pKa and Ka explained

Below you will find an explanation of both Ka and pKa. 

Ka is named the ionization constant. It is the equilibrium constant for the ionization of an acid. The ionization of an acid is the same as an acid splittings of a hydrogen ion.

This ionization of an acid is easily illustrated by some examples:

H2CO3 => HCO3- + H+    (H+ is often written as H3O+, but it's basically the same) 
CH3COOH => CH3COO- + H+
HCOOH => COOH + H+  

As you can see H+ is split of from the starting compound. The tendency of this "splitting off H+ action" is quantified by the Ka value. The larger the Ka value is the bigger is the tendency towards splitting off H+.

The best way to learn the meaning of Ka-values is to look at an example.

HA + H2O <=> H+ + A+

HA is an acid that splits of an H+ to create a conjugate (A-) and a free acidifying H+.

The Ka-value is defined the following way when HA is the acid and A is the conjugate of the acid.

 

The larger Ka is the more will be on the ionized form which is the same as most of the acid (HA) has split off its protons (H+).

Sometimes it can be difficult to look at Ka-values and guess how much ionized and unionized acids exists. For this reason the pKa values was invented. The pKa value makes it easier to handle equilibriums. pKa is defined as:

or

 

If we have AH in equilibrium with A- in a solution we know that half is on the form AH and half is on the form A- when pKa equals pH. 

This comes from the calculations done in the section about the NH3 / NH4+ equilibrium where its shown how to calculate the concentration of NH3 when we know ([NH3] + [NH4+]). From this equation (below) it's easy to see that we have half NH3 and half  NH4+ if [H+] equals Ka, equivalent to pH equals pKa. 

NH3 =

Notice that when pH equals pKa or [H+] equals Ka the second part of the denominator is 1, just like the first part of the denominator - hence divided by two.

If you want to go more into details about pKa- and Ka-values I suggest you visit the resource section or watch the flash presentation about pKa- and Ka-values.

 

By the way:

The acids from the table above have the following Ka-values:

H2CO3  Ka: 4.5 E-7
H2CO3  Ka: 1.74 E-5
HCOOH  Ka: 1.78 E-4

 

Since the Ka value of HCOOH is larger than the Ka-values of the other acids, HCOOH is he strongest acid and has the biggest tendency towards splitting off hydrogen. 

Temperature and salinity

Ka is both dependent on temperature and salinity. In many textbooks about chemistry this is not considered. In the salinity and Ka section you can learn more about how salinity affects Ka values. In the section about temperature and Ka you will learn how temperature effects Ka-values.

 

 

 

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