the bond in HA is also more polar. The H atom will have a greater partial positive charge, which creates a stronger hydrogen bond between the H atom and the O atom of the H2O molecule. Because it is then easier for the proton, H, to be transferred, HA is a stronger acid.

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Re: Electronegativity and acid strength

Postby Chem_Mod » Sat Nov 28, 2015 11:09 am

Adriana's observation is correct:

When comparing acids in which there is just one atom besides H (like HF, HCl, HBr, etc.) looking at the bond length/strength is a good measure of how acidic the acid is: The stronger/shorter the bond, the less likely it is to dissociate and the weaker the acid (i.e. HF is the weakest acid here).

When comparing acids with more than H and one other atom, and the same type of atom directly attached to the acidic proton (like HBrO2 vs. HClO2 where the O-H bond is broken for both--i.e. they're both oxoacids) it is best to look at the stability of the conjugate base. In this case, BrO2- is less stable because Br is less electronegative than Cl so the charge of BrO2- is less spread out than in ClO2-.

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