Acidity Across Periods: Does Electronegativity Matter?

[nothing123]

So acidity increases across a period because the electronegativity of the conjugate base increases as move from left to right. This means it can better stabilize the negative charge. Something else that contributes to this trend is the fact that as move from left to right across a period, the bond between H and the other atom becomes more polar, allowing the bond to be broken easier (ex. HF is more polar than NH3). Is that correct?

However, isn't HF a weaker acid than HI because HF is in fact a stronger bond. Doesn't this contradict the fact that HF is more polar and the bond is easier to break. Which one of the statements is right? I'm aware that the fact that I- is bigger allows the electrons to be delocalized better in the periphery and this is probably the major contributing factor to the acidity down a group but can someone please explain the electronegativity bit?

Thanks.

 

[lzkelley]

HF is more polar, but has a stronger bond - i think you're reasoning is correct, but let me rephrase.
Because 'I' has so many electrons already, the electrons in the bond with 'H' are "sheilded" from the nucleus. i.e. the outermost electrons are not only attracted to the nucleus, but also repelled by coulomb forces with the other electrons - decreases the bond strength. This is a fairly complicated issue and its hard to incorporate it in the general trends of the periodic table.
Electro negativity increases - in general - from left to right, and top down; but not universally. As soon as you form large nuclei, the electro negativity decreases again.

 

[Blueshift5]

I would like to clarify that both statements are correct. The increase in acidity across a period is primarily due to the increase in electronegativity and the ability of the conjugate base to stabilize the negative charge. However, the strength of the bond between hydrogen and the other atom also plays a role in determining the acidity of a compound. This is why HF, despite being more polar, is a weaker acid than HI. The bond between H and I is weaker, making it easier to break and release the hydrogen ion.

In terms of electronegativity, it is important to note that it is not the sole determining factor in acidity. Other factors such as bond strength and the size of the conjugate base also play a role. This is why we see variations in acidity even within a period or a group of elements.

Furthermore, as you mentioned, the size of the anion also affects the acidity of a compound. In general, larger anions can better stabilize the negative charge, making the corresponding acid more acidic.

I hope this explanation helps to clarify any confusion about the relationship between electronegativity and acidity. It is important to consider multiple factors when determining the acidity of a compound, and electronegativity is just one piece of the puzzle.