Why is the Degree of Dissociation less satisfactory than the Acid Dissociation Constant as a measure of the strength of acid?
- I am not sure the exact difference but as first glance they seemed related yet somehow degree of dissociation is more like a percentage value whereas Ka is an absolute value. Absolute values are surely more accurate right?
- Can it be because Ka measures the exact amount of H+ and the conjugate base relative to the amount of the weak acid, but degree of dissocation can be inaccurate especially for dibasic and polybasic acids?
They are of course, intimately related. Degree of dissociation is to be calculated or derived from Ka, the acid dissociation constant, which is fixed for any given acid, at a particular temperature.
Your 1st point is correct, but it is really an issue of technicality rather than Chemistry concept. Ka is direct, dissociation degree is indirect and derived as a result. Hence, Ka values are more useful.
Your 2nd point is partially correct; there will necessarily be different degrees of dissociation for dibasic (diprotic) and polybasic (polyprotic) acids, depending on whether you're referring to the 1st acidic proton, the 2nd, and so on. Just as there is Ka1, Ka2, etc.
As far as the H2 syllabus is concerned, you mainly need to work with Ka. Possibly, the question (in the 'A' level exam) might ask you to obtain the degree of dissociation, given the Ka value.
Simply use 'alpha' or 'a' for degree of dissociation in the ICE table; where c = molarity/concentration in mol/dm3 of the acid.
_____________HA ---> H+ + A-
Initial c 0 0
Change -ca +ca +ca
Equilibrium c(1-a) ca ca
Hence, substituting Ka value into the Ka expression (that is based on the equilibrium molarities of the ICE table above), you can work out the value of the degree of dissociation.
There are quite a number of acid-base equilibria 'A' level questions that can be done by two alternative working pathways. One alternative working involves degree of dissociation, the other (alternative working) without using degree of dissociation, but in moles directly (ie. not percentage dissociation). Both alternative methods will give you the correct answer, it's up to you which you prefer to use in the 'A' levels.