ionization

Question 1

theory of ionization

Answer 1

Ionization theory was presented by Arrhenius in 1887 which consist of following postulates:
 The substances called electrolytes are believed to contain electrically charged particles called ions.
 These charges are positive for H+

ion or ions derived from metals and negative for the ions derived
from non-metals. Number of electrical charges carried by an ion is equal to the valency of
corresponding atom.
 Molecules of electrolytes (acids, bases and salts) dissociate into oppositely charged ions on dissolution
in water, e.g.

NaCl ⇌ Na+ + Cl−
HCl ⇌ H
+ + Cl−
NaOH ⇌ Na+ + OH−

 The number of positive and negative charges on the ions must be equal so that the solution as a whole
remains neutral.
 In solution, the ions are in a state of disorderly or random motion. Upon colliding they may combine
to give unionized molecules.
 Thus ionization is a reversible process in which the solution contains ions of electrolyte together with
unionized molecules.

H2SO4(aq) ⇌ 2H
+
(aq) + SO4
2−
(aq)

 The extent of ionization or the degree of ionization depends upon the nature of electrolyte. Strong
electrolytes such as HCl etc. ionize completely in water. Weak electrolytes such as acetic acid
(CH3COOH) ionize only slightly
 Ionization is not affected by electric current.

Question 2

types of solutes

Answer 2

On the basis of accepting or donating proton, solvents can be classified into four groups.
i. Protophillic
ii. Protogenic
iii. Amphiprotic
iv. Aprotic

Question 3

protophillic

Answer 3

 Any solvent that can accept proton (H+

) from solute is called Protophillic.

 For example, liquids like (acetones, ether & liquid ammonia) are Protophillic solvents.
 Also called basic solvents.

Question 4

protogenic

Answer 4

 Those solvents which can donate proton (H+

) are called Protogenic solvents.

 They are usually acids in nature.
 E.g. H2SO4, HCl, CH3COOH, etc.

Question 5

amphiprotic

Answer 5

 Those solvents which can donate or accept the proton are around into amphiphiprotic solvents.
 This group contains H2O, alcohols.

Question 6

aprotic

Answer 6

 Those solvents which cannot donate or accept protons are said to Aprotic.
 They are used to study acidic and basic reactions of other compounds.
 Hydrocarbons are grouped in this class of solvents.

Question 7

law of mass action

Answer 7

 Law of mass action is stated by Guldberg and Waage. It states about the influence of the concentration
of the reactants on the rate of reaction.
 Law of mass action states that the rate at which substance reacts is proportional to its active mass and
the rate of chemical reaction is proportional to the product of the active masses of the reactants.
 Active mass is the number of moles per liter.
 It is represented by placing the chemical formula of the substance in square brackets. For example,
HCl is represented as [HCl].e

Question 8

equilibrium

Answer 8

The state at which two opposing forces or actions are balancing each other is called equilibrium.

Question 9

chemical equilibrium

Answer 9

The state of reversible reaction when the two opposing reactions occur at the same rate and the
concentration of reactants and products do not change with time is called chemical equilibrium.

Question 10

law of chemical equilibrium

Answer 10

 When the above stated law of mass action is applied to a reaction in equilibrium, the result is termed
as the law of chemical equilibrium.
 For example, the reaction,

aA + bB ↔ cC + dD

 The law of chemical equilibrium states the product of molar concentration of the products raised to the
power equal to its co-efficient, divided by the product of the molar concentration of the reactants raised
to its co-efficient, is constant at constant temperature and is termed as equilibrium constant.

Kc =
[C]
c
[D]
d
[A]
a[B]
Where “Kc” is called constant of law of mass action, and is equilibrium constant for a specific chemical
reaction.

Question 11

characteristics of equilibrium constant

Answer 11

 Its value remains constant at a given temperature irrespective of the direction of approach.
 The value of the equilibrium constant remains constant at given temperature and pressure irrespective
of the concentration of the reactants and products.
 The value of equilibrium constant depends on the nature and temperature of the reaction but it remains
unaffected in the presence or absence of catalyst.
 It gives information about the reaction proceeding in a particular direction at a given temperature.

Question 12

pH

Answer 12

pH is a measure of the acidity or basicity of an aqueous solution. Solutions with a pH less than 7 are
said to be acidic and solutions with a pH greater than 7 are basic or alkaline. Pure water has a pH very close
to 7.

OR

It is defined as the decimal logarithm of the reciprocal of the H+

ion acidity; aH
+
in a solution.

pH = − log10(aH

+) = log10 (
1
aH
+
)

Question 13

pH scale

Answer 13

 Indicators may be considered as weak acid or weak basis that act like buffers and also exhibit color
changes as heir degree of dissociation varies with pH. E.g. methyl red shows its fuel alkaline color
yellow at pH about 6 and full acid color red at pH 4.
 So indicator offers a convenient way of calorimetric method of determining the pH of solution.
 The dissociation of an acid indicator can be expressed as:

Kin =
[H3O
+][ln−]
[Hln]

 HIn is unionized form and Inis the ionize form, unionize form gives acid color and ionize form gives
basic colour. When an acid is added to the solution of indicator, the H+

ion concentration increases and

HIn predomination and give acid colour. When a base is added [H3O
+
] is reduced and more ionized

form is produce so the color changes.

pH = pKin +
[Base]
[Acid]

Question 14

sorensons pH scale

Answer 14

 Hydrogen ion concentration are typically very small number, it is b/w 1 (in one molar strong acidic
solution) and 1 × 10-14 (in one molar strong basic solution). So, it is very difficult to handle such small
calculations.
 A chemist Sorensen established a method to express hydrogen ion concentration of a solution. This
method is called Sorensen’s pH scale: and it is defined as:

“The logarithm of reciprocal of hydronium ion is called pH.”

pH = − log[H]
+

 The pH has values b/w 0 & 14 with the help of pH value, the nature of solution (acidic or basic) can
be determined.
 If the solution has pH below 7 to 0, it will be acidic in nature. And the value at which hydrogen ion
concentration is equal to hydroxyl ion concentration, then the solution has pH 7.47 (at 0oC) and 6.15
(at 100oC) and will be neutral in nature.
o Neutral solution [H+
] = [OH-
]
o Acidic solution [H+
] > [OH-
]
o Basic solution [H+
] < [OH-
]

And
o pH + pOH = 14

Question 15

pka

Answer 15

pKa of an acid is the –ve log of its acid dissociation constant. Just as pH that is used to find H+
of a
solution, pKa can be used to describe the dissociation constant of a weak acid. The higher the pKa, the weaker
is the acid. E.g.

o pKa of CH3COOH is 4.76.
o pKa of HNO3 is –1.45.
o pKa of (COOH)2 is 1.27.
o pKa of CF3COOH is 0.25.

Question 16

applications

Answer 16

drug solubility
drug stability
drug activity
drug absorption
enzyme activity
therapetuic efficacy
permeability of the drug throuhg biological membranes

Question 17

drug solubility

Answer 17

 As many drugs are either weak acids or bases, so their solubility is affected by the change in pH.
 The weakly acidic drugs (e.g. aspirin) are more soluble in alkaline solution because in alkaline solution
they are converted into salt form, which is more soluble than in acidic form conversely the weakly
basic drugs are more soluble in acidic solution.
 And if the pH is lowered then acidic drug will be precipitated. So for the solubility of acidic drug pH
should be higher than 7 & for basic drug lower than 7.

Question 18

drug stabilit

Answer 18

 Drugs are only stable at a certain pH. So for this purpose the pH of drugs must be constant otherwise
the ionization of drug may take place, which decreases the therapeutic effect of the drug.
 For example, cocaine HCl (Salt) is for two months at P H 5.7. But during this period P H is decreased
to 4.2. So stability is decreased. If P H is increased from 5.7 to 6, its decomposition into its ions is
decreased & stability increased.

Question 19

drug activity

Answer 19

 The activity of drug also depends upon their pH and its ionized or unionized form.
 For example, mandelic acid, benzoic acid etc. are acidic drugs. They are more effective in unionized
form than ionized form. So such acidic drugs need acidic medium (low pH) to get unionized form &
for better effectiveness.

Question 20

drug absorption

Answer 20

According to P H partition theory:
“The acidic drugs are absorbed by the stomach (with low pH) and basic drugs are absorbed by intestine

(having high pH).”

 On the basis of this theory, we can conclude that for the absorption of an acidic drug (e.g. aspirin) the
medium should have high concentration of hydrogen ions (low pH) & for the absorption of basic drug.
(e.g. paracetamol) the absorption medium should have low concentration of hydrogen ions (having
high pH).

Question 21

enzyme activity

Answer 21

The activity of enzymes present in our body is influenced by pH. Some enzymes are active in acidic
medium, some in alkaline and other in neutral medium. And beyond that limit enzymes become inactive and
may be destroyed. E.g. pepsin works at 1.5 – 1.6.

Question 22

therapeutic effeciacy

Answer 22

Solutions to be applied to tissues or administrative parenterally are liable to cause irritation of their pH
is greater than the relevant body fluid e.g. pH of ophthalmic and nasal solution should be maintained.

Question 23

permeabilt of the drug through biologicla membranes

Answer 23

This depends on the extent of ionization of drugs. The pH of solution can affect the extent of ionization
of weakly acid / weakly basic drug. A drug in the non-ionized form is more permeable than its ionized form.