Base Dissociation Constant

 Hello! yesterday we started talking about the whole idea of the ionization or dissociation of a base. We said that if a base dissociates completely in solution, then it is a strong base. If the base does not dissociate completely in solution, then it is a weak base. We also established that the extent of dissociation or ionization of a weak base is often very minimal. We ended by saying that there is a constant that can be used to deduce the extent of dissociation of a base and that is called the base dissociation constant(Kb).


Imagine for a moment that B is a solution of a weak base and the weak base is dissolved in water. The reaction of the weak base with water can be seen as;

:B(aq) + H2O(l) <-> HB^+(aq) + OH^-(aq)

[The base accepts a proton from water according to the Bronsted – Lowry definition of bases]


Just as I explained while discussing the dissociation of acids (I asked readers to review the posts on acid dissociation constant especially the post on Jun.19, 2023 on the page timeline), we would have that the base dissociation constant could be written as;

Kb = [HB^+] [OH^-]/[B]

Why did I exclude H2O from the equilibrium expression? Well, you would find the answer in the earlier post about the acid dissociation constant Ka that I just referred to above. 


This implies that when dynamic equilibrium is attained in the system(dynamic equilibrium in a chemical system implies that the rate of forward reaction is equal to the rate of reverse reaction), we are going to have some of the ionized base as well as a large proportion of the unionized base molecules.


Hence, the Kb is a physical constant the shows the extent of ionization of a weak base in solution at a given temperature. Recall that Kb is an equilibrium constant hence its value only changes with change in the temperature of the system. Since most observations of base dissociation occur at room temperature (25oC or 298 K), the values of Kb published in standard chemical literature often suffice.


Tomorrow we shall begin to consider mathematical problems that involve the base dissociation constant. Do not forget to reach out to me via messenger, WhatsApp, telegram, email, phone call or text message if you have any confusion that you would want me to help you clear up.


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