Ionic Equilibrium


  • In aqueous solutions, some substances conduct electricity and are called electrolytes, while others do not and are referred to as non-electrolytes.
  • Electrolytes are further classified into strong and weak electrolytes. Strong electrolytes are almost completely ionized when dissolved in water, while weak electrolytes are only partially dissociated.
  • Ionic equilibrium is established in weak electrolytes between ions and unionized molecules.
  • Acids, bases, and salts are examples of electrolytes and can act as either strong or weak electrolytes.


  • Acids, bases, and salts are common substances found in nature.
  • Hydrochloric acid, present in gastric juice, is essential for digestive processes.
  • Acetic acid is the main component of vinegar, while citric, ascorbic, and tartaric acids are found in lemon, orange, and tamarind.
  • Acids have a sour taste and turn blue litmus paper red, while bases taste bitter, feel soapy, and turn red litmus paper blue.
  • When mixed in the right proportions, acids and bases react to form salts.
  • Common salts include sodium chloride, barium sulfate, and sodium nitrate.
  • Sodium chloride exists in a solid state as a cluster of positively charged sodium ions and negatively charged chloride ions held together by electrostatic interactions.
  • The ionization of hydrochloric acid is complete, while that of acetic acid is only partial (< 5%).
  • The extent of ionization depends on the strength of the bond and the extent of solvation of ions produced.
  • Dissociation refers to the separation of ions already present in the solid state, while ionization refers to the process in which a neutral molecule splits into charged ions in the solution.


Arrhenius’s Concept of Acids and bases

The Arrhenius concept of acids and bases was introduced by Swedish chemist Svante Arrhenius in the late 19th century. According to this concept, acids are substances that ionize in water to produce hydrogen ions (H+), while bases are substances that ionize in water to produce hydroxide ions (OH-).

Arrhenius defined an acid as any substance that increases the concentration of H+ ions when dissolved in water. The H+ ion is also known as a proton. When an acid dissolves in water, it donates a proton to a water molecule, forming a hydronium ion (H3O+). For example, hydrochloric acid (HCl) dissolves in water to produce H+ and Cl- ions:

HCl (aq) → H+ (aq) + Cl- (aq)

Similarly, Arrhenius defined a base as any substance that increases the concentration of OH- ions when dissolved in water. For example, sodium hydroxide (NaOH) dissolves in water to produce Na+ and OH- ions:

NaOH (aq) → Na+ (aq) + OH- (aq)

The Arrhenius concept is limited to aqueous solutions and only applies to substances that ionize in water. It does not explain the acidic behavior of some substances that do not contain hydrogen, such as boron trifluoride . In addition, it does not account for the fact that some substances can act as both acids and bases, depending on the conditions. For example, water can act as an acid (donating a proton) in the presence of a stronger base, or as a base (accepting a proton) in the presence of a stronger acid.

Brönsted-Lowry Acids and Bases

 The Danish chemist, Johannes Brönsted and the English chemist, Thomas M. Lowry gave a more general definition of acids and bases. According to Brönsted-Lowry theory, acid is a substance that is capable of donating a hydrogen ion H+ and bases are substances capable of accepting a hydrogen ion, H+. In short, acids are proton donors and bases are proton acceptors.

Consider the example of dissolution of NH3 in H2O represented by the following equation:

The basic solution is formed due to the presence of hydroxyl ions. In this reaction, water molecule acts as proton donor and ammonia molecule acts as proton acceptor and are thus, called Lowry-Brönsted acid and base, respectively. In the reverse reaction, H+ is transferred from NH4+ to OH. In this case, NH4+ acts as a Bronsted acid while OH acted as a Brönsted base. The acid-base pair that differs only by one proton is called a conjugate acid-base pair. Therefore, OH is called the conjugate base of an acid H2O and NH4+ is called conjugate acid of the base NH3. If Brönsted acid is a strong acid then its conjugate base is a weak base and vice-versa It may be noted that conjugate acid has one extra proton and each conjugate base has one less proton.

Consider the example of ionization of hydrochloric acid in water. HCl(aq) acts as an acid by donating a proton to H2O molecule which acts as a base.

It can be seen in the above equation, that water acts as a base because it accepts the proton. The species H3O+ is produced when water accepts a proton from HCl. Therefore, Cl is a conjugate base of HCl and HCl is the conjugate acid of base Cl. Similarly, H2O is a conjugate base of an acid H3O+ and H3O+ is a conjugate acid of base H2O.

Conjugate acid and base

In acid-base chemistry, the terms "conjugate acid" and "conjugate base" refer to two substances that are related by the loss or gain of a proton. When an acid donates a proton (H+) to a base, the acid is transformed into its conjugate base, and the base is transformed into its conjugate acid. For example, in the reaction between hydrochloric acid (HCl) and water (H2O), HCl donates a proton to water, forming the hydronium ion (H3O+) and the chloride ion (Cl-):

HCl + H2O H3O+ + Cl-

In this reaction, HCl is the acid and Cl- is its conjugate base, while H2O is the base and H3O+ is its conjugate acid.

Similarly, in the reaction between ammonia (NH3) and water, ammonia accepts a proton from water, forming the ammonium ion (NH4+) and the hydroxide ion (OH-):

NH3 + H2O NH4+ + OH-

In this reaction, NH3 is the base and NH4+ is its conjugate acid, while H2O is the acid and OH- is its conjugate base.

The conjugate acid and base pairs are related to each other by the transfer of a single proton. They have similar chemical properties, but with opposite charges. The strength of an acid or base is related to the strength of its conjugate base or acid, respectively. A strong acid has a weak conjugate base, while a strong base has a weak conjugate acid. Similarly, a weak acid has a strong conjugate base, while a weak base has a strong conjugate acid.

Lewis Acids and Bases

The Lewis acid-base theory, proposed by American chemist Gilbert N. Lewis, defines an acid as an electron pair acceptor and a base as an electron pair donor. According to this theory, a Lewis acid is a substance that can accept a pair of electrons, while a Lewis base is a substance that can donate a pair of electrons.

In the Lewis acid-base theory, the acid-base reaction involves the transfer of an electron pair from the Lewis base to the Lewis acid, resulting in the formation of a coordinate covalent bond. The Lewis acid is then said to be coordinated to the Lewis base, forming a coordination complex.

Examples of Lewis acids include metal ions, such as Al3+, Fe3+, and Cu2+, and molecules with incomplete octets, such as BF3, AlCl3, and SO3. These compounds are electron deficient and can accept an electron pair from a Lewis base to form a coordinate covalent bond.

Examples of Lewis bases include molecules with lone pairs of electrons, such as NH3, H2O, and OH-. These compounds areelectron rich and can donate an electron pair to a Lewis acid to form a coordinate covalent bond.

The Lewis acid-base theory is a more general concept of acids and bases compared to the Arrhenius and Bronsted-Lowry theories, as it does not require the presence of hydrogen ions in solution. It is widely used in coordination chemistry, where the formation of coordination complexes involves the interaction of Lewis acids and bases.