PHYSICAL CHEMISTRY OF MILK

Lesson 21. SALTS AND THEIR HYDROLYSIS

Module 9. Buffers

Lesson 21
SALTS AND THEIR HYDROLYSIS

21.1 Introduction

Salts are ionic compounds containing cations and anions except hydrogen and hydroxide ions. All ions in aqueous solution are hydrated i.e., they are surrounded by water molecules in aqueous solution some ions undergo only hydration while other ions react with the water molecules and result in the formation of acidic and alkaline solutions . Salt is formed when an acid comes in contact with a base and when dissolved in water it gets dissociated and forms ions. This process is known as hydrolysis.

21.2 Definition of Salt

Salts are defined as ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of positively charged cations and negatively charged anions in such a proportion that the product formed is electrically neutral i.e without a net charge.

There are several varieties of salts. Salts that hydrolyze to produce hydroxide ions when dissolved in water are basic salts and salts that hydrolyze to produce hydronium ions in water are acid salts. Neutral salts are those that are neither acid nor basic salts.

21.3 Hydrolysis of Salts

Dissolving salts in water would result in the interaction of its ions viz. cations and anions with water. This is called hydrolysis. This interaction would affect the pH of the solution. The cations (e.g., Na⁺, K⁺, Ca²⁺, Ba²⁺, etc.) of strong bases and anions (e.g., Cl^–,SO₄, Br^–, NO₃^–, ClO₄^– etc.) of strong acids simply get hydrated but do not hydrolyse. Therefore the solutions of salts formed from strong acids and bases are neutral i.e., their pH is 7 for e.g.,. Na₂S0₄, KC1, KN0₃ etc. Salts formed by reactions of strong acid and weak -base will give acidic aqueous solutions and for e.g.,nh₄ci, CuSO₄, Al(N0₃)₃ etc. This is due to the interaction of the ions produced from the salt with the ions formed from water. Hydrolysis is quite opposite of neutralization process.

21.4 Neutralization

Reaction between acid and base would result in salt formation. This reaction is known as neutralization. In this reaction water is also formed along with salt This reaction could be represented as follows

Depending on the solubility of the salt, it may remain in ionized form in the solution or it may precipitate out of solution. Neutralization reactions usually proceed to completion.

21.5 Types of Combinations for the Salt Formation

The interaction between acid and base could be grouped in to four categories basing on their type These categoriese are

a. Strong acid and strong base

b. Strong acid and weak base

c. Strong base and weak acid

d. Weak base and weak acid

21.5.1 Strong acid with a strong base

Interaction between a strong acid and a strong base would result in the formation of salt. When strong acid like hydrochloric acid (HCl) reacts with an equally strong base like sodium hydroxide( NaOH) sodium chloride (NaCl) would be formed .

The acid and base neutralize each other with the resulting solution will be neutral (pH=7) and there is no interaction between the ions and water.

21.5.2 Strong acid with a weak base

The interaction between a strong acid and a weak base would also result in the formation of salt. The following equation expresses this reaction. :

Ammonia ions formed in this reaction would result in the lowering of pH since they react with water. As such the resulting salt which is in the form of a solution is acidic.

Similarly when hydrochloric acid (HCl) reacts with a weak base like ammonia would form ammonium chloride which is a salt, but in this reaction water molecule is not formed. The solvent water interacts with the ammonia which is the cation of the salt to reform the weak base :

The reaction between a strong acid and a weak base also produces a salt, but water is not usually formed because weak bases tend not to be hydroxides. In this case, the water solvent will react with the cation of the salt to reform the weak base. The equation below represents this reaction. :

21.5.3 Weak acid with a strong base

Interaction between a weak acid like bleaching powder with a strong base like sodium hydroxide would result in the formation of sodium hypochloride which is a salt. The basicity of this slat is due to the hydrolysis of the conjugate base of the (weak) acid used in this reaction. Similarly sodium acetate formed by the interaction between acetic acid which is a weak acid and sodium hydroxide (NaOH) a strong base will be formed which is a basic salt. When salt is dissolved, ionization takes place: The bleaching powder being a weak acid when it reacts with sodium hydroxide

When a weak acid reacts with a strong base the resulting solution will be basic. The salt will be hydrolyzed to form the acid, together with the formation of the hydroxide ion from the hydrolyzed water molecules.

21.5.4 By the reaction of a weak acid with weak base

The reaction between the bleaching powder with a weak base like ammonia would form ammonium hypochloride

pH of the solution formed from the reaction of a weak acid with a weak base depends on the relative strengths of the reactants. For example, if the acid HClO has a K_a of 3.4 x 10^-8 and the base NH₃ has a K_b = 1.6 x 10^-5, then the aqueous solution of HClO and NH₃ will be basic because the K_a of HClO is less than the K_a of NH₃.

When a salt is formed between a weak acid and a weak base it could be neutral, acidic, or basic depending on the relative strengths of the acid and base.

If K_a(cation) > K_b(anion), the solution of the salt is acidic.

If K_a(cation) = K_b(anion), the solution of the salt is neutral.

If K_a(cation) < K_b(anion), the solution of the salt is base