Deeper Introduction to Acid Dissociation Constant

 Hello fam, lets take the matter a little deeper from where we left it off yesterday. Remember that we defined the acid dissociation constant as the equilibrium constant that can be used to describe the dissociation or ionization of an acid in water. 


Let us now assume that we have a weak acid that has the chemical formula HA. We are using a weak acid here to illustrate the implication of the equilibrium process. We did not use a strong acid because it would dissociate completely in solution and there would be no acid molecules left to set up a dynamic equilibrium. 


When we dissolve this acid in water, we are going to notice that the reaction would be reversible because the acid is a weak acid. 

The equation that describes this process is;

HA(aq) <--> H+(aq) + A-(aq)


The reason why the reaction is reversible is that the acid does not dissociate or ionize to a large extent in solution. Hence, at every point in time, we would have the species HA, H+ and A- in solution. This implies that a dynamic equilibrium would be established in the solution. The rate at which HA is converting to H+ and A- would be the same as the rate at which H+ and A- are combining to form HA again.


Due to the fact that the weak acid would only ionize to a small extent in solution, we would have more of the HA than the H+ and the A- which are the ions that compose the acid. In order to describe the equilibrium expression for this reaction, we have to involve the idea of the acid dissociation constant which I introduced in yesterday’s lesson. This is the constant that tells us something about how much the HA has dissociated to form A- and H+. The acid dissociation constant is represented as Ka.


For the acid that we are considering in this context we can write that;

Ka = [H+] [A-]/[HA]   (refer to the earlier lesson on the meaning of equilibrium constant)


If the value of Ka is large, then we know that the acid dissociates to a large extent, and that the forward reaction is favored so we would have more of the ions that compose the acid than the original acid molecule. If the Ka is small, then we would have more of the original acid molecules that did not break up when the acid was added to water than the ions that compose the acid.


The key importance of the Ka value is that it gives us a yard stick to differentiate a weak acid form a strong acid. A weak acid would have a very small Ka (usually in the order of 10^-5 or lower) because the acid molecules do not break up to give the ions that compose the acid while the strong acid would have a very high Ka indicating that the acid molecules were completely broken up into its constituent ions when dissolved in water.


SALIENT POINT:

The Ka is the value that chemists use to distinguish between a strong acid and a weak acid.


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