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Some Concepts of Acids and Bases

Some of the most important equilibria in chemistry are those involving acids and bases. With the advancement of science, various attempts have been made to define these acids and bases, starting from the phenomenological basis to the molecular composition and structure of the compound. A few of the currently accepted concepts are discussed below.

Arrhenius Concept

Arrhenius classified a substance into an acid or a base in terms of the characteristic ions of water which it produces in aqueous solution. Thus, an acid is a substance which ionizes in water to produce H+(aq) or the hydronium ion; a base is a substance which produces hydroxide ion, OH-(aq). The strength of an acid is defined in terms of the concentration of H+(aq) that is present in the aqueous solution of a given concentration of the acid. Likewise, the strength of a base depends upon the relative concentration of OH-(aq) in an aqueous solution of the base.

Bronsted-Lowry Concept

J. Bronsted and T. Lowry independently proposed a broader concept of acids and bases. According to them, "a substance is known as an acid if it can donate a proton and as a base if it can accept a proton". The substance may be a molecule or an ion. The reaction of an acid with a base constitutes the transfer of a proton from the acid to the base.

The dissolution of ammonia in water may be represented by the equation

In this reaction, H2O is serving as an acid as it donates a proton to the base NH3. If a solution of an ammonium salt is made strongly alkaline, ammonia gas is released. The reaction is therefore reversible and the system exists in equilibrium.

In the reverse reaction, is serving as an acid and OH- as a base. It follows, then, that in this Bronsted acid-base reaction, two acids (H2O and ) and two bases (OH- and NH3) are involved. The base NH3 gains a proton and thereby forms the acid which on the loss of a proton forms the base NH3. Such an acid-base pair, related through the loss or gain of a proton, is called a conjugate pair. NH is the conjugate acid of the base NH3, and NH3 is the conjugate base of the acid . Similarly, the acid H2O and the base OH- constitute a second conjugate pair in the preceding reaction. We may indicate conjugate relationships by the use of subscripts in the following manner:

  1. Acid1  Base2  Acid2   Base1
    A few other examples are
  2. CH3COOH + H2O H3O+ + CH3COO-  
    Acid1        Base2    Acid2       Base1
  3. HCl + H2O H3O+ + Cl
    Acid1   Base 2  Acid2   Base1
    Acid1  Base2  Acid2 Base1
    Similar types of reactions can be written in a nonaqueous medium

  5. Base2 Acid1 Acid2 Base1
    There are many molecules and ions that can function as acids in certain reactions and as base in other reactions. Such species are called amphiprotic. For example, H2O acts as an acid in reactions (1) and (4) and as a base in reactions (2) and (3).
    Arrhenius acid-base type reactions are also covered by the Bronsted-Lowry reactions:
  6. H3O+ + OH-⇔  H2O + H2O
    Acid1   Base2     Acid2   Base1

  7. Acid1  Base2  Acid2  Base1 `
Strength of Bronsted Acids and Bases.The strength of a Bronsted acid is determined by its tendency to donate a proton, and the strength of Bronsted base is determined by its tendency to accept a proton. Consider the reaction
HCl + H2O   H3O+ + Cl-
Acid1  Base2   Acid2   Base1
This reaction proceeds virtually to completion, i.e. the reactants are completely, i.e. the reactants are completely converted into products. It can be concluded that:
  1. The acid HCl is stronger than H3O+ ion, since HCl has a greater tendency to lose a proton than the H3O+ ion.
  2. The base H2O is stronger than Cl- ions, since in the competition for holding the proton, H2O shows a larger tendency and is thereby converted to H3O+.
Thus, we see that the strong acid HCl has a weak conjugate base Cl- ion and the strong base H2O has a weak conjugate acid H3O+. The fact that a strong acid has a weak conjugate base and vice versa is always true since a strong acid with a high tendency to lose a proton is necessarily conjugate to a weak base with a small tendency to gain and hold a proton. Similarly, a strong base with a strong tendency to attract a proton is necessarily conjugate to a weak acid with a lower tendency to lose a proton.

An aqueous solution of 0.1mol dm-3 acetic acid is only 1.33 per cent ionized at 25 ° C. The reaction is
Acid1   Base2    Acid2   Base1

or briefly

HAc + H2O H3O+ + Ac-
Acid1 Base2   Acid2    Base1

which is largely displaced to the left. Once again, it can be concluded that:
  1. The H3O+ is a stronger acid than HAc since at equilibrium more H3O+ ions than HAc molecules have lost protons.
  2. The Ac- is a stronger base than water since it shows greater tendency to attract the proton than H2O, thus converting itself into HAc.
In the above example, we again notice that the stronger acid H3O+ is conjugate to the weaker base H2O and the stronger base Ac- is conjugate to the weaker acid HAc. From the above two examples, one more conclusion can be drawn. This can be stated as follows: In a given reaction, the position of equilibrium favours the formation of a weak acid and a weak base.

Thus, in the reaction between HCl and H2O, the equilibrium concentrations of the weaker species H3O+ and Cl- are high, whereas in the solution of acetic acid, the equilibrium concentrations of the stronger species H3O+ and are low.

The Arrhenius concept of a strong and a weak acid in aqueous solution can be explained by the Bronsted concept. In Arrhenius theory, a strong acid is virtually 100 % ionized in aqueous solution and thus produces a very high concentration of H3O+. In the Bronsted system, acids are classified according to their ability to donate protons to a specific base under consideration. It is obvious that if water is used as the specific base, then the acid strengths of the Arrhenius concept can be explained in terms of the Bronsted concept. Thus, strong Arrhenius acids are those substances that are stronger acids than H3O+ and weak Arrhenius acids are those substances that are weaker acids than H3O+.

Problem (a) What is the conjugate base of
(i) HS-, (ii) H3O+, (iii) HOCl and (iv) CH3OH?
(b) What is the conjugate acid of
(i) F-, (ii) OH-, (iii) , (iv) NH3, (v) , (vi) HS-
(a) The following relations give us the conjugate base.
(i) HS- + H2O   H3O+ + S2-
     Acid                       Base
(ii) HOCl + H2O H3O+ + OCl-
      Acid                         Base
(iii) CH3OH + H2O H3O+ + CH3O-
       Acid                          Base
(b) The following relations give us conjugate acid.
(i) H2O + F-   HF +  OH-
     Base         Acid
      Base         Acid
      Base       Acid
      Base       Acid
(v) H2O + HS- H2S + OH-
       Base           Acid

Identify the Bronsted acids and bases in the following:
(c) H3O+ HS- H2S + H2O
The identifications of Bronsted acids and bases in the given reactions are given below:
  Base1 Acid2 Acid1 Base2
 Acid1  Base2 Base1  Acid2
(c) H3O+ + HS- H2S + H2O
   Acid1 Base2     Acid2   Base1
  Acid1     Base2  Acid2 Base1

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