Ionic equilibrium

Lec.9

Introduction • Ionization: a process in which unionized molecules are

changed into ions.• Types of electrolytes:

Strong electrolytes Weak electrolytescomplete ionization partial ionization

All Salts, strong acids & strong bases Weak acids and weak bases

HCl H+ +Cl-

NaOH Na+ +OH-

Na Cl Na+ + Cl-

CH3 COOH CH3 COO - + H+

NH4OH NH4+ + OH-

No equilibrium take places ,so no equilibrium constant

The reaction stop when reached equilibrium(IONIC Equilibrium ), so it has equilibrium constant (K)

Where: Ka = acid ionization constant

Kb = base ionization constant

Ostwald Law(1888):•Described the relation between the extent of ionization (α) and concentration (C) mole/ liter for solutions.•Weak electrolyte dissociates in water to ions .

Where : [A-], [B+] : concentrations of the ions produced [AB] :the undissociated form at equilibrium state. K : ionization or dissociation constant

Suppose V = volumeConcentration = number of moles/ volume (L)

No.of moles

In case of very weak electrolyteα = very small value and can be neglected i.e. (1- α) = 1

Where : C =1/V

Very weak electrolyte

weak electrolyte

Degree of Ionization

Determination of (α) Experimentally:

• Determine the value of( Ʌ v ) : equivalent conductance with

different weak electrolyte concentrations and calculated K in

each case.

• K was always constant and independent of the dilution only in

case of weak electrolytes.

• Determine the value of(Ʌ o )= equivalent conductance at

infinite dilution.

•   If λ v ↑ α ↑ ---------- λ v α (α)

α= Ʌ v / Ʌ o

•  •

Example (1):

The acid can be considered very weak, [α ‹‹‹‹ 1] •Concentration of hydrogen ion:

The Ion Product of Water• Water is very slightly ionized

( Very weak electrolyte)

H2O is very weak electrolyte

[H2O] = constant value

[H2O]*K=constant value

[H2O]*K=KW= ion product of water

[H+] * [OH-]= KW= 10-14 at constant temperature

For example: at samples of H2O:

1- When [H+] = 10-5> 10-7 (acidic solution)2-When [H+] = 10-9< 10-7 ( alkaline solution)4- When [OH-] =10-11<10-7 (acidic solution)

The Hydrogen Ion Exponent:( pH)

• pH is defined as the negative exponent of 10 which gives the hydrogen ion concentration , pH= - log 10[H+]

• pOH = - log [OH-]

• [H+][OH-] = Kw =10-14

• -log [H+]-log[OH-]=-log10-14

• p H +p OH =14

Example (1):What is the pH value of:•Pure water.•HCl (0.01 mole/L).•NaOH (0.001 mole/L).

Example (2):The ionization constant of acetic acid at 25 0C is 1.82x10-5. Calculate the pH of 0.1 mole/ L acid.

Another method of calculating [H+]: According to Ostwald’s dilution law,

pH= -log [H+]= 2.87

Example (3):If the pH value of 0.1 mole/L acetic acid is 2.872, calculate the ionization constant of the acid.

AnswerC=0.1 pH= 2.872 K=?pH= -log [H+]2.872 = -log [H+][H+]= 1.353x10-3

COOHCH3

HCOOCH3