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Electronegativity affects the relative strength of an acid because the more electronegative an atom is, the more likely it wants to "hold on" to the hydrogen atom attached to it, therefore making it harder for the acid to deprotonate and for the H+ ion to detach from it. For example, comparing HF and HCl, fluoride is more electronegative, so it is stronger in holding to the hydrogen atom, but chloride is less electronegative, so it would be easier to pull the H+ ion away for it than it would be from the fluoride.
For an oxoacid, the rules would be slightly different as it is dependent on if the resulting anion is stabilized by delocalizing the negative charge. Basically, if the compound is Cl-O-H, it will be more acidic than I-O-H because it is more stabilized.The Cl is more electronegative than I, so it pulls more electrons away from the O than does the I. This would stabilize the charge more in the respective compound. So in this case, the rule regarding longer bonds being stronger acids does not necessarily apply because in the presence of oxygen, the strength of the acid is more dependent on the stability of the negative charge than the bond length.
Why is it a rule that the more electronegative an atom is that is attached to hydrogen, the stronger the acid (in reference to molecules like HCl vs. H2S)? This seems counterintuitive, because a higher electronegativity difference should mean a more ionic and therefore a stronger bond.
Strong acids and bases completely dissociate in solution. Therefore, they must have a small change in electronegativity between atoms in order for the molecule to completely dissociate in solution. On the other hand, weak acids and bases may partially dissociate in solution, but they do not completely dissociate.
As you go down the group of the periodic table, acidity increases with the decrease of electronegativity (since the size of the atom increases). The smaller the atom, the shorter the bond and thus harder for them to dissociate in water. In my notes from class it says that HF<HCl<HBr<HI which goes from less acidic (short, strong bond) to more acidic (long, weaker bond).
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