Physical Chemistry of Milk

Module 8. Acids and bases

Lesson 18
LEWIS CONCEPTS OF ACIDS

18.1 Introduction

Various scientists have attempted to explain the concept of acids and bases. Lewis tried to give his conceptual definition to these acids and bases basing upon their electronic configuration. Similarly Arrhenius and Bronsted and Lewis have also explained the concept of acids and bases taking in to consideration several other parameters of the reactions between an acid and a base. The concepts proposed by Arhenius and Bronsted and Lowry are limited to such substance which contain protons where as the Lewis proposed more general and broader concept for acids and bases.

18.2 Lewis Concepts

According to this concept an acid is any species which is capable of accepting a pair of electrons and a base is a species having lone pair of electrons for donation. Thus all the Bronsted - Lowry Acids and bases are also Lewis acids and Lewis bases but all the Lewis acids and bases are not Bronsted - Lowry Acids and bases respectively. Lewis acids also include some other compounds like BF₃, AlCl­₃, ZnCl_­2, FeCl₃, SO₃ etc. According to Lewis an acid may also be a species without any proton.

18.2.1 Types of lewi's acid

Lewis acids have been grouped based on certain criteria. The criteria for such grouping are discussed here under.

• Compounds whose central atom has an incomplete octet. In other words Lewi's acids of this type are the electro deficient molecules such as BF₃, AlCl₃, GaCl₃ etc.

• Compounds in which the central atom has available d-orbital and may acquire more than that of an octet of valence electrons.
  

• Some other examples of this type are PF₃, PF₅, SF₄, SeF₄, TeCl₄ etc.
• All simple cations e.g; Na⁺, Ag⁺, Cu²⁺, Al³⁺,Fe³⁺ They can combine with an electron pair

Molecules with a multiple bond between atoms of dissimilar electro negative charge.

O=C=O, O=S=O

18.2.2 Lewis bases

Bases are also being grouped similar to acids on the basis of certain criteria. The criteria and the groups of bases are discussed here under. These are the following types

* All the simple negative ions such as Cl^-, F^- etc.
* Molecules with one or two unshared pair of electrons ex: H₂O, NH₃ , ROH , R₂O, R₂S Pyridine etc.
* Multiple bonded compounds which form coordination compounds with transition metals Eg. CO, NO, Ethylene, Acetylene.

18.3 Bronsted - Lowry Concept

Bronsted - Lowry defined an acid, as a substance which gives up proton (proton donor) and a base as a substance which accepts proton (Proton acceptor). In the following cases the species B accepts a proton and thus behaves as a base while the species BH⁺ gives up a proton and thus acts as an acid

Such related pairs of an acid and a base is said to conjugate to one another i.e in the above example B is the conjugate base of the acid B⁺H and B⁺ H is the conjugate acid of the base B. On the whole we can say that every base has its conjugate acid and every acid has a conjugate base. Hence, strong acid has a weak conjugate base and a weak acid has a strong conjugate base some examples of conjugate acids and bases is given in Table 18.1

Table 18.1 Conjugate acids and bases

Thus according to Bronsted -Lowry concept neutralization is a process in which a proton is transferred from an acid to a base.

A substance acts as an acid only when another substance capable of accepting a proton (i.e Base) is present. For eg. HCl or CH₃COOH solution in benzene is not acidic because benzene is not in a position to take up protons. On the other hand HCl or CH₃COOH solutions in water are definitely acidic because HCl or CH₃COOH can ionize in water.
Water the most common solvent can act both as an acid as well as a base because it can give up a proton as well as can receive a proton Such compounds as water can function both as an acid and base is are known as amphoteric or amphiprotic. Its dual behaviour may be represented by the following equation