pKa in Chemistry : Definition

Definition

Ka, acid dissociation constant, is a ratio that indicates the extent of dissociation of acid under a specific temperature, i.e. how strong the acid is.

Consider this reaction: AH2(aq) → A-(aq) + 2H+(aq)

This equation denotes the ionization of an acid in water. For example, 10 M of A- and 10M of H+ presents in the solution, while only 1 M of AH remains. After AH is dissolves in water, the Ka of AH is : [100]^2 * [100] / [100] = 10000 (M^2)

However, using Ka to compare the strength of acids is not convenient, as the number would often get confusingly too big or too small, etc. So we will use pKa instead, to make the number neater. Just like converting [H+(aq)] to pH, simply time the Ka with “ -log “, and pKa of the acid can be obtained.

Hence, pKa of AH = -log(10000) = -4

The smaller the pKa, the more able a molecule to give out hydrogen ions and vice versa. However, unlike pH, a high pKa doesn’t mean the molecule is highly acidic and alkaline, as ethane has a pKa of over 50, but it has a pH of 7.

Relationship between Pka and ka

• pKa is the negative logarithm of a dissociation constant (Ka).
• This affects the protonation or charge state of molecules in solution. At a pH equivalent to the pKa half of the molecules are charged, which is protonated (bases) or deprotonated (acids). One pH unit below (bases) or above (acids) a pKa value 90% of the molecules are charged. Two pH units from the pKa 99% of the molecules are charged and so on.

Ka and pKA

Ka is the dissociation value for a compound in water; relationship with pKa is: high Ka value indicates that something dissociates in H20 well, inversely, a low pKa value indicates that the compound in an aqueous solution. Because of the inverse log relationship to [H+], a higher [H+] indicates a lower pH value, i.e. more [H+] = acidic solution. the same applies for pOH except you are measuring [OH-] ions in solution.

pKa, mathematically, is equal to the -log(Ka), where Ka is the acid dissociation constant. When Ka is large, the acid dissociates more in water. The more the acid can dissociate in water, the stronger it is. Therefore, we can use pKa as a relative measure of acid strength. But take note though that when Ka is high, pKa is low. Thus, the lower the pKa, the stronger the acid.

Determination of pka

pKa or

pKa = -log(Ka)

is often called the dissociation constant.

It depends on

• Temperature
• Ionic strength
• Solvent. (polar, non-polar, can it form hydrogen bonds)

Henderson-Hasselbalch equation

The most commonly used equation to measure pKa is via the Henderson–Hasselbalch equation.

pH=pKa+log10([A−][HA])

Experimentally, we can determine pKa since we know that when a weak acid is half-neutralized, at that precise point in the titration, pH = pKa.

So, we need to prepare two equal size samples of the acid. Titrate one to the endpoint, such as with phenolphthalein indicator. Note the volume of base added. Now add precisely half that amount of base to the second sample of the acid and measure pH.

Hence, pH = pKa.