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Acid strength, anion size, and bond energy

Video transcript

let's say we have a binary acid H X 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 and 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 3 hydrochloric acid with an approximate pKa of negative 7 hydrobromic acid at negative 9 and hydroiodic acid at negative 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 - so here we have all 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 H I the iodide and I must be the most stable because H I 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 an ion instead of ion here so we increase in the size and the size of the eeee 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 on the iodide anion and we have this charge spread out over a large volume of space and 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 and so that destabilizes this anion compared to the iodide anion and the iodine anion becomes the most stable and therefore H I is the most likely to donate a proton and therefore H 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 fluorines are 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 right 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 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 right 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 dissociation energies so you 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 right 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 and I should say these are our proximate bond energies and you'll see several different values in different textbooks so if the H I bond is the is the easiest to break that means when you're thinking about the acids right 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 and hydroiodic acid is the strongest out of these four binary acids