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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: a_huynh00 on April 17, 2009, 12:10:51 AM

Title: How does electronegativity affect the strength of an acid?
Post by: a_huynh00 on April 17, 2009, 12:10:51 AM

My book says that more oxygen because of its electronegativity increases the strength of an acid. For example HClO is a weak acid compared to HClO4.

Is it because the oxygen attracts the H more to the oxygen thus the H has a weak bond to the rest of the atom. So it ionizes easier to break up like a strong acid?

I'm confused.

Title: Re: How does electronegativity affect the strength of an acid?
Post by: nj_bartel on April 17, 2009, 12:24:32 AM

The increased dipole moment caused by the electronegative oxygens pulls electron density away from the hydrogen, which has a low electronegativity.  This makes the hydrogen more prone to dissociate.  The other factor that increases acidity is resonance stability of the conjugate base - I'll go into this if you've discussed resonance before.

Title: Re: How does electronegativity affect the strength of an acid?
Post by: a_huynh00 on April 17, 2009, 12:27:53 AM

Yup we talked about resonance. How does resonance effect the strength of an acid though?

Title: Re: How does electronegativity affect the strength of an acid?
Post by: nj_bartel on April 17, 2009, 12:45:18 AM

Ok, I can give a truer answer then.

The acidity of an acid depends on the stability of it's conjugate base - the more stable the conjugate base, the stronger the acid.  One of the biggest factors contributing to stability is electron delocalization.  By electron delocalization, I mean the electrons have more space to move around.

http://www.chemguide.co.uk/inorganic/period3/clo4a.gif

That's the structure of the perchlorate anion.  Due to resonance effects, the negative charge is actually shared equally between all 4 oxygens, to give this structure

http://upload.wikimedia.org/wikipedia/commons/7/7c/Perchlorate-2D-dimensions.png

The negative charge (electrons) are spread out over a large area on several electronegative atoms (which are happy to accept the partial negative charge, due to their electron affinity).  The combination of these things leads to a stable conjugate base.

Applying this info to my previous post - perchloric acid exhibits resonance too - there just exists a formal positive charge on the oxygen attached to the hydrogen and a formal negative charge on the other oxygens.  The other oxygens are resonance electron withdrawers, which means there is less electron density near the hydrogen, making it more prone to dissociate.