strength increases Similarly, H S is stronger acid than H O. But, when we discuss elements in the same row of the periodic table, H-A bond polarity becomes the deciding factor for determining the acid strength. As the electronegativity of A increases, the strength of the acid also increases. For example, Electronegativity of A increases CH < NH < H O < HF Acid strength increases .
. Common Ion Effect in the Ionization of Acids and Bases Consider an example of acetic acid dissociation equilibrium represented as: CH COOH(aq) H + (aq) + CH COO – (aq) or HAc(aq) H + (aq) + Ac – (aq) K a = [H + ][Ac – ] / [HAc] Addition of acetate ions to an acetic acid solution results in decreasing the concentration of hydrogen ions, [H + ]. Also, if H + ions are added from an external source then the equilibrium moves in the direction of undissociated acetic acid i.e., in a direction of reducing the concentration of hydrogen ions, [H + ]. This phenomenon is an example Table .
The Ionization Constants of Some Common Polyprotic Acids (298K) It can be seen that higher order ionization constants ( K a , K a ) are smaller than the lower order ionization constant ( K a ) of a polyprotic acid. The reason for this is that it is more difficult to remove a positively charged proton from a negative ion due to electrostatic forces. This can be seen in the case of removing a proton from the uncharged H CO as compared from a negatively charged HCO – . Similarly, it is more difficult to remove a proton from a doubly charged HPO – anion as compared to H PO – .
Polyprotic acid solutions contain a mixture of acids like H A, HA – and A – in case of a diprotic acid. H A being a strong acid, the primary reaction involves the dissociation of H A, and H