The acid dissociation constant Ka

• Strong and weak acids are terms used to describe the extent of dissociation of an acid
• A strong acid, such as HCl, will completely dissociate in aqueous solution
• A weak acid, such as ethanoic acid, CH3COOH, partially dissociates in aqueous solution
• The acid dissociation constant Ka is one of several special equilibrium constants used for acid-base equilibria. All the constants are just versions of the equilibrium constant Kc in terms of concentrations in mol dm-3
• The dissociation of any weak acid, HA can be shown as a general form: HA --> HA+ + H+
• The acid dissociation constant Ka is calculated as: Ka = [H+][A-]/[HA] with units: (mol dm-3) x (mol dm-3) / mol dm-3 = mol dm-3
• As with all equilibrium constants, Ka changes with temp and recorded Ka values are usually standardised at 25 degrees
• The larger the numerical value of Ka, the further the equilibrium value is to the right
• If you look at the Ka expression, the concentration of the dissociated ions are on top and so the larger the Ka value, the greater the dissociation and the greater the acid strength
• The Ka values highlight a potential problem - it is difficult to compare numbers with negative indices. As with the pH scale, the problem has been solved by converting Ka value into a negative logarithm called pKa
• pKa = -logKa
• Ka = 10^-pKa
• pKa values are much more manageable than Ka, and it is much easier to compare relative acidic strengths using pKa values than Ka values
• The stronger the acid, the larger the Ka and the smaller the pKa
• The weaker the acid, the smaller the Ka and the larger the pKa
• pKa values are used extensively for comparing the strengths of weak acids, particularly in biological systems
• [H+(aq)] depends on

  1. the concentration of the acid [HA(aq)]
  2. the acid dissociation constant Ka
• When HA molecules dissociate H+ and A- ions are formed in equal quantities.