Hydrolysis of Salts

pH Chart

In our last post: Types, Preparation & Uses of Salts, we studied that salts are the products of the neutralization reactions between acids and bases. Following this fact, all salts are expected to have a neutral pH of 7 in solution. Interestingly, some salts produce solutions with pH > 7 (basic solutions), while others produce acidic solutions with pH < 7. Such salts, which are the products of either weak acids and strong bases or strong acids and weak bases, decompose in water to form two products. This phenomenon is known as hydrolysis.

Recall that from our post on pH in Acids & Bases (Part III), we learnt that water contains equal concentration of hydrogen and hydroxide ions. Therefore, the dissolution of a salt in water is likely to upset this ionic equilibrium based on the salt's composition. This is the underlying principle of hydrolysis of salts. So, in this post, we shall be looking at the hydrolysis of different types of salts based on their formation. To fully understand this topic, you may also need to read our posts on Acids & Bases (Part I) and Acids & Bases (Part II).

Hydrolysis of Salts of Weak Acids & Strong Bases

An example of a salt that falls into this category is sodium ethanoate, CH3COONa, formed from ethanoic acid and sodium hydroxide. In solution, sodium ethanoate ionizes completely to produce ethanoate and sodium ions.

CH3COONa(aq) ----------> CH3COO–(aq) + Na+(aq)

Being a weak acid/base, the water molecules undergo partial dissociation as discussed in Acids & Bases (Part I).

H2O(l) <------------____> H+(aq) + OH–(aq)

Note that the reversible arrows points longer to the undissociated molecules, H2O, showing that they are the favoured species in the dissociation process.

The four ions in solution undergo interaction with one another, and being the conjugate base of a weak acid, the ethanoate ions from the salt, combine with the hydrogen ions from the water to form undissociated ethanoic acid molecules.

CH3COO–(aq) + H+(aq) <-----_______> CH3COOH(aq)

According to Le Chatelier's Principle, this will distort the chemical equilibrium of the system. Consequently, in order to replace the lost hydrogen ions, the undissociated water molecules must undergo further dissociation to produce hydrogen and hydroxide ions. This process continues until all the hydrogen and ethanoate ions are removed from the solution, leaving only sodium and hydroxide ions, which are products of the ionization of sodium hydroxide - a strong base.

NaOH(aq) ---------> Na+(aq) + OH-(aq)

These ions remain in solution, and account for the pH of the resulting solution of sodium ethanoate being basic (pH > 7). Other examples of salts that fall into this category (solution pH > 7), and their constituent acids and bases are:

1. Sodium trioxocarbonate (IV), Na2CO3 - a product of trioxocarbonate (IV) acid, H2CO3, and sodium hydroxide, NaOH.

2. Potassium ethanoate, CH3COOK -  a product of ethanoic acid, CH3COOH, and potassium hydroxide, KOH.

3. Sodium trioxosulphate (IV), Na2SO3 - a product of trioxosulphate (IV) acid, H2SO3, and sodium hydroxide, NaOH.

In general, salts of weak acids and strong bases undergo hydrolysis to give basic or alkaline solutions.

Hydrolysis of Salts of Strong Acids & Weak Bases

An example of a salt of a strong acid and a weak base is ammonium chloride, NH4Cl, which is a product of aqueous ammonia and hydrochloric acid and ionizes in solution to produce ammonium and chloride ions.

NH4Cl(aq) ----------> NH4+(aq) + Cl-(aq)

Accordingly, there are also hydrogen and hydroxide ions from the partial dissociation of water.

H2O(l) <------------____> H+(aq) + OH–(aq)

The hydroxide ions combine with the ammonium ions to form ammonium hydroxide molecules. Being a weak base, the formation of the molecules is favoured over the formation of dissociated ions.

NH4+(aq) + OH–(aq) <-----_______> NH3.H2O(aq)

Expectedly, this action destabilizes the equilibrium of the system, and causes more water molecules to dissociate to replace the hydroxide ions, which had been removed as aqueous ammonia molecules. As this dissociation occurs, more hydrogen ions are also introduced into the system. This process occurs, until all the hydroxide and ammonium ions are eventually removed from the solution, leaving behind hydrogen and chloride ions. Being products of a strong acid, they remain in solution as ions, and this accounts for the acidic nature of the of the ammonium chloride solution (pH < 7).

HCl(aq) -------------> H+(aq) + Cl-(aq)

Other salts, whose solutions will produce pH < 7, and their constituent acids and bases include:

1. Aluminium chloride, AlCl3 - a product of aluminium hydroxide, Al(OH)3 and hydrochloric acid, HCl.

2. Copper (II) tetraoxosulphate (VI), CuSO4 - a product of copper hydroxide and tetraoxosulphate (VI) acid, H2SO4.

3. Zinc trioxonitrate (V), Zn(NO3)2 - a product of zinc hydroxide, Zn(OH)2 and trioxonitrate (V) acid, HNO3.

Generally, salts of strong acids and weak bases undergo hydrolysis to produce acidic solutions.

Hydrolysis of Salts of Strong Acids & Strong Bases

A typical example of a salt of this category is sodium chloride, NaCl. It ionizes in water to produce sodium and chloride ions.

NaCl(aq) ---------------> Na+(aq) + Cl–(aq)

Ordinarily, one would expect the hydroxide ions from the water molecules to be removed as sodium hydroxide, and the hydrogen ions as hydrochloric acid. However, being a product of a strong acid and a strong base, the four ions remain in solution due to the complete ionization of acid and the base.

HCl(aq) ----------------> H+(aq) + Cl–(aq)

NaOH(aq) ---------------> Na+(aq) + OH–(aq)

Consequently, the ionic equilibrium of water is not disturbed as there is no hydrolysis. Therefore, giving a neutral solution with a pH = 7.Other salts with solutions of neutral pH are:

1. Potassium chloride, KCl - a product of potassium hydroxide, KOH, and hydrochloric acid, HCl.

2. Sodium tetraoxosulphate (VI), Na2SO4 - a product of sodium hydroxide, NaOH, and tetraoxosulphate (VI) acid, H2SO4.

3. Potassium trioxonitrate (V), KNO3 - a product of potassium hydroxide, KOH, and trioxonitrate (V) acid, HNO3.

Generally, salts of strong acids and strong bases do not undergo hydrolysis, and are the only category of salts that exhibit this property.

Note that if any sodium salt exhibits a particular property, the chances that the corresponding potassium salt will behave similarly are very high. E.g, NaCl and KCl, Na2SO4 and K2SO4, Na2CO3 and K2CO3 etc.

Hydrolysis of Weak Acids & Weak Bases

An example of a salt formed from a weak acid and a weak base is ammonium ethanoate, CH3COONH4 - a product of aqueous ammonia, NH3.H2O, and ethanoic acid, CH3COOH. It ionizes in solution to form ammonium and ethanoate ions.

CH3COONH4(aq) ----------> CH3COO–(aq) + NH4+(aq)

Expectedly, the water molecules in the solution undergo partial dissociation to give hydrogen and hydroxide ions.

H2O(l) <------------____> H+(aq) + OH–(aq)

On reaction, the ammonium and hydroxide ions combine to produce aqueous ammonia molecules, while the ethanoate ions combine with the hydrogen ions to form ethanoic acid molecules.

NH4+(aq) + OH–(aq) <----_______> NH3.H2O(aq)

CH3COO–(aq) + H+(aq) <-----_______> CH3COOH(aq)

The formation of these molecules lead to the removal of the hydrogen and hydroxide ions, and creates an imbalance in the ionic equilibrium of water. In response to this, more water molecules undergo dissociation to replace the lost ions. This process continues until all the ammonium and ethanoate ions are converted to undissociated molecules. Although the salt is strongly hydrolysed in solution, the pH of the solution remains almost neutral (if not neutral).

Other examples of salts of weak acids and weak bases, and their constituent acids and bases are:

1. Ammonium trioxocarbonate (IV), (NH4)2CO3 - a product of aqueous ammonia, NH3.H2O, and trioxocarbonate (IV) acid, H2CO3.

2. Magnesium trioxosulphate (IV), MgSO3 - a product of magnesium hydroxide, Mg(OH)2, and trioxosulphate (IV) acid, H2SO3.

3. Calcium tetraoxophosphate (V), Ca3(PO4)2 - a product of calcium hydroxide, Ca(OH)2, and tetraoxophosphate (V) acid, H3PO4.

Do These

Question 1
Classify these salts according to the strength of the acids and bases they are formed from. Briefly discuss whether hydrolysis occurs in each case:

a) Potassium ethanoate
b) Ammonium trioxocarbonate (IV)
c) Potassium chloride
d) Ammonium tetraoxosulphate (VI)

Question 2
Consider the following salts:
KNO3, CH3COONa, NH4Cl
Which one of them would form:
a) an acidic solution
b) a basic solution
c) a neutral solution

Twitter: @gmtacademy
WhatsApp: 07034776117
Facebook: www.facebook.com/greatermindstutors