Lecture Five covers the following points:-
Types of neutralization reaction
Types of indicator used
Which one will be used in Quantitative titration
Types of neutralization reaction
Strong Acid + Strong Base = Salt + Water
HCl + NaOH = NaCl + H2O
Strong Acid + Weak Base = Salt + Water
HCl + NH4OH = NH4Cl + H2O
Weak Acid + Strong Base = Salt + Water
CH3COOH + NaOH = CH3COONa + H2O
Weak Acid + Weak Base = Salt + Water
CH3COOH + NH4OH = CH3COONH4 + H2O
Neutralization Curves
Preequivalence
Equivalence
Postequivalence
Titrating a strong acid with a strong base:
We will be calculate hypothetical titration curves of pH versus volume of titrant
50 ml of 0.05 M HCl with 0.1 M NaOH
Initial point:
pH= - log [H3O+]
= - log 0.05 = 1.3
Preequivalance:
After adding 10 ml of NaOH
CHCl = no. mmol HCl –no. mmol NaOH/ total V
= (M x V)HCl _ ( M x V )NaOH / Vtotal
= (0.05 x 50 ) _ ( 0.1 x 10 ) / 60
= 2.5 x 10-2
pH = - log (2.5 x10-2) = 1.6
After addition of 24.9 ml NaOH
CHCl = (M x V )HCl - ( M x V )NaOH / Vtotal
= ( 0.05 x 50 ) – ( 0.1 x 24.9) / 74.9
= 2.5 - 2.49 / 74.9 = 0.01/ 74.9
= 1.33 x10-4
pH = - log ( 1.33 x 10-4 ) = 3.87
Equivalence point:
[ H3O+] = ( Kw )1/2 = ( 1 x 10-14)1/2 = 1 x 10-7
pH = - log ( 1 x 10-7 ) = 7
Postequivalence :
After addition of 25.1 ml NaOH
CNaOH = (M x V )NaOH - ( M x V )HCl / Vtotal
= ( 0.1 x 25.1 ) – ( 0.05 x 50) / 75.1
= 0.01/ 75.1 = 1.33 x 10-4
pOH = - log ( 1.33 x 10-4 ) = 3.87
pH = 14 – 3.87 = 10.13
After addition of 40 ml NaOH
CNaOH = (M x V )NaOH - ( M x V )HCl / Vtotal
= ( 0.1 x 40 ) – ( 0.05 x 50) / 90
= 4 – 2.5 / 90 = 1.5 / 90 = 0.0166
pOH = - log ( 0.0166 ) = 1.77
pH = 14 – 1.77 = 12.23
Acid –Base Indicator
Many substances both naturaly occurring and synthetic, display colors that depend upon the pH of the solutions in which they are dissolved.
An acid – base indicator is a weak organic acid or weak organic base whose undissociated form differs in color from its conjugate base or its conjugate acid form. For example, the behavior of an acid type indicator, HIn , is described by the equilibrium:
HIn + H2O = In- + H3O+
Here , internal structural changes accompany dissociation and cause the color change. The equilibrium for a base type indicator In, is:
In + H2O = InH+ + OH-
In the following paragraphs we focus on the behavior of acid type indicators. The discussion, however, can be readily extended to base type indicator as well.
THE EQUILIBRIUM CONSTANT EXPRESSION FOR DISSOCIATION OF AN ACID INDICATOR TAKES THE FORM:
Ka = [H3O+] [ In- ] / [ HIn]
Rearranging leads to
[H3O+] = Ka [HIn ] / [ In- ]
- log [H3O+] = - log Ka - log [HIn ] / [ In- ]
pH = pKa - log [HIn ] / [ In- ]
The human eye is not very sensitive to color differences in a solution containing a mixture of In- and HIn , particularly. When the ration [In-] / [HIn] is greater than about 10 or smaller than 0.1. Consequently, the color imparted to a solution by a typical indicator appears to the average observer to change rapidly only within the limited concentration ratio of approximately 10 to 0.1.
To obtain the indicator pH range:
pH ( acid color) = pKa - 1
pH ( basic color) = pKa + 1