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200 chemistry K = {[H ][X ]} / [HX ] In general, when strength of H-A bond a2 decreases, that is, the energy required to break Here, Ka1nd K aa2 called the first and second the bond decreases, HA becomes a stronger ionization constants respectively of the acid 2 acid. Also, when the H-A bond becomes more X. Similarly, for tribasic acids like H PO we 3 4 polar i.e., the electronegativity difference have three ionization constants. The values between the atoms H and A increases and of the ionization constants for some common there is marked charge separation, cleavage polyprotic acids are given in Table 6.8. of the bond becomes easier thereby increasing Table 6.8 T he Ionization Constants of Some the acidity. Common Polyprotic Acids (298K) Butitshouldbenotedthatwhilecomparing elements in the same group of the periodic table, H-A bond strength is a more important factor in determining acidity than its polar nature. As the size of A increases down the group, H-A bond strength decreases and so the acid strength increases. For example, Size increases It can be seen that higher order ionization HF << HCl << HBr << HI constants (K , K ) are smaller than the Acid strength increases 2 3 Similarly, H S is stronger acid than H O. lower order ionization constant (K a ) of a 2 2 polyprotic acid. The reason for this is that But,whenwediscusselementsinthesame it is more difficult to remove a positively row of the periodic table, H-A bond polarity charged proton from a negative ion due to becomes the deciding factor for determining electrostatic forces. This can be seen in the the acid strength. As the electronegativity case of removing a proton from the uncharged of A increases, the strength of the acid also H CO as compared from a negatively charged increases. For example, 2 –3 HCO 3. Similarly, it is more difficult2–o remove Electronegativity of A increases a proton from a doubly charged HPO 4 anion as compared to H PO .– CH < NH < H O < HF 2 4 4 3 2 Polyprotic acid solutions contain a Acid strength increases mixture of acids like H A, HA and A in case 2 6.11.8 Common Ion Effect in the of a diprotic acid. 2A being a strong acid, the primary reaction involves the dissociation of Ionization of Acids and Bases H A, and H O in the solution comes mainly Consideranexampleofaceticaciddissociation 2 3 from the first dissociation step. equilibrium represented as: + – 6.11.7 Factors Affecting Acid Strength CH C3OH(aq) H (aq) + CH C3O (aq) or HAc(aq) H (aq) + Ac (aq) Having discussed quantitatively the strengths of acids and bases, we come to a stage where K = [H ][Ac ] / [HAc] a we can calculate the pH of a given acid Addition of acetate ions to an acetic solution. But, the curiosity rises about why should some acids be stronger than others? acid solution results in decreasing the concentration of hydrogen ions, [H +]. Also, What factors are responsible for making if H ions are added from an external source them stronger? The answer lies in its being a complex phenomenon. But, broadly speaking then the equilibrium moves in the direction of undissociated acetic acid i.e., in a direction we can say that the extent of dissociation of an acid depends on the strength and polarity of reducing the concentration of hydrogen of the H-A bond. ions, [H ]. This phenomenon is an example Reprint 2026-27