Strength of Acids

[Uyenvy Nguyen 1D]

The bigger an anion in an acid, the stronger the acid is, right? For #11 on the week 10 Sapling homework, the acids given were HClO, HBrO, and HIO. It asked to list the acids from most acidic to least acidic. How come they considered the electronegativity of Cl, Br, and I instead of considering the size of them when determining the strengths?

[LarisaAssadourian2K]

From what I've understood, since there is an oxygen present in the compound and the Cl/Br/I are bonded to that, the electronegativity of the atom is used rather than the atomic size in determining the acid strength. I hope this helps!

[Isaac Wen]

I'm not entirely sure either, but I remember reading something about how if you're comparing elements down a column, bond length matters more whereas electronegativity matters more when comparing elements across a period.

[BrittneyMyint1D]

From my understanding, the H in each of those acids (HClO, HBrO, and HIO) are bonded to the O, so you have to consider the electronegativity rather than the size. However, if it was HCl, HBr, and HI, since the H are bonded to the anions, you can consider the size. Hope this helps!

[Hailey Qasawadish 2J]

You can only use the size of the atom to determine the relative acidity of two atoms bonded together, for example, HF vs HCl. Since Cl is bigger than F, the bond between HCl is longer and weaker than the bond between HF. Thus, the weaker bond is easier to break and the acid is stronger. When you have molecules such as HClO and HBrO, you can no longer use size. This is because the bond that breaks is the bond between the O and the H which is the same in both of these molecules. So instead, we have to look at the electronegativity of the atoms Cl and Br. Since Cl is more electronegative than Br, Cl has more electron-withdrawing power. This makes HClO a stronger acid than HBrO. Hopefully this makes sense!

[BoparaiAdeshsurjit2F]

Since there is oxygen present the acidity is now based on what has the highest electronegativity. There are way too many rules for acids and bases lol

[MMorcus2E]

From my understanding, you determine whether you look at electronegativity or size by looking at what the H is bonded to. Since all molecules have more than 2 atoms, you can't look at the atomic radius. Therefore, your next step would be to look at the electronegativity. Since electronegativity increases up a column, then Cl would be the most electronegative followed by Br and I respectively. Just remember to look at the number of atoms in the molecule and what the H is bonded to first!

[Evelyn Silva 3J]

In this case you would have to consider the electronegativities of each atom because they are not being bonded to H. You only consider atomic radius when you have a binary acid.

[reyvalui_3g]

In oxo acids the stronger acid is the one with the more electronegative atom attached to the OH group. This is because the atom will pull the electrons from the oxygen closer to it, making the OH bond more polar. In turn the molecule can deprotonate much easier.