Acid-Base Equilibria: Theory and Applications - Prof. Abdo, Lecture notes of Analytical Chemistry

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Acid – Base Equilibria

Prof. Dr. Elham Y. Hashem

Acid – Base theories:-

1- Arrhenius Theory

1894 – Nobel Price – applicable only in water

Is any substance that ionizes (partially or completely) to give hydrogen ions which associate with water to give hydronium ion H 3 O+

H A  H O  H O^  A 2 3

An Acid

A Base

Ionizes in water to give hydroxyl ions.

(weak base) B^ ^ H 2 O  BH^ OH

(strong base) M (OH)n  M n ^ nOH

H Acid

Base

H Base

Acid

Acid 1  Base 2  Acid 2  Base 1

Example

CH 3 COOH NH 3 NH 4 CH 3 COOH

HOAC NH 3 NH 4 OA c

  

    ) كمذيباألمونيافى^ (

An acid

A base

3- Lewis Theory

(1923) the electronic theory

Is a substance that can accept an electron pair.

Is a substance that can donate an electron pair.

H :NH 3 H:NH 3

AlCl 3 + (^) O

R

R

Cl 3 Al OR 2

Autoprotolysis:

Is the self-ionization of a solvent to give a cation and anion. e.g.

CH 3 OH 2  CH 3 O



2 CH 3 OH

Pure water ionizes slighty , or undergoes autoprotolysis.

2H 2 O H 3 O+ + OH-

a H O

aH O. OH

K

2

2

a o 3 w

 

The activity of water in dilute solvation is constant

K  H O. aOH

3

o a w o Kw is the thermodynamic autoprotolysis constant. Or Self – ionization constant.

Molar Concentration:

Will be represented by square brackets [ ] around the species. Simplified equations for the above reactions are.

HCl  H Cl

H   Ac

     HAc 

H Ac Ka

 

H^  OH

Kw  H OH 

HAc

H 2 O

Ka &Kw^ are the molar equilibrium constants.

pOH  log OH 

p Kw  log Kw

p Kw  pH pOH

at 25C

pH + pOH = 14

The pH Scale

The pH of a solution was defined by Sorenson as

pH   log H 

p anything = -log [anything] وعلية فإن يمكن القول

Ex. 1: calculate the pH

of a 2 10 -3^ M HCl soln.

Ex. 2: Calculate the pOH and the pH of a 5 10 -4^ M soln. of NaOH.

Ex. 4: The pH of a soln. is 9.67, calculate the hydrogen ion concentration in soln.

Ex. 3: Calculate the pH of a soln. prepared by mixing 2ml of a strong acid soln. of pH 3 and 3 ml of a strong base soln. of pH 10.0.

Example