 Acids and Bases and their Ionisations

Strong acid : 100% dissociation
HClO4, H2SO4, HNO3, HI, HBr
HCl + H_{2}O \rightarrow H^{+} + Cl^{-}

Strong base :
NaOH, KOH, RbOH, CsOH, Ba(OH)2

Weak acid : Which undergoes partial dissociation Hydrogen ion concentration : K_{a} = \frac{C{\alpha} \ C{\alpha}}{C - C{\alpha}} = \frac{C\alpha^{2}}{1 - \alpha}
Ka = Cα2 (α < < < 1, 1 − α = 1)
\alpha = \sqrt{\frac{K_{a}}{C}}
[H^{+}] = C\alpha = C. \sqrt{\frac{K_{a}}{C}} = \sqrt{K_{a}.C}
[H^{+}] = C\alpha = \sqrt{K_{a}.C}

Arrhenius acid - base theory :
Strong acid : Which produce more no. of H+ (HClO4, H2SO4, HCl)
Weak acid : Which produce less no. of H+(CH3COOH, HCN, H2S)
Strong base : Which produce more no. of OH (NaOH, KOH)

Bronsted - Lowry acid - base theory :
Acid : Proton donor [HCl , H2SO4, CH3COOH ......]
Base : Proton acceptor[NaOH, KOH, NH3 .......]
Salt : Neither proton acceptor nor proton donor. [C6H6, CCl4 ..... aprotic]

Conjugate acid - base pair :
A Bronsted - Lowry acid - base pair which differ by only one proton is called conjugate acid - base pair.
Acid − H+ → conjugate base
Base + H+ → conjugate acid. Ionic product of water : (KW)
H_{2}O + H_{2}O \rightleftharpoons H_{3}O^{+}+ OH^{-}
K = \frac{[H_{3}O^{+}][OH^{-}]}{[H_{2}O]^{2}}
K{[H_{2}O]^{2}} = [H_{3}O^{+}][OH^{-}]
K_{W} = [H_{3}O^{+}][OH^{-}]

Kw → acid, ΔT = 25°C
Kw = 1.0 × 10-14 mol2 lit-2
[H+][OH-] = 10-14, [H+] = 1.0 × 10-7 mol/lit

Degree of dissociation :
\alpha = \frac{1}{55.5 \times 10^{7}} = \frac{10^{-7}}{55.5} = \frac{10^{-7}}{\left(\frac{1000}{18}\right)}
α = 1.8 × 10-9
% dissociation = 1.8 × 10-7 , \alpha = \frac{10^{-7}}{55.5}

Part3: View the Topic in this Video from 0:10 to 9:04

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1. Degree of ionisation :
\tt \alpha = \frac{number \ of \ molecules\ ionised \ or \ dissociated}{total \ number \ of \ molecules \ taken}
For strong electrolytes, α = 1
For weak electrolytes α < 1

2. Ostwald's Dilution law :
k = \frac{C\alpha^{2}}{1 - \alpha}
If α is very small 1 - α ≈ 1 ⇒ K = Cα2
or \alpha = \sqrt{\frac{K}{C}} \Rightarrow \alpha \propto \frac{1}{\sqrt{C}}
Here, K is dissociation constant and C is molar concentration of the solution.

3. Dissociation constant of acid, K_{a} = \frac{\left[H^{+}\right]\left[A^{-}\right]}{\left[HA\right]} =\frac{C\alpha^{2}}{\left(1 - \alpha\right)}

4. Dissociation constant of the base K_{b} = \frac{\left[B^{+}\right]\left[OH^{-}\right]}{\left[BOH\right]} =\frac{C\alpha^{2}}{\left(1 - \alpha\right)}