Acid-Base Equilibria (Including Multistage Ionization) and Ionization Constants

Ionization of a compound is referred to as the splitting of neutral compounds into ions. As per the Arrhenius theory, the compounds that form H+(hydrogen) ions after dissociation in the medium (aqueous) are called acids. On the other hand, the compounds that form OH- ions after dissociation in the aqueous medium are called bases. The extent of dissociation, at which the acids and bases dissociate into ions, is known as the degree of dissociation. Both acids and bases reach the state of equilibrium, the state comes to be known as acid and base equilibrium. There are a lot of things that are clubbed with the extent of reaction. Let’s have a look at the acid-base equilibria (including multi-stage ionization) and ionization constants in detail! 

Acid and Base equilibrium 

Both acids and bases have an equilibrium state when the rate of forward and backward reactions becomes equal. But this doesn’t mean that there is no reaction taking place. Alternatively, it means that the concentration of both reactants and products doesn’t change due to the equal concentration of reactant and product. Determination of acid-base equilibrium plays a vital role in a specific reaction. It helps in – 

• Predicting the products obtained in a specific reaction. 

• The relative concentration of the products formed. 

• Helps in the identification of the weaker acid and base.

• Weak acids and bases are little common in their nature.  

Arrhenius Concept Of Acid & Base Dissociation 

The Arrhenius concept plays a critical role in explaining the concept of degree of dissociation. As ionization takes place in an aqueous solution, Arrhenius’s theory fits perfectly to explain the dissociation process and acid-base equilibria (including multi-stage ionization) and ionization constants. It is a way to determine the strength of acids and bases involved in the reaction. However, the strength of particular acid and base depends entirely on the ionization power. The more acid or base gets ionized in the particular reaction, the more is the ionization constant of the reaction. In shorter words, we can say that the acids that have high power to dissolve into their constituent ions are known as strong acids. Thus, they have the capability of donating protons instantly. Hence they are known as good proton donors. On the other hand, strong bases are termed good proton acceptors because they are good at accepting protons. 

Strong Acid & Bases 

In an aqueous solution, the strong acid is capable of dissolving. It dissociates completely in the aqueous solution resulting in the formation of H+ or hydrogen ions. This concept is based on Arrhenius’s theory. Similarly, when the strong base is made to dissolve into the aqueous solution, it completely dissolves into OH- ions. 

In short terms, an acid that has the power to dissolve 100% in the aqueous solution, is known as a very very strong acid. And the base that has the power to dissolve 100 % in the aqueous medium, they are termed as a very very strong base. Let’s understand them by taking a specific example.  

Let’s first talk about a very very strong acid, HCl.  

HCl + H2O → H3O+ + Cl- 

When HCl is dissolved in an aqueous solution, water, the acid dissolves to form H3O+ ions. The acid completely dissolves which indicates that the hydrochloric acid is quite strong.  

Let’s now move to the example of a very very strong base, NaOH.  

NaOH + H2O  → Na + + OH- 

In this reaction, the aqueous medium is mixed with NaOH, it dissolves completely into sodium ions and OH- ions.  

A few examples of strong acids are Hydrochloric Acid, Hydrobromic Acid, Hydroiodic Acid, Nitric Acid, Perchloric Acid, Sulfuric Acid, etc.  

A few examples of strong bases are Barium Hydroxide, Calcium Hydroxide, Potassium Hydroxide, Sodium Hydroxide, etc.  

Weak Acids & Bases 

Alternative to strong acids and bases, the weak acids and bases don’t completely dissolve in the aqueous medium. Let’s have a look at the general format of dissociation of a weak acid. 

HA + H2O ⇌ H3O+ + A-

In such a reaction, the arrow is double-sided. This indicates that the reaction is capable of either moving forward to form the ions or it can move to the backward direction to increase the concentration of the reactant. This type of reaction is also known as a recombining reaction. 

Equilibrium constant acid-base

Also known as ionization constants, or dissociation constants, acids, and bases have their equilibrium constants denoted by Ka and Kb respectively. The equilibrium constant acid-base is known to determine the relative strength of the chemical compounds. It’s important to determine the equilibrium constant to know the rate of dissociation and the extent of dissociation of chemical compounds. If you know the equilibrium constant of a particular reaction, it’s easy to calculate the ionization percentage along with the pH value of a reaction. 

Proton Transfer 

In a reaction, when the proton gets transferred from acid to base, the pair formed is known as conjugate acid-base pairs.  

HBr + H2O → Br- + H3O+

 In the above reaction, we have a strong acid, HBr, and a strong base, H2O. When the reaction occurs, it leads to the formation of Brand H3O+ ions. Meaning, the proton transfer takes place from Hydrobromic Acid to water. Thus, in this reaction, water and hydronium ions are referred to as conjugate acid-base pairs. One thing to note is that water can act as both acid and base, depending on the reaction and the conditions. In other words, water has an amphoteric nature (the compound that can act as acid and base).  

Equilibrium constant acid-base reaction  

Acids:- 

The equilibrium constant in an acidic reaction can be written in the form – 

Ka = [H3O +] [A-] /[HA], 

Where [H3O +] is the concentration of hydronium ions

[A-] is the concentration of the conjugate base 

[HA] is the concentration of acids  

Bases:- 

To define the equilibrium constant of the base here is the simple formula  

Kb = [BH +] [OH-] /[B], 

Where [BH +] is the concentration of conjugate acids 

[OH-] is the concentration of Hydroxide ion 

[B] is the concentration of the base.  

Conclusion

So this was all about acid-base equilibria (including multi-stage ionization) and ionization constants. We hope this article will help you to get a good knowledge of acid and base and how they undergo dissociation. The Arrhenius concept is a great way to get the whole knowledge of acid-base equilibrium. And how they affect the reaction as a whole.