Why sulfuric acid is stronger than phosphoric acid
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Why sulfuric acid is stronger than phosphoric acid
I was just wondering why phosphoric acid is weaker than sulfuric acid. When I tried looking at this online, most sources just listed the pKa's, and I don't think that is a sufficient way to answer my question.
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Re: Why sulfuric acid is stronger than phosphoric acid
To understand why acids are stronger or weaker relative to each other, it is important to look at the stabilities of the conjugate bases. In general, the more stable a conjugate base is, the stronger its acid is. Often times, resonance is one of strongest contributors to stability because it can offer the delocalization of the negative charge.
With the case of H2SO4 and and H3PO4, you’d notice after you remove the first acidic proton (remember they are polyprotic), the corresponding conjugate base of H2SO4, which is HSO4-, has more resonance structures than the conjugate base of H3PO4, which is H2PO4-. That means that HSO4- is able to delocalize its -1 charge over 3 oxygen atoms whereas H2PO4- can only delocalize its -1 over 2 oxygen atoms.
Since HSO4- has increased resonance stabilization, it is less likely than H2PO4- to regain a proton (as a base would) to reform the acid.
With the case of H2SO4 and and H3PO4, you’d notice after you remove the first acidic proton (remember they are polyprotic), the corresponding conjugate base of H2SO4, which is HSO4-, has more resonance structures than the conjugate base of H3PO4, which is H2PO4-. That means that HSO4- is able to delocalize its -1 charge over 3 oxygen atoms whereas H2PO4- can only delocalize its -1 over 2 oxygen atoms.
Since HSO4- has increased resonance stabilization, it is less likely than H2PO4- to regain a proton (as a base would) to reform the acid.
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Re: Why sulfuric acid is stronger than phosphoric acid
it has to do with the formal charge of both; sulfur in H2SO4 has a charge of +2, while phosphorus has a +1 charge. Sulfur has a higher charge, so it'll withdraw e- density more from the surrounding oxygens than phosphorus. (e- density decreases so more H atoms in H2SO4 will disassociate). hope that helps somewhat, I'd say this is a partway explanation ?
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Re: Why sulfuric acid is stronger than phosphoric acid
How did you post the images there? I've tried to do the same to post replies but don't know how.
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Re: Why sulfuric acid is stronger than phosphoric acid
Nathan Rothschild_3D wrote:How did you post the images there? I've tried to do the same to post replies but don't know how.
When you find an image on google images or wherever, instead of clicking on "Copy," click on "Copy image address. Then you would paste the image address inside after clicking on "Img" in the reply box. Using substitution, let's say the image address is "1234". So that means the entry would be [img]1234[/img]. Hope this helps!
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