How does polarity affect acid strength

An oxoacid sometimes called an oxyacid is an acid that contains oxygen. To be more specific, an oxoacid is an acid that:. Sulphuric acid : Drops of the concentrated oxoacid sulfuric acid sulphuric acid rapidly dehydrate a piece of cotton towel. These acids can be arranged in order of their pK a values and, by extension, their relative strengths:.

Recall that smaller values of pKa correspond to greater acid strength. Therefore, HOCl is the strongest acid and HOI is weakest, and acid strength decreases as the central halogen descends on the periodic table. Because Cl is the most electronegative, it draws the bulk of the electrons in the HOCl molecule toward itself; because H and Cl are on opposite ends of the molecule, Cl pulls at the electrons in the H-O bond, thereby weakening it.

Consider the family of chlorooxoacids, which are arranged below in order of pK a values:. The strongest acid is perchloric acid on the left, and the weakest is hypochlorous acid on the far right. Notice that the only difference between these acids is the number of oxygens bonded to chlorine. As the number of oxygens increases, so does the acid strength; again, this has to do with electronegativity.

Carboxylic acids are an important subclass of organic oxoacids, characterized by the presence of at least one carboxyl group. For the halogen-containing acids above, HF has the strongest bond and is the weakest acid.

The strong bond between the more similarly-sized 'H' and 'F' atoms doesn't want to break and allow the 'H' to transfer. HI, on the other hand, is a very strong acid. The big 'I' atom overpowers the helpless little 'H' and the H-I bond is very weak. When comparing acids that have 'A' atoms in the same row, bond polarity differences are more important in determining acid strength.

That's because bond strength differences are much smaller between atoms nearby each other in the same row.

Bond polarity is largely determined by the electronegativity difference between the two atoms involved in the bond. Electronegativity is basically how much an atom wants electrons. Think of electronegativity as a measure of an atom's electron crush.

Yes, we're talking about the boy band kind of crush. For bonds that involve an atom that has a huge electron crush and an atom that just isn't feeling the electron love, the bond tends to be really polar. The electrons reciprocate the crush and go for the atom that likes them the most.

Isn't that nice? The hydrofluoric acid HF bond is polar because F really loves electrons. Let's compare this to CH 4. Carbon is in the same row as fluorine check out the figure above , but HF is a much stronger acid then CH 4. The C-H bonds are not polar compared to the H-F bond.

Consider the following K a data, for example. The acidity of these oxyacids increases significantly as the oxidation state of the central atom becomes larger. This trend is easiest to see in the four oxyacids of chlorine. This factor of 10 11 difference in the value of K a for hypochlorous acid HOCl and perchloric acid HOClO 3 can be traced to the fact that there is only one value for the electronegativity of an element, but the tendency of an atom to draw electrons toward itself increases as the oxidation number of the atom increases.

As the oxidation number of the chlorine atom increases, the atom becomes more electronegative. This tends to draw electrons away from the oxygen atoms that surround the chlorine, thereby making the oxygen atoms more electronegative as well, as shown in the figure below.

As a result, the O-H bond becomes more polar, and the compound becomes more acidic. Ernest Z. Mar 18, Explanation: When shared electrons in an "H-X" bond spend most of their time close to "X" atom, the bond becomes polar. It also becomes weaker. It is easier for the proton to leave the molecule, so the compound becomes more acidic.