Acid strength, anion size, and bond energy

- Let's say we have a binary acid, HX, where X is equal to a halogen. So X is equal to Fluorine, Chlorine, Bromine, or Iodine. If HX donates this proton, we're left with the conjugate base, which is X minus. We saw from the previous video the more stable the conjugate base, the more likely HX is to donate a proton. So the more stable the conjugate base, the stronger the acid. If we take a look at these four binary acids here, we have hydrofluoric acid with a pKa value of approximately positive three, hydrochloric acid with an approximate pKa of negative seven, hydrobromic acid at negative nine, and hydroiodic acid at -10. We know the lower the pKa value, the stronger the acid. So as we go down this way, we are decreasing in pKa values, and therefore, we are increasing in acid strength. So we're increasing in acidity. Therefore, hydroiodic acid is the strongest acid out of these four because hydroiodic acid has the lowest value for the pKa. If hydroiodic acid is our strongest acid, the conjugate base must be the most stable. So the conjugate base to hydroiodic acid would be the iodide anion, I minus. So here we have all of the different conjugate bases. We'd have the fluoride anion, which is the conjugate base to HF. We have the chloride anion, which is the conjugate base to HCl. We have the bromide anion, which is the conjugate base to HBr, and of course, again, the iodide anion, the conjugate base to HI. The iodide anion must be the most stable because HI is our strongest acid. So we can explain the stability of this conjugate base in terms of the size of the ion. Remember as you go down a group on the periodic table, you would increase in the size of the anion. So let me go ahead and write anion instead of ion here. So we increase in the size, in the size of the anion. So why does that help to stabilize the conjugate base? Well we need to think about this negative charge here. So we have a negative charge in the iodide anion, and we have this charge spread out over a large volume of space. That makes the anion more stable. So remember electrons repel each other, but if you can spread out the negative charge over a large amount of space, then you can better stabilize that negative charge. So this is more stable than, for example, the fluoride anion. The fluoride anion has a negative charge that's concentrated in a small volume of space. So that destabilizes this anion compared to the iodide anion. The iodide anion becomes the most stable, and therefore, HI is the most likely to donate a proton, and therefore, HI is our strongest acid out of these four. Notice this is different from the previous video where we talked about electronegativity. There we were comparing elements in the same period on the periodic table. So we were moving horizontally across our periodic table this way. And in that video, the fluoride anion was the most stable one because fluorine's our most electronegative element, and therefore, best able to stabilize a negative charge. But as you go down a group on the periodic table, your electronegativity decreases. So that can't be the dominant trend because if your electronegativity decreases as you go down, just thinking about electronegativity, that would predict HF to be the strongest acid, and that's not what we observe. So as you go down a group on the periodic table, it's the size of the anion that determines the stability of the conjugate base. So the larger the anion, the better it is to stabilize a negative charge, and therefore, the more stable the conjugate base. The more stable the conjugate base, the more likely HX is to donate a proton, and therefore, the stronger the acid. Another very important factor to think about is the strength of the bond. We've already said that hydroiodic acid is our strongest acid with the lowest pKa value. So this bond right here must be the easiest to break. If it's easy to break this bond, that makes it easy to donate this proton. So we can get an idea of the bond strengths for our binary acids by looking at bond association energy. So we could also call these bond energies or bond enthalpies. So remember bond dissociation energy measures the amount of energy that's needed to break a bond in the gaseous state. So if we look at our hydrogen halides and we think about our bonds, notice what happens to the bond energy. It's the hardest to break this bond, the bond between hydrogen and fluorine. This takes the most energy to break this bond, and as you go down, we see we decrease in bond dissociation energy. So it only takes 299 kilojoules per mole to break this bond between H and I. I should say these are approximate bond energies and you'll see several different values in different textbooks. So if the HI bond is the easiest to break, that means when you're thinking about the acids, this bond is the easiest to break, therefore, it's the most likely to donate a proton, and therefore, it has the lowest value for the pKa. Hydroiodic acid is the strongest out of these four binary acids.