Acids And Bases

ACIDS AND BASES Definition: Arrhenius theory: According to this theory an acid is a substance that produce H+ or H3 O+ produce in water; for example: HCl, HNO3, H2 SO 4 and a base is a substance that can produce OH- in water ; for example : NaOH, LiOH, Ca(OH)2. Bronsted Lowry theory: According to this theory acids are those substances which can donate proton H+; for example: NH4+, HSO4, and all arrhenius acids. Bases are those which can accept proton for example: NH3, H2O, CO3-2. Lewis theory: According to this theory acids are those which can accept electron pair for example: H+, positive ions – Al+3, Fe+2 etc and bases are those which can donate a pair of electrons for example: OH-, negative ionsF-, O-2, N-3. Properties of Acids and Bases: ACIDS: •

When dissolved in water produce Hydrogen ions

•

Neutralize bases to produce water and salts

•

Turns blue litmus into red

•

Sour in taste

•

Electrolytes in nature

BASES: •

When dissolved in water produce Hydroxyl ions

•

Neutralize acids to produce water and salts

•

Turns red litmus into blue

•

Bitter in taste

•

Electrolytes in nature, conduct electricity

Acid Base Reaction: NEUTRALIZATION: This is the reaction between H+ or H3 O+ from an acid and the OH- from base to form water. The neutralization reaction is exothermic and release approximately 56KJ / mole of energy. H+ + OH-

------------ H2 O ΔH˚ = - 55.9KJ

Acid + Base

------------ Salt + Water

e.g. HCl+ NaOH

------------ NaCl + H2 O

STRONG AND WEAK ACIDS AND BASES: Strong acids:

A strong acid completely dissociate into ions (conjugate acid and base) in water or from salt but a weak acid dissociates partially. Strong acid:

HA + H20 ------------ H3O+

+

A-

e.g.

HCl + H20 ------------ H3O

+

+

Cl-

(acid)

(base)

(conjugate acid)

(base)

Weak acids: A weak acid dissociate very slightly into ions in water unlike that of strong acids.

Weak acid:

--------- H3O+ + A-

HA + H20

H3O+

CH3COOH + H20 --------- (acid)

(base)

CH3COO-

+

(conjugate acid)

(base)

In a dilute solution of weak acid major molecules are undissociated. In general a strong acid has a weak base while weak acid has strong base. Examples: Strong Acid:

Acid + conjugate base (weak)

Weak Acid:

Acid + conjugate base (strong)

CH3COOH

H+

+ CH3COO-

HNO3

H+

+ NO3-

HCl

H

+

+

Cl

H2SO4

H+

+

HSO4

-

Strong base: A strong base can dissociate completely into ions (conjugate base) and acid water Strong base: XOH

---------

Base

Example: Weak base:

X

+

conjugate acid

NaOH

---------

Na

+

OH-

+

conjugate base

+

OH-

A weak base partially dissociates or weakly dissociated in dilute form, into ions in water.

Examples of acids and bases: SL.NO

STRONG ACID

WEAK ACID

STRONG BASE

WEAK BASE

1

HCl

HCO3

LIOH

NH3

2

HNO3

H3PO4

NaOH

AMINES

3

H2SO4

HF

KOH

4

HClO4

HCN

Ca(OH)2

5

HBr

H2SO3

Ba(OH)3

6

HI

Degree of dissociation: The extent of dissociation or ionization of weak acids and weak bases is called as Degree of dissociation or ionization. It is always less than 1. HA

---------

H+ + A-

[H+][A-] Ka =

[HA]

Ka = Dissociation constant of acid or Acid constant HYDROGEN ION CONCENTRATION (pH)

•

Sorenson in 1909 introduced pH and defined it as negative logarithm of Hydrogen ion concentration. A scale pH id used to express the H+ ion concentration in water.

•

pH = - log [H+] or pH= 1/ log [H+]

•

When an acid gets dissociated it releases ions

---------

HA

H+ + A-

[H+][A-] [HA]

Ka =

Ka = Dissociation constant of acid or Acid constant -log Ka = pKa pH = pKa+ log [A-]/[HA]

•

The acidic or basic nature of solution is measured by H+ ion concentration.

•

In case of base the pH is expressed in terms of pOH which is pOH = -log [OH-] or 1/ log [OH-]

•

The dissociation constant of base is given by

XOH

---------

Kb =

[oH-] [x+]

OH- + X+

[xOH] Kb = Dissociation constant of base or base constant pOH = pKb+ log [Conjugate cation]/[Base] (or)

pOH = pKb+ log [x+]/[xOH]

•

The pH scale ranges from 0 to 14

•

pH less than 7.0 is said to be acidic, pH 7.0 is neutral and pH above 7.1 is basic

•

Pure water has an equal concentration of H+ & OH – ions. That means 10-7 each. So pure water is neutral. In a neutral condition the relation between pH and pOH is pH + pOH = 14.

Importance of pH in biological fluids:

•

Most of the biological reactions require appropriate pH (optimum pH) the change in which stops the reaction.

•

Microorganisms require optimum pH for their growth.

•

Most of the crops best grown for better yield at specific pH of soil.

Some important pH values of biological fluids: Biological Fluid

pH

Biological Fluid

Pancreatic juice

7.5 -8.0

Gastric juice

1.5 -2.0

Blood plasma

7.35 – 7.45

Saliva

6.4 -7.0

Tears

7.2 -7.4

Urine

5.0 -7.5

Human milk

7.2 -7-4

pH

WATER Introduction

•

Water is the most abundant substance in living systems, making up to 70% or more of the weight of most of the organisms.

•

The first living organism coecerovates arose from aqueous environment

•

It is regarded as the SOLVENT OF LIFE

•

The structure and function of bio molecules (carbohydrates, proteins, nucleic acid etc) depends upon the attractive forces between the water molecules and their ionization property.

Properties of water

•

The Hydrogen bonds between water molecules provide the cohesive forces that make water a liquid at room temperature.

•

Polar bio molecules dissolve readily in water because they can replace water- water interactions with more energetically favorable water -solute interactions.

•

Non polar bio molecules interfere with water – water interactions but are unable to form water solute interactions; non polar molecules are poorly soluble in water. In aqueous solutions non polar molecules tend to cluster together.

•

Hydrogen bonds, ionic bond and van der waals interactions are individually weak but collectively they influence on 3 Dimensional structure of proteins, Nucleic acids, Polysaccharides and membrane lipids.

Bonding in water molecules: •

Water molecules exist in a bent geometry.

•

The bond angle is 104.5˚ slightly less than tetrahedron (109.5˚) and the O –H distance is 0.958 A˚

•

There exists electrical property in water due to the electro negativity difference between H & O. The unequal sharing results in two electric dipoles in the water molecules. Each Hydrogen bears partial positive charge (δ+) and oxygen bears partial negative charge (δ-). So water has an electric dipole. δ+ δ+

H 104.5˚

O δ-----------

H

H

H H Hydrogen bonding of water • •

Water has a higher Melting point, boiling point and heat of vaporization than other solvents. Each Hydrogen atom of water molecule shares an electron pair with central oxygen atom.