What is a Bronsted-Lowry Base in Inorganic Chemistry? 

Bronsted-Lowry base in inorganic chemistry is any chemical substance that can accept a proton from the other chemical substance it is reacting with.  

What is the Reason for Bronsted-Lowry Acid-Base Theory?  

Bronsted-Lowry acid-base theory was introduced in 1923 by J. N. Bronsted and T. M. Lowry based on the ability of a substance to accept or donate protons or hydrogen ion (H⁺) and is also called as proton theory of acids and bases. This theory was explained to overcome the limitations of Arrhenius theory. So it is an extension of the Arrhenius theory of acids and bases. The major limitation of Arrhenius theory was that it is explained only in aqueous solutions.  

What is a Bronsted-Lowry Acid and Base?  

Based on the Bronsted-Lowry acid-base theory, the Arrhenius definition was over ruled by explaining that there is no need to have H⁺ or OH^- ion to act as an acid or a base. Thus a Bronsted-Lowry acid is defined as a proton (H⁺) donor and a Bronsted-Lowry base is a proton (H⁺) acceptor.  

Eg:  HCl (aq)   +    NH₃ (aq) ⟶ NH₄⁺ (aq)    +    Cl^- (aq)  

In the above-given example, hydrochloric acid (HCl) is donating a proton i.e. H⁺ ion, to ammonia (NH₃). As HCl is donating a proton, it is a Bronsted-Lowry acid and as NH₃ is accepting a proton, it is a Bronsted-Lowry base. Further, NH₃ accepts a proton and forms NH₄⁺ ion, and HCl losses a proton and forms Cl^- ion. In this particular example, Cl^- is termed as the conjugate base of the Bronsted-Lowry acid (HCl) and NH₄⁺ is termed as the conjugate acid of the Bronsted-Lowry base (NH₃).  

Acid ↔ H⁺ + Conjugate base

Base   +   H⁺    ↔      Conjugate acid  

Bronsted-Lowry theory is extended towards large number of compounds to be classified as acids and bases which includes not only the neutral molecules such as sulphuric acid, acetic acid, nitric acid and other alkali metal hydroxides but also include certain charged species such as cations and anions. A Bronsted-Lowry acid always contains a proton in its structure and it may be a neutral or positive or negative ions. 

Bronsted-Lowry Base  

The bronsted-Lowry base is a substance that has the tendency to accept a proton and forms its conjugate acid. A Bronsted-Lowry base is a substance with at least one lone pair of electrons present in it and it may be a neutral or a negative ion.  

NH₃   +    H₂O   ↔      NH₄⁺    +    OH^-

In the above-given example, ammonia (NH₃) accepts a proton and forms NH₄⁺ and thus ammonia is a Bronsted-Lowry base and NH₄⁺ ion is its conjugated acid.  

Based on the given reaction or the type of reactant, the Bronsted-Lowry base is identified. For example, compounds like water (H₂O), ammonia (NH₃) act as both Bronsted-Lowry base and also acid depending upon the other reactant present. 

H₂SO₄   +   H₂O    ↔        HSO₄^-     +    H₃O⁺ 

H₂O   +    NH₃   ↔     OH^-    +    NH₄⁺ 

In the above given equations, in the first equation the sulphuric acid (H₂SO₄) donates a proton (H⁺ ion) to water molecule and forms its conjugate base (HSO₄^-). In this case, water molecule accepts a proton and acts as Bronsted-Lowry base and forms its respective conjugate acid (H₃O⁺).   

In the given second equation, the water molecule donates a proton to ammonia and forms its conjugate base OH- ion. The ammonia molecule accepts a proton and forms its respective Bronsted-Lowry acid NH4+. Thus, in this case, a water molecule donates a proton and acts as Bronsted-Lowry acid.  

What are the Factors that Affect the Bronsted-Lowry Basicity? 

Some of the major factors that affect the basicity of the Bronsted-Lowry base are electron affinity or the charge on the atom, delocalization of electrons and presence of electron withdrawing group within the molecule of the study.  

Electron affinity is defined as the quantity of energy released when an electron is added to a neutral atom to form a negative ion. If a molecule has high electron affinity or the overall charge on the atom is high, then it has fewer tendencies to donate its electrons to the incoming proton. When it is not able to donate electrons to the proton, then it cannot accept the proton to act as Bronsted-Lowry base. Thus, an atom or molecule with less electron affinity and less charge on the atom has greater tendency to act as Bronsted-Lowry base.  

Delocalization of electrons is the electrons which are delocalized within the atom or molecule i.e. they are freely moving and are not localized on a particular bond. When the delocalization of electrons is high, the electrons are less available for the proton and thus the atom or molecule with high delocalization has less tendency to act as Bronsted-Lowry base.  

When a molecule has electron withdrawing group present in it, the lone pair electrons or the delocalized electrons present in the molecule are dragged towards this group within the molecule. Due to this the electrons are not available on the particular atom for the proton to accept it. Thus, due to the presence of electron withdrawing group present in the molecule, the tendency to act as Bronsted-Lowry base decreases drastically.  

Polarizability is the distribution of charge within the atom or molecule. In a large atom, the charge is easily polarized around a great volume when compared to a small atom in which it is distributed in a small volume. If the Polarizability is high, then the electrons are not bound tightly by the nucleus and are donated easily to the approaching proton. Thus, as polarizability if high, the atom or molecule has greater tendency to act as Bronsted-Lowry base.  

Thus, strong Bronsted-Lowry base are the molecules or ions that have greater tendency to accept a proton by donating a pair of electrons and form its corresponding conjugate acid which is weak in nature. Similarly, weak Bronsted-Lowry base are the molecule or ions which has less tendency to accept a proton by donating electrons and form its corresponding conjugate acid which is relatively strong.  

Below given are some of the Bronsted-Lowry base in the form of anions which accept protons to form their respective conjugate acids.  

Solved Questions 

1. In the equation given below which is Bronsted-Lowry base? Explain.  

H₂PO₄^-   +   OH^-   ↔    HPO₄^2-   +     H₂O 

A) H₂PO₄^- B) OH^- C) HPO₄^2- D) H₂O 

In the given equation, the OH^- ion accepts a proton. So it is the Bronsted-Lowry base and forms its conjugate acid H₂O. H₂PO₄^- donates a proton and acts as Bronsted-Lowry acid and forms its conjugate base HPO₄^2-. Thus, the correct option is option B.  

Context and Applications 

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

• B.Sc. in Chemistry   
• Chemical Engineering  
• M.Sc in Chemistry 
• B.Tech Biochemistry.