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Topic: acid strength  (Read 26127 times)

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Offline dnbwise

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acid strength
« on: July 26, 2005, 11:02:31 PM »
I thought that a couple rules to follow regarding the strength of acids were:

1) for a nonmetal acid strength increases across a period (left to right) and down a group
2) acid strength increases with EN of nonmetal and as O atoms increase

One problem I came across asks which acid is stronger: H3AsO4 or H3PO4. The answer is H3PO4, but I thought since As further down the group then it would be the stronger of the two. Can someone please explain where my thinking is wrong.

Offline Eric

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Re:acid strength
« Reply #1 on: July 27, 2005, 02:46:47 AM »
Hmmm in this situation lets look at this electrically.  Electronegativity increases towards Flourine, so P will want to hold onto its electrons more than As.  These too only differ in the number of atomic particles, as oxidation number is the same.  H3PO4 will be more polar that H3AsO4 and therefore want to lose an H+.  And as far as that trend goes, I seem to recall that HF is weak and there is no Acid trend.   Here are the acid rules i know:  Binary acids (like HCl, HBr) are strong except for HF.  In oxoacids, when there are more hydrogens the acid is likely stronger.  Non metal oxides form acids in water.  There may be some im missing.  It is best to look at it in all terms possible.  

*side: HF although "weak" definition-wise, can dissolve glass and not even HCl can do that.

Offline Winga

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Re:acid strength
« Reply #2 on: July 27, 2005, 03:16:17 AM »
First thing I want to point out is that the electronegativity difference between P and As is only by 0.01.

Second, both P and As are not directly bonded to H.

I am not sure my answer is correct, but I think it can be explained in this way.

There is p(pi)-d(pi) dative bonding in H3PO4 (and its depronated forms) which some e- density from O p-orbitals can donate to vacant P d-orbitals, therefore, O is less basic.

While As d-orbitals (4d) are higher in energy so that this dative bonding is not quite favour.

Offline Eric

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Re:acid strength
« Reply #3 on: July 27, 2005, 03:45:08 AM »
.01 is a not lot, but we are not saying that H3PO4 is strong and H3AsO4 is weak, only that the first is strongER.  so cant we just say that P will draw the H electron more so than As and cause a larger dissociation rate?  This is unrelated to direct bonds.  Its not a binary acid. right? they would have the same lewis structure, but based on trends we should assume P is stronger.  no numbers given.
« Last Edit: July 27, 2005, 03:46:49 AM by Skimaster12 »

Offline Borek

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Re:acid strength
« Reply #4 on: July 27, 2005, 04:16:58 AM »
For the same electronegativity think about radius of central atom in the acid group.
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Offline sdekivit

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Re:acid strength
« Reply #5 on: July 27, 2005, 04:31:10 AM »
I thought that a couple rules to follow regarding the strength of acids were:

1) for a nonmetal acid strength increases across a period (left to right) and down a group
2) acid strength increases with EN of nonmetal and as O atoms increase

One problem I came across asks which acid is stronger: H3AsO4 or H3PO4. The answer is H3PO4, but I thought since As further down the group then it would be the stronger of the two. Can someone please explain where my thinking is wrong.

you should check for resonance hybrids. The conjugated base that has that has the most canonical structures and thus is the most stable will be the strongest acid.

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Re:acid strength
« Reply #6 on: July 27, 2005, 11:44:11 AM »
Acid strength depends on the charge separation of the electronegative atom and hydrogen (in facilitating the positive charge character on the hydrogen and thus the energy for such the acid base reaction) and inversely on the bond strength, both are somewhat dependeent on electronegativity.

The above trend has some exceptions, particularly in reference to the first element of each column/group.

You need to reread your text, I really don't know where you got those rules of thumb mentioned in the op...

After you reread your text on the strength of acids you should be able to understand why phosphoric acid is the stronger acid.
« Last Edit: July 27, 2005, 11:45:41 AM by GCT »

Offline dnbwise

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Re:acid strength
« Reply #7 on: July 27, 2005, 07:05:56 PM »
My text reads as follows:
the strength of an acid depends on its ability to donate a proton. Two factors determine how easily a proton is released from a binary non-metal  hydride:
1. Across a period, non-metal  hydride acid strength increases.
2. Down a group, non-metal hydride acid strength increases.

For oxoacids:
1. For oxoacids with the same number of oxygens around E (the central nonmetal), acid strength increases with the electronegativity of E.
2. For oxoacids with a different number of oxygen atoms around E, acid strength increases with the number of O atoms.

I think I was applying the non-metal hydride rules for H3AsO4, which is an oxoacid. With P being more electronegative than As (because electronegativity decreases down a group) this explains why H3PO4 is the stronger of the two.

Thanks to all.
« Last Edit: July 27, 2005, 07:07:09 PM by dnbwise »

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