What are Acids and Bases?  

Acids and bases play an important role in our day-to-day lives. We recognize acids for their sour taste, and bases for their bitter taste. The word acid was coined from the Latin word “acidus” which means “sharp” and the word base is synonymous to “alkali” which is Arabic origin but coined from Latin “Kalium” which means “potash”. We notice the presence of acids in many of our daily foods like in lemons as citric acid, in grapes as ascorbic acid, in milk products as lactic acid etc. Our body contains hydrochloric acid in our stomach which helps in digestion process. Similarly, our body also contains base named as antacid which contains bases like sodium bicarbonate or milk of magnesia to neutralize the excess of stomach acid. 

What are the Characteristics of Acids and Bases?

 The distinct characteristics of acids and bases are 

• Acids possess a characteristic taste of sourness, while bases are bitter. 

• Acids have the ability to change the blue litmus paper to red, whereas bases change the red litmus paper to blue. 

• An acid reacts with a bases and vice-versa to yield salts and water as by-product. 

Theories of Acid and Base 

In modern chemistry, three main theories are used to explain acid base concepts. 

• Arrhenius acid base theory
• Bronsted-Lowry conjugates acid base theory
• Lewis’s theory of lone pair of electrons  

What is an Acid and Base Based on Arrhenius Theory?

Arrhenius definition of acid and base was published in 1887 and is based on water. In acid-base chemistry, the theory has got its own importance. It is the simplest theory and can be explained in two major points. According to Arrhenius, an acid is one that liberates hydrogen ions $\left({\text{H}}^{+}\right)$by dissociating in water and a base liberates hydroxide ions $\left({\text{OH}}^{-}\right)$by dissociating in water. Likewise, Acid is an ion or molecule which liberates ${\text{H}}^{+}$ ions and makes the solution acidic and a base is a molecule or ion which liberates ${\text{OH}}^{-}$ ions and makes the solution basic in nature. The reaction between an acid (hydrochloric acid,$\text{HCl}$) and base (sodium hydroxide,$\text{NaOH}$) occurs to deposit salt$\left(\text{NaCl}\right)$.

Eg. Arrhenius acid: $\text{HCl}$, ${\text{HNO}}_3,{\text{H}}_2{\text{SO}}_4,{\text{H}}_3{\text{PO}}_4,\text{HF},\text{HBr},\text{HI},{\text{H}}_2\text{S}$

Arrhenius base: $\text{KOH},\text{NaOH},\text{Ca}{\left(\text{OH}\right)}_2,\text{Mg}{\left(\text{OH}\right)}_2$

$\mathrm{HCI}\left(aq\right) \to {\mathrm{H}}^{+}\left(aq\right) + {\mathrm{CI}}^{-}\left(aq\right)$

HCI ionizes in water, producing ${\mathrm{H}}^{+}$ and CI ions.

$\mathrm{NaOH}\left(aq\right) \to {\mathrm{Na}}^{+}\left(aq\right) + {\mathrm{OH}}^{-}\left(aq\right)$

NaOH dissociates in water, producing ${\mathrm{Na}}^{+}$ and OH.

Limitations of Arrhenius Theory

Arrhenius theory fails to explain the basic nature of bases such as ammonia, pyridine, amines etc., as they do not contain ${\text{OH}}^{-}$ions and the acidic nature of ${\text{CO}}_2,{\text{SO}}_2$etc., as they do not liberate ${\text{H}}^{+}$ ions. It also does not explain the nature of compounds in non-aqueous solvents such as benzene, acetone etc. The acid base reaction between $\text{HCl}$ and${\text{NH}}_3$ cannot be explained by this theory.

Bronsted-Lowry Theory

According to Bronsted-Lowry theory, all Arrhenius acid and base are considered to be Bronsted-Lowry acid and base but not necessarily. Thus, an acid based on Bronsted-Lowry theory is a molecule or ion from which a proton can be detached and a base is a molecule or ion by which a proton can be accepted. Thus, a Bronsted-Lowry acid acts as proton donor and base act as proton acceptor. 

Example: 

Bronsted-Lowry acid:

$\text{HCl},\text{HBr},{\text{HNO}}_3,{\text{HC}}_2{\text{H}}_3{\text{O}}_2,{\text{HNO}}_2$

Bronsted-Lowry base:

${\text{NH}}_3,{\text{ H}}_2\text{O}$

, Acetone, methylamine, ethylene

When a Bronsted-Lowry acid-base pair reacts with each other, they form a conjugate pair i.e., an acid donates proton and forms a conjugate base and a base accepts proton and forms a conjugate acid. 

Thus, an acid differs from its conjugate base or a base differs from its conjugate acid with only one proton which are named as conjugate acid base pairs. Bronsted-Lowry acid base theory is almost acceptable till date and in general chemistry both acid and base are defined as Bronsted-Lowry acid base.   

Lewis’s Theory

Lewis’s theory of acid and base is a very general theory which does not explain in terms of solubility or solvent or the protons. Based on Lewis theory, an acid is an ion or molecule which has an empty orbital to accept a lone pair of electrons and a base is a molecule or ion which gives off a lone pair of electrons to the Lewis acid. As the other acid base pairs, Lewis’s acid reacts with its base pair to obtain an adduct. An adduct is a compound formed by sharing the lone pair of electrons between the acid-base pair. 

Example:

Lewis’s acid: ${\text{Fe}}^{2+},{\text{Fe}}^{3+},{\text{Cu}}^{2+},{\text{H}}^{+},{\text{BF}}_3,{\text{AlF}}_3$

Lewis’s base: ${\text{H}}^{\text{-}}{\text{,F}}^{\text{-}}{\text{,H}}_{\text{2}}{\text{O,NH}}_{\text{3}}{\text{,OH}}^{\text{-}}$

In general, it is said that any acid-base which acts as Lewis acid-base may not be a Bronsted-Lowry acid-base. 

As Lewis acids take up a pair of electrons, they are called as “electrophile” originated from Greek which means “electron lover” and a Lewis bases give away a pair of electrons and are called as “nucleophile” which means “nuclei lover”. Electrophile is deficient of electrons and so attracts negative charge and Nucleophile is electron rich which attracts positive charge. The transfer of electrons from an electron rich nucleophile or a base to electron poor electrophile or an acid is generally showed by a curved arrow. 

What is an Amphoteric Substance? 

An amphoteric substance is an ion or molecule which has the behavior of both as acid or as base, based on the surroundings it is present. The best example for an amphoteric substance is water i.e., it ionizes to hydronium $\left({\text{H}}_3{\text{O}}^{+}\right)$ ion and hydroxide $\left({\text{OH}}^{-}\right)$ion which acts as an acid and a base, respectively.

$\begin{array}{l}{\text{H}}_2\text{O}\leftrightarrow {\text{H}}^{+}+{\text{OH}}^{-}\\ {\text{2H}}_2\text{O}\leftrightarrow {\text{H}}_3{\text{O}}^{+}+{\text{OH}}^{-}\end{array}$

As, the water dissociates, the presence of ${\text{H}}^{+}$ ions makes acid and the presence of ${\text{OH}}^{-}$ makes it a base. Thus water is said to be neutral. ${\text{H}}^{+}$ ion does not exist in water, rather it forms ${\text{H}}_3{\text{O}}^{+}$ ion by bonding to another water molecule.

Another well-known amphoteric behavior substance is ammonia (${\text{NH}}_3$). In general, ammonia is a base, while based on its surroundings it also possesses an acid behavior.

$\begin{array}{l}{\text{NH}}_3+\text{HCI}\to {\text{NaH}}_4^{+}+{\text{CI}}^{-}\\ {\text{LiN+2NH}}_{\text{3}}\leftrightarrow {\text{3Li}}^{\text{+}}{\text{+3NH}}_{\text{3}}{\text{O}}_{\text{2}}^{\text{+}}\end{array}$

In the first equation, ammonia takes up a proton from hydrochloric acid leading to the formation of ammonium ion, acting as a base. While, in the second equation it donates a proton to form an amide, acting as an acid.  

Strengths of Acids and Bases 

Substance undergoes dissociation in water and form ions. Number of moles of ${\text{H}}_3{\text{O}}^{+}$ produced for each mole of a substance gives the strength of an acid and number of moles of ${\text{OH}}^{-}$gives the strength of a base, respectively. Thus, strong acid and base dissociate completely when compared to weak acid and weak base which do not dissociate completely. Strong acids act as strong electrolytes as they donate ${\text{H}}^{+}$ ions by dissociating completely and weak acids act as weak electrolytes as they dissociate partially and produce only few ${\text{H}}_3{\text{O}}^{+}$ ions. Dissociation of strong acid is irreversible reaction while dissociation of weak acid is a reversible reaction due to the presence of strong conjugate base formed from weak acid.

In few reactions such as neutralization, the strength of acids and bases are explained in terms of pH shown by the solution. 

In general, pH measures the acidic, basic and neutrality of a solution. It ranges from $0$ to $14$ i.e. if $\text{pH}$is$7$, the solution is said to be neutral, if it is less than $7$, it is acidic solution and if it is more than $7$ then a basic solution. A $\text{pH}$ scale is logarithm based and donates the amount or quantity of hydrogen ions present in a given solution.

Practice Problem:

In the given equation which is the conjugate acid base pair? 

${\text{HNO}}_{\text{3}}{\text{+NH}}_{\text{3}}\leftrightarrow {\text{NO}}_{\text{3}}{}^{\text{-}}{\text{+NH}}_{\text{4}}{}^{\text{+}}$

${\text{HNO}}_{\text{3}}{\text{/NH}}_{\text{3}}{\text{ and NO}}_{\text{3}}{}^{\text{-}}{\text{/NH}}_{\text{4}}{}^{\text{+}}$

${\text{HNO}}_{\text{3}}{\text{/NO}}_{\text{3}}{}^{\text{-}}{\text{and NH}}_{\text{4}}{}^{\text{+}}{\text{/NH}}_{\text{3}}$

${\text{NH}}_{\text{3}}{\text{/NO}}_{\text{3}}{}^{\text{-}}{\text{and HNO}}_{\text{3}}{\text{/NH}}_{\text{4}}{}^{\text{+}}$

${\text{NO}}_{\text{3}}{}^{\text{-}}{\text{/HNO}}_{\text{3}}{\text{and NH}}_{\text{3}}{\text{/NH}}_{\text{4}}{}^{\text{+}}$

Solution:

The acid

HNO

3

losses a proton to form a conjugate base

${{\text{NO}}_3}^{-}$

The base

NH

3

takes up a proton and forms a conjugate acid

${{\text{NH}}_4}^{+}$

Thus, the answer is Option B.

${\text{HNO}}_{\text{3}}{{\text{/NO}}_{\text{3}}}^{\text{-}}{{\text{and NH}}_{\text{4}}}^{\text{+}}{\text{/NH}}_{\text{3}}$

It is to be noted that Option D may also be right but it is conjugate base acid pair and not acid base pair.

Context and Applications   

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for 

• B.Sc. in Chemistry   
• M.Sc. in Chemistry