Acidic Strength

“The extent to which an acid transfers proton to a base, is a measure of its strength as an Acid.”

• It is the measure of the %age of ionization and not concentration.
• A larger value of $K_a$ signifies a stronger acid. $K_a=\frac{[H_3{O}^{+}][A^{-}]}{[HA]}$
• A larger value of $p{K}_a$ signifies a weaker acid. $p{K}_a=-log[K_a]$
• Stronger the acid, weaker its conjugate base.
• Any factor that stabilizes the conjugate base of an acid increases its acidity.

“The strength of a Bronsted-Lowry Acid depends on the extent to which a proton can be separated from it and transferred to a base.” Removing the proton involves breaking a bond, and making the conjugate base more electrically negative.

• Down a group, acidic strength increases. This is due to the decreasing effectiveness of orbital overlap between smaller and larger elements. Lesser the effectiveness of orbital overlap, weaker is the bond. With the bond being weaker, it is easier for it to lose a proton and hence is more acidic in nature.
• Left to right on a period, acidic strength increases. Due to electronegativity increasing, the bond is polarized. Not only that, but due to the electronegative nature, the anion (conjugate base) formed is able to retain the electron gained readily.
• With increase in $s$-character, electronegativity increases and hence so does acidity.