Now, using the relation for conjugate acid-base pair, K a × K b = K w using the value of K b of NH from Table . . We can determine the concentration of conjugate acid NH + K a = K w / K b = – / . × – = .
× – . . . Di- and Polybasic Acids and Di- and Polyacidic Bases Some of the acids like oxalic acid, sulphuric acid and phosphoric acids have more than one ionizable proton per molecule of the acid.
Such acids are known as polybasic or polyprotic acids. The ionization reactions for example for a dibasic acid H X are represented by the equations: H X(aq) H + (aq) + HX – (aq) HX – (aq) H + (aq) + X – (aq) And the corresponding equilibrium constants are given below: K a = {[H + ][HX – ]} / [H X] and K a = {[H + ][X - ]} / [HX - ] Here, K a and K a are called the first and second ionization constants respectively of the acid H X. Similarly, for tribasic acids like H PO we have three ionization constants. The values of the ionization constants for some common polyprotic acids are given in Table .
. In general, when strength of H-A bond decreases, that is, the energy required to break the bond decreases, HA becomes a stronger acid. Also, when the H-A bond becomes more polar i.e., the electronegativity difference between the atoms H and A increases and there is marked charge separation, cleavage of the bond becomes easier thereby increasing the acidity. But it should be noted that while comparing 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 HF << HCl << HBr << HI Acid