The Acid Dissociation Constant, Ka
Weak acids
A weak acid is an acid that partially (or incompletely) dissociates in aqueous solutions
- Eg. all organic acids e.g. ethanoic acid and some inorganic acids HCN (hydrocyanic acid), H2S (hydrogen sulfide) and H2CO3 (carbonic acid)
- The position of the equilibrium is more over to the left and an equilibrium is established
The diagram shows the partial dissociation of a weak acid in aqueous solution
- As this is an equilibrium we can write an equilibrium constant expression for the reaction
- This constant is called the acid dissociation constant, Ka, and has the units mol dm-3
- Values of Ka are very small, for example for ethanoic acid Ka = 1.74 x 10-5 mol dm-3
- When writing the equilibrium expression for weak acids, the following assumptions are made:
- The concentration of hydrogen ions due to the ionisation of water is negligible
- The value of Ka indicates the extent of dissociation
- The higher the value of Ka the more dissociated the acid and the stronger it is
- The lower the value of Ka the weaker the acid
pKa
- The range of values of Ka is very large and for weak acids, the values themselves are very small numbers
Table of Ka values
- For this reason it is easier to work with another term called pKa
- The pKa is the negative log of the Ka value, so the concept is analogous to converting [H+] into pH values
pKa = -logKa
- Looking at the pKa values for the same acids:
Table of pKa values
- The range of pKa values for most weak acids lies between 3 and 7