A large value of Ka means the acid is a strong acid, and a small value of Ka means the acid is a weaker acid. If the Ka is greater than 10, the acid will be completely dissociated in water 3.3B ACIDITY AND pKa Chemists usually express the acidity constant, Ka, as its negative logarithm, pKa pKa=-logKa For acetic acid the pKa is 4.75 pKa=4og(1.76×1035)=-(-4.76)=476 Notice that there is an inverse relationship between the magnitude of the pKa and the strength of the acid
3.3B ACIDITY AND pKa A large value of Ka means the acid is a strong acid, and a small value of Ka means the acid is a weaker acid. If the Ka is greater than 10, the acid will be completely dissociated in water. Chemists usually express the acidity constant, Ka, as its negative logarithm, pKa. pKa = - logKa For acetic acid the pKa is 4.75: Notice that there is an inverse relationship between the magnitude of the pKa and the strength of the acid: pKa = -log(1.76 ×10 -5 ) = - (-4.76) = 4.76
The larger the value of the pka, the weaker is the acid CHr Coh< CF2 COH< HC pKa=4.76 pKa=0 pKa=-7 Weak acid Very strong acid increasing acid strength In pure water at 25C, the concentrations of hy fronium and hydroxide ons are equal to 10M. Since the concentration of water in pure water is 55.5M. we can calculate the Ka for water H3O[OH](0)(10 18×1016 (555) pKa=15.7
The larger the value of the pKa, the weaker is the acid. CH3 CO2 H ﹤ CF3 CO2 H ﹤ HCl pKa = 4.76 pKa = 0 pKa = -7 Weak acid Very strong acid increasing acid strength In pure water at 25℃, the concentrations of hyfronium and hydroxide Ions are equal to 10 -7 M. Since the concentration of water in pure water is 55.5 M, we can calculate the Ka for water Ka = [H3O + ] [OH - ] [H2 O] = (10 -7 )(10 -7 ) (55.5) = 1.8 ×10 -16 pKa = 15.7
3.3C PREDICTING THE STRENGTH OF BASES The principle which allows us to estimate the strengths of bases the stronger the acid the weaker will be its conjugated base So relate the strength of a base to the pKa of its conjugate acid The larger the pKa of the conjugate acid, the stronger is the base increasing base strength CH2CO OH Very weak base pKa of conjugate Strong base pKa of conjugate acid(CH3 H) pKa of conjugate acid(HCI)=-7 4.75 acid(H2O)=15.7
3.3C PREDICTING THE STRENGTH OF BASES The principle which allows us to estimate the strengths of bases: the stronger the acid, the weaker will be its conjugated base. So relate the strength of a base to the pKa of its conjugate acid. The larger the pKa of the conjugate acid, the stronger is the base. increasing base strength Cl - CH3 CO2 - OH - Very weak base pKa of conjugate acid (HCl) = - 7 pKa of conjugate acid (CH3 CO2 H) = 4.75 Strong base pKa of conjugate acid (H2 O) = 15.7