DISSOCIATION OF WEAK ACID AND BASES

Q: Arrhenius Base definition?

The substance is dissociated into hydroxyl ions (OH–) when dissolved in water.

Q: What is an Example of Conjugate Acid-Base?

HCl+H2O⇌H3O++Cl− Here, HCl donates a proton to water and therefore acts as an acid, and water accepts a proton and thus acts as a base. But in reverse, chlorine ion accepts a proton, so it behaves as a base. Pairs of substance that gain or loss of proton is called conjugate acid-base.

Q: What is an Example of LEWIS Acid-Base?

BF3+NH3→F3BNH3 Here, NH3 behave as a base because of the tendency to donate lone pair of electron on the nitrogen atom to BF3, and BF3 accept a pair of an electron, so it acts as an acid.

Table of Contents

DISSOCIATION OF WEAK ACID

INTRODUCTION

There are different concepts put forward for acids and bases, such as Arrhenius, Bronsted – Lowry, and Lewis concepts.

WHAT IS DISSOCIATION?

Dissociation means substances dissolved in the solution break the bonds and are converted into ionic forms known as dissociation.

ARRHENIUS CONCEPT:

According to him, dissociation is considered that substance dissociates and is dissolved in water giving hydrogen ions.

For example, acetic acid dissociates and is dissolved in water which gives hydrogen ions.

\[\displaystyle C{{H}_{3}}COOH+water\rightleftharpoons H_{{\left( {aq} \right)}}^{+}+C{{H}_{3}}COO_{{\left( {aq} \right)}}^{-}\]

The same for base substance is dissociated into hydroxyl ions (OH^–) when dissolved in water.

\[\displaystyle Ca{{\left( {OH} \right)}_{2}}+water\rightleftharpoons Ca_{{\left( {aq} \right)}}^{{+2}}+2OH_{{\left( {aq} \right)}}^{-}\]

BRONSTED – LOWRY CONCEPT:

Acid: substance tends to donate a proton.

Base: substance tends to accept a proton.

\[\displaystyle HCl+{{H}_{2}}O\rightleftharpoons {{H}_{3}}{{O}^{+}}+C{{l}^{-}}\]

Here, HCl donates a proton to water and therefore acts as an acid, and water accepts a proton and thus acts as a base. But in reverse, chlorine ion accepts a proton, so it behaves as a base. Pairs of substance that gain or loss of proton is called conjugate acid-base.

LEWIS CONCEPT:

In 1923 G.N. Lewis defined acids and bases. According to him species that accepts a pair of electrons is known as acid, and a species that donates a pair of electrons is known as a base.

e.g.

\[\displaystyle B{{F}_{3}}+N{{H}_{3}}\to {{F}_{3}}BN{{H}_{3}}\]

Here, NH₃ behave as a base because of the tendency to donate lone pair of electron on the nitrogen atom to BF₃, and BF₃ accept a pair of an electron, so it acts as an acid.

Dissociation of a weak acid monobasic acid HA in water following equation as,

\[\displaystyle HA+{{H}_{2}}O\rightleftharpoons {{H}_{3}}{{O}^{+}}+{{A}^{-}}\]

By applying the law of chemical equilibrium and expression given by the equilibrium constant K_C is,

\[\displaystyle {{K}_{c}}=\frac{{\left[ {{{H}_{3}}{{O}^{+}}} \right]\left[ {{{A}^{-}}} \right]}}{{\left[ {HA} \right]\left[ {{{H}_{2}}O} \right]}}-------(1)\]

[ ]= Represent concentration in mole/liter

In a dilute solution, water is a significant excess, so the water concentration denoted by K, and [H₃O⁺] means hydrogen ion is hydrated, so replaced with H⁺ and above equation (1) put this value and rewrite equation as,

\[\displaystyle {{K}_{c}}=\frac{{\left[ {{{H}^{+}}} \right]\left[ {{{A}^{-}}} \right]}}{{\left[ {HA} \right]\times k}}-------(2)\]

The product of two constants K_C and K = another constant it is denoted as K_a, above (2) write as

\[\displaystyle {{K}_{a}}=\frac{{\left[ {{{H}^{+}}} \right]\left[ {{{A}^{-}}} \right]}}{{\left[ {HA} \right]}}-------(3)\]

The constant (K_a) is the acid concerned and is called the acid’s dissociation constant.

RELATIVE STRENGTH OF WEAK ACID

According to equation (3) dissociation constant of the weak acid is also expressed in terms of the degree of dissociation (∝) and total molar concentration (c) of the acid.

The dissociation of acetic acid is denoted as,

\[\displaystyle =\frac{{{{C}_{\alpha }}\times {{C}_{\alpha }}}}{{C\left( {1-\alpha } \right)}}\]

\[\displaystyle =\frac{{C_{\alpha }^{2}}}{{1-\alpha }}------(4)\]

For weak acid, (∝) = very small, therefore 1- ∝ is was taken as 1, the above equation (4) written as,

\[\displaystyle {{K}_{a}}=\frac{{C_{\alpha }^{2}}}{1}\]

\[\displaystyle \alpha =\sqrt{{\frac{{{{K}_{a}}}}{c}}}------(5)\]

\[\displaystyle {{C}^{2}}={{u}^{2}}+{{v}^{2}}+{{w}^{2}}---(1)\]

Here, ∝₁ and ∝₂ = degree of dissociation of the two acids.

DISSOCIATION OF WEAK BASE

For weak monoacid base as BOH, its dissociation according to Arrhenius concept,

\[\displaystyle BOH\rightleftharpoons {{B}^{+}}+O{{H}^{-}}\]

The equilibrium law of equation, dissociation constant K_b of the base as,

\[\displaystyle {{K}_{b}}=\frac{{\left[ {{{B}^{+}}} \right]\left[ {O{{H}^{-}}} \right]}}{{\left[ {BOH} \right]}}------(1)\]

C moles/litre is the initial concentration of the base and ∝ is the degree of dissociation, then

\[\displaystyle =\frac{{C_{\alpha }^{2}}}{{1-\alpha }}------(2)\]

For the weak base, ∝ is very small as compared to 1, so equation (2) is written as,

\[\displaystyle {{K}_{b}}=C_{\alpha }^{2}\]

\[\displaystyle \alpha =\sqrt{{\frac{{{{K}_{b}}}}{c}}}\]

\[\displaystyle \left[ {O{{H}^{-}}} \right]={{C}_{\alpha }}\]

\[\displaystyle =C\sqrt{{\frac{{{{K}_{b}}}}{c}}}\]

\[\displaystyle =\sqrt{{c{{K}_{b}}}}------(3)\]

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DISSOCIATION OF WEAK ACID AND BASES

DISSOCIATION OF WEAK ACID

RELATIVE STRENGTH OF WEAK ACID

DISSOCIATION OF WEAK BASE

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Bhoomika Sheladiya

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