K_a to pK_a

What is Ka?

The acid ionisation constant, Ka, is the equilibrium constant for chemical reactions in an aqueous solution involving weak acids. The magnitude of acid dissociation is predicted using Ka’s numerical value.

What is pKa?

The pKa value is a number that describes the acidity of a specific molecule. The strength of an acid is determined by how tightly a proton is held by a Bronsted acid.

In chemistry, the terms pH, pKa, pKb, Ka, and Kb are used to characterise how acidic or basic a solution is, as well as to measure the strength of acids and bases. Although the pH scale is the most commonly used indicator of acidity and basicity, pKa, pKb, Ka, and Kb are more accurate predictors of acid and base strength and reactions. Here are definitions for each term, as well as basic formulas for calculating them and an explanation of how they differ.

Table of Contents

• Understanding Ka and pKa
• Converting From pKa to Ka
• Acidity Dissociation Constant Ka
• Frequently Asked Questions – FAQs

Understanding Ka and pKa

The acid dissociation constant is Ka. The – log of this constant is pKa. The general equation for acid dissociation is:

HA + H₂O ⇌ A^– + H₃O⁺

Where,

Ka = [H3O⁺][A^–]/ [HA]

pKa = – log Ka

At half the equivalence point,

pH = pKa = – log Ka

Because an acid dissociates primarily into its ions, a high Ka value implies a powerful acid. A high Ka value indicates that the reaction arrow promotes product formation. A low Ka value, on the other hand, indicates that just a little amount of acid dissociates, indicating a weak acid. A low Ka value indicates that the reaction prefers the reactants above the products. The Ka values of most weak acids range from 10^-2 to 10^-14.

The same information is provided by pKa, but in a different format. The stronger the acid, the lower the pKa value. Alternatively, the weaker the acid, the higher the pKa value. The pKa value of weak acids is usually between 2 and 14.

Converting From pKa to Ka

The conversion of pKa to Ka is given below.

pKa = – log Ka

– pKa = log Ka

10^-(pKa) = Ka

Ka = 10^-(pKa)

Consider the reactions below.

HA(aq) + H₂O(l) ⇌ H₃O⁺(aq) + A^-(aq)

Ka = [A^–][H₃O⁺] / [HA]

We can calculate the Keq (equilibrium constant) or Ka of a reversible acid-base reaction. The temperature is the only variable that affects the Ka.

The ability of a strong acid to completely dissociate in an aqueous solution distinguishes it from a weak acid. If the acid-base reaction favoured the left (greater [HA]), the concentrations of both [A^–] and [H₃O⁺] would be lower, resulting in a lower Ka value. As a result, the stronger the acid is, the greater the Ka. The weaker the acid, the lower the Ka.

pKa = – log[Ka]

As a result, the lower the pKa, the more powerful the acid. The weaker the acid, the greater the pKa.

Acidity Dissociation Constant Ka

The reaction of an acid HA in water can be written in a number of ways. The acid dissociation constant, denoted by Ka, is the equilibrium constant of an acid’s dissociation reaction. This constant represents the strength of an acid in solution in numerical values. Ka is often stated in mol/L units. For quick reference, there are tables of acid dissociation constants. The usual form of the equilibrium reaction in an aqueous solution is

HA + H₂O ⇆ A^– + H₃O⁺

where HA is an acid that dissociates in the conjugate base of the acid A^– and a hydrogen ion that forms the hydronium ion H₃O⁺ when it comes into contact with water. The reaction is at equilibrium when the concentrations of HA, A^–, and H₃O⁺ do not change over time, and the dissociation constant can be computed.

Ka = [A^–][H₃O⁺] / [HA][H₂O]

The concentration is indicated by the square brackets. The equation is simplified by keeping the concentration of water constant unless the acid is sufficiently concentrated:

HA ⇆ A^– + H⁺

Ka = [A^–][H⁺]/[HA]

The acidity constant or acid-ionisation constant is another name for the acid dissociation constant.