Factors Affecting Strength of Acid

If the acid is strong, its value of K will be high and the value of pH will be low. The dissociation of acid will depend on strength of acid and the polarity of H-A bond. As the strength of H-A bond decreases, the energy required for breaking that bond will decrease and HA will be stronger. When difference between electronegativities of A and B will increase, apparently ionisation will occur and will be easy to break the structure of the bond. Hence, acidity will increase.

For this reason, H₂S is stronger acid than HO, but if we discuss the elements in the same period of periodic table, the polarity of H-A bond will determine the strength of acid. As the electronegativity of A increases, the strength of acid will increase.

Ø Common ion effect on the ionisation of acids and bases:

Consider an example of acetic acid dissociation equilibrium represented as:

CCOO ⇋ + CCO

or HA ⇋ + A

Addition of acetate ions to an acetic acid solution results in decreasing the concentration of hydrogen ions, []. Also, if H+ ions are added from an external source then the equilibrium moves in the direction of undissociated acetic acid i.e., in a direction of reducing the concentration of hydrogen ions, []. This phenomenon is an example of common ion effect. It can be defined as a shift in equilibrium on adding a substance that provides more of an ionic species already present in the dissociation equilibrium. Thus, we can say that common ion effect is a phenomenon based on the Le Chatelier’s principle

In order to evaluate the pH of the solution resulting on addition of 0.05M acetate ion to 0.05M acetic acid solution, we shall consider the acetic acid dissociation equilibrium once again,

HA ⇋ + A

Initial concentration (M)

0.05 0 0.05

Let x be the extent of ionization of acetic acid.

Change in concentration (M)

–x +x +x

Equilibrium concentration (M)

0.05-x x 0.05+x

Therefore, = [][A]/[H Ac] = {(0.05+x)(x)}/(0.05-x)

As is small for a very weak acid, <<0.05.

Hence, (0.05 + ) ≈ (0.05 – ) ≈ 0.05

Thus,

1.8 × =

= = [] = 1.8 × M

pH = – log(1.8 × ) = 4.74

Ø Hydrolysis of salts and the pH of their solutions:

Salt is obtained by combination of acid and base in definite proportion. When the salt is dissolved in water, ionisation occurs. The following types of salts are obtained depending on acid or base strong or weak. Salt formed from strong base and strong acid is neutral and so its pH is 7.0. e.g., Nacl.

But if salt is formed from strong acid and weak base, it will be acidic and pH of its aqueous solution will be less than 7 e.g. NCl. Similarly, salt formed from weak acid and strong base is basic and its pH in aqueous solution will be more 7, e.g. CCOONa. The reason is that salt reacts with water and undergoes hydrolysis reaction.

Acid	Base	Salt	Property	Example
Strong	Strong	Neutral	Neutral	NaOH + HCl → NaCl + O
Strong	Weak	Acidic	Acidic	HCl + NOH ⇋ NCl + O
Weak	Strong	Basic	Basic	CCOOH +NaOH ⇋ CCOONa + O
Weak	Weak	Neutral Or Acidic Or basic	Neutral Or Acidic Or basic	CCOOH + NOH → CCOON + O HCOOH + NOH ⇋ HCOON + O COOH + NOH ⇋ COO N + O

Hydrolysis reaction is an equilibrium reaction and so its corresponding equilibrium constant can be calculated which is known as hydrolysis constant (). It can be determined with the help of the example of weak acid and strong base e.g. CCOONa

(1) For salt of weak acid and strong base:

CCOON + ⇋ CCOO + NaO

(2) For salt of strong acid and weak base:

NC + ⇋ NaO + HC

(3) For salt of weak acid and weak base:

CCOON + ⇋ CCOO + NO

Earlier you have studied about K and K. The equation of K. can be obtained from them as shown below :

Salt	Hydrolysis of water	pH of solution
Strong acid weak base	= =	>7
Weak acid strong base	= =	< 7
Weak acid weak base	= =	=7

Thus, from the nature of the salt pH of its aqueous solution can be calculated.

Ø Buffer solution:

The pH of the fluids like blood in our body and urine is almost constant. If there is change in this pH, it affects biochemical reaction in the body. The pH of chemical and biochemical reactions in our body are constant, viz. the pH of human saliva is 6.4. In addition, hydrochloric acid is present in human stomach which helps in digestion. The pH of cosmetics are also kept constant. Hence, the question arises that how pH in any solution can be kept constant. Such solutions are called buffer solutions. Its definition can be given as below:

"The solution which resists the change in pH carried out by addition of acid or base in small proportion to them or are being diluted, and the values of their pH remain constant are called buffer solutions". Buffer solutions can be acidic or basic. If p of weak acid and p of weak base are known, buffer solutions of known pH can be prepared. Buffer solutions can be of three types as follows:

(i) Acidic buffer solution :

Acidic buffer solution can be prepared by mixture of weak acid and its salt with strong base.

(ii) Basic buffer solution :

Basic buffer solution can be prepared by mixture of weak base and its salt with strong acid.

(iii) Neutral buffer solution :

Neutral buffer solution can be prepared by neutralisation of weak acid and weak base. These types of buffer solutions are shown below:

Type	Substance	Value of pH
Acidic	CCOOH + CCOONa	<7
Basic	NOH + NCl	>7
Neutral	CCOOH + NOH	≈7

Buffer solution of known pH can be prepared by using the following Henderson- Hasselbalch equation.

For acidic solution,

pH = p +

Where [acid] is concentration of weak acid and its dissociation constant is and [salt] is concentration of the salt of this weak acid with strong base. For an acidic buffer solution it can be written as

pH = p +

Similarly, for basic buffer solution e.g. NOH + can be written that

pH = p +

Such buffer solutions can be used in chemical and biochemical reactions and especially in analytical chemistry. In human body buffer solutions containing [HC] and [C] as well as [P] and [HP ] are present.