PHYSICAL CHEMISTRY 1.2 – ACIDS, BASES AND SALTS
Arrhenius concept of acids and bases.
What is an acid? (according to Arrhenius concept of acids and bases)
Ø Arrhenius considered that an acid is a substance which when dissolved in water dissociates to produce H+ ions as the only positively charged ions, i.e.
e.g.
Ø He considered a base to be a substance which produces hydroxyl ions when dissolved in water as the only negatively charged ions, i.e.
e.g.
The neutralization of acid with a base yields a salt and water.
e.g.
+ 
+ 
+ 
Weakness of Arrhenius equation
- i) This concept is limited to water. It refers to H+ and OH– ions derived from water. A true general concept of acid and base should be appropriate to other solvents like liquid NH3 and alcohols.
- ii) The concept does not provide room for acids and bases which do not contain H+ ions and OH– ions.
Bronsted–Lowry concept of acids and bases
- Bronsted and Lowry proposed a theory of acids and bases applicable to all solvents.
- They proposed that an acid is any substance that can donate a proton to any other substance.
e.g.
- A base is a substance that can accept a proton from any other substance.
e.g.
Monoprotic and polyprotic acids and bases
- They are called monoprotic acids: acids which donate only one proton. E.g. HNO3, HCl.
- Diprotic acids can donate two protons. E.g. H2SO4.
- Polyprotic acids can donate more than one proton. E.g. H2SO4, H3PO3, H2C2O4.
- Polyprotic bases can accept more than one proton. E.g.
- Monoprotic bases can accept only one proton. E.g.
Note: HCl and Cl– are acid-base conjugate pairs. Another example is HNO3 and NO3–.
Amphoteric (amphiprotic) acids and bases
These behave as Bronsted–Lowry acids or bases.
Conjugate acid-base pair
For every acid, there is a corresponding (conjugate) base to accept a proton.
e.g.
Acid base (Proton donor) (Proton acceptor)
HA and A– are conjugate pairs, i.e. HA is a conjugate acid of A– and A– is a conjugate base of HA.
® In a solution, there must be a base to accept a proton.
e.g.
Acid Base Acid Base
Note: From Bronsted–Lowry concept of acids and bases, the stronger the acid, the weaker its conjugate base, and the stronger the base, the weaker its conjugate acid.
- CH3COOH is a weak acid, but its conjugate base i.e. CH3COO– is a strong base.
- H2O is a weak base, but H3O+ is a strong acid.
Advantage of Bronsted–Lowry concept over Arrhenius
- It can apply to any solvent, not necessarily water. Here the definition of bases is much wider.
Weakness:
Since the concept is based on proton transfer, it does not consider other compounds which do not contain hydrogen, e.g. AlCl3, BF3, SO3.
In contrast to Arrhenius theory, acids and bases are no longer related to salts (by neutralization).
Question 1:
- Define
- Conjugate acid-base pair.
- Conjugate base.
- For the following pairs, write down the equation to show the conjugate acid-base pair.
/ - /
/ 
/ 
Question 2:
- Write the formula and give the name of the conjugate base for each of the following acids.
- Write the name and formula of the conjugate acids for each of the following bases.
- NH3
- Br–
- HS–
Question 3:
In each of the following acid-base reactions, identify the acid and the base on the left and their conjugate partners on the right.
ANSWERS:
HCN, CN– is a conjugate pair and NH3, NH4+ is a conjugate pair.
- HSO4–, SO42- is a conjugate pair.
Acid Base Base Acid
[Al(H2O)3]3+, [Al(H2O)5OH]2+ is a conjugate pair.
OH– and H2O is a conjugate pair.
LEWIS CONCEPT OF ACIDS AND BASES
- Lewis proposed an even broader concept of acids and bases focusing on electron transfer rather than proton transfer.
- According to Lewis, an acid is a substance that can accept a pair of electrons. Therefore, an acid is an electron pair acceptor.
- A base is a substance that can donate a pair of electrons, i.e., a base is an electron pair donor.
NOTE: Bronsted–Lowry acids e.g. HCl, H2SO4, HNO3 are not Lewis acids. Thus, an acid-base reaction can occur when a base provides a pair of electrons to share with an acid resulting in a coordinate compound or complex.
Therefore, ammonia chloride ions (AlCl3) are Lewis bases, while H+, BF3 are Lewis acids.
NOTE: A Bronsted–Lowry base (like NH3) reacts by donating an electron pair to a proton. Therefore, Bronsted–Lowry bases are also Lewis bases.
Reason: This is because upon donating a pair of electrons, it would have accepted a proton. Therefore, Bronsted–Lowry bases are also Lewis bases.
i. F– fluoride ion, SO42- sulphate ion, NH4+ ammonium ion, HBr hydrogen bromide, H2S hydrogen sulphide.
IONIC EQUILIBRIUM OF ACIDS AND BASES
Most acids and bases are weak, i.e., they do not ionize fully when dissolved in water. Thus, apart from water equilibrium, they also establish equilibrium.
e.g.
Ammonia, which is a typical weak base, ionizes as follows:
But the ionization of weak acids/bases generally occurs to a greater extent than that of water.
STRENGTH OF WEAK ACIDS AND BASES
The position of equilibrium of a reaction between the acid and water varies from one weak acid to another. The further to the left it lies, the weaker the acid is.
The equilibrium constant is written as:
But H2O is constant at constant temperature.
Putting the constant on the same side:
Where Ka = dissociation/ionization constant of an acid.
Similarly, for weak bases, the position of equilibrium varies from base to base. The further to the left it lies, the weaker the base is.
Where Kb = dissociation/ionization constant of a base.
The Ka and Kb values are used to determine the strength of acids and bases, i.e., Ka and Kb values are quite small for very weak acids/bases reflecting very little ionization of these acids/bases in solution.
Example:
The Kb value for C6H5NH2 is 4.17 × 10-10, NH3 is 1.78 × 10-5. Indicate which base is stronger than the other.
NH3 is stronger than C6H5NH2.
The strength of weak acids and bases can also be determined from its degree of dissociation (Ostwald’s dilution law).
Since Ka and Kb values are inconvenient to handle, usually pKa and pKb are used.
For example:
The lower the value, the stronger the acid/base respectively and vice versa.
THE RELATIONSHIP BETWEEN pKa AND pKb FOR A CONJUGATE ACID–BASE PAIR
Consider the equilibrium:
The product of Ka and Kb gives:
Example:
Formic acid (HCOOH) has a Ka of 1.78 × 10-14 moles. Calculate the [H3O+] and the pH of 0.1M solution of HCOOH.
Since Ka value is small, the expression:
NOTE: The approximation is done when [HA]₀ is greater than 100 Ka.
But if initial concentration [HA]₀ is less than 100 Ka, then the exact expression formed must be solved.
Calculate [H3O+] and pH in which has Ka value of moles/dm3.
IONIC PRODUCT OF WATER AND PH
- Water auto-ionizes, i.e., transfers a proton from one water molecule to another producing H3O+ and OH– ions.
- Base Acid
But the concentration of H2O is much larger than the two ions and is constant at constant temperature. Since the concentration of H3O+ is constant, it is made part of the constant.
Since the concentrations are equal, this implies that pure water is neutral.
Scale
- Is a scale which shows degree of acidity or alkalinity of a solution.
= 7
i.e., pH of 7 is neutral point.
NOTE:
In acidic solution, the concentration of H3O+ is greater than [OH–].
i.e.,
In basic solution, the concentration of OH– is greater than [H3O+].
STRONG ACIDS AND BASES
When an acid is added to water as an aqueous solution of HCl, in addition to self-ionization of water, the acid also ionizes.
Due to common ion effect of hydroxonium ion as HCl is fully ionized, it suppresses the ionization of water; hence [H3O+] and [OH–] ion from water will be less than usual.
It is generally acceptable to consider the ionization of HCl to be the sole source of hydroxonium ions. This is also applicable in strong bases for OH– ions.
Example 1:
If 0.001M of NaOH is added to enough amount of 1L of water, what is the concentration of OH– and H3O+ ions?
Solution:
BUFFER SOLUTIONS
A buffer solution is a solution which maintains its pH when a small amount of an acid or alkali is added to it.
Or
It is one that resists a change in pH when a small amount of acid or alkali is added to the solution.
A buffer solution usually consists of a weak acid and one of its salts or a weak base and one of its salts.
Types of buffer solutions
- Acidic buffer solution
This is the buffer solution which keeps the pH below 7.
They are formed by mixing a weak acid and its salt (of a strong base).
e.g.
How does the buffer system work?
Consider:
Since the salt is strong, it dissociates completely into ions, increasing the concentration of acetate ions, shifting the equilibrium to the left hand side, suppressing the dissociation of acetic acid due to the common ion effect. Hence [CH3COOH] is equal to the salt concentration and due to the common ion effect, [CH3COO–] becomes equal to the initial concentration of the acid.
Therefore, the solution will contain these important species:
- A lot of unionized acid
- A lot of acetate ions from salt
- Enough to make the solution acidic
When little H+ ions are added, the following reaction occurs:
® The acetate ions concentration from the salt are large enough to consume the added hydrogen ions; therefore, there will be no accumulation of H+ in the solution.
® If OH– ions are added, the following reaction occurs:
This decreases [H+] in the solution, shifting the equilibrium to the right hand side to replace H+ used to neutralize OH– added. Therefore, no accumulation of OH– in the solution.
NOTE: Addition of H+ to acidic buffer increases the acid concentration but decreases the salt concentration by the same amount of H+ added.
Addition of OH– to acidic buffer decreases the acid concentration but increases the salt concentration by the same amount of OH– added.
SALT HYDROLYSIS
A salt is a compound which contains metallic or radical or positive radical rather than hydrogen (H+) and acidic or negative or anion radical rather than hydroxyl ion (OH–).
CLASSIFICATION OF SALTS
Salts are categorized into four major classes:
- Normal salt (strong salts)
- Salts with strong cation and weak anion
- Salts with weak cation and strong anion
- Salts with weak cation and weak anion
i) STRONG SALTS
These are salts with strong cation and strong anions.
ii) SALTS WITH STRONG CATION AND WEAK ANIONS
These are also known as basic salts, mostly organic salts or salts of carboxylic acid.
iii) SALTS WITH WEAK CATION AND STRONG ANION
These salts are termed acidic salts.
iv) SALT WITH WEAK CATION AND WEAK ANION
This salt undergoes both cationic and anionic salt hydrolysis because both weak ions will react with water to give acid and base.
HYDROLYSIS OF CLASSES OF SALTS
1. Hydrolysis of normal salts
The salts having strong cation and strong anion do not undergo salt hydrolysis process, rather they ionize in solution to give free ions.
2. Hydrolysis of basic salts
The salt having strong cation and weak anion undergoes the type of hydrolysis termed as anionic salt hydrolysis.
Definition:
Anionic salt hydrolysis is the reaction between water and salt with strong cation and weak anion whereby the weak anion reacts with water to give acid and base.
Example:
Initially the salt will ionize:
Anion will react with H2O:
iii) Hydrolysis constant
Hydrolysis constant is the ratio of the product molar concentration to the reactant concentration raised to their powers which is equal to the balancing number in the hydrolysis equation.
Normally it is denoted by Kh.
Hydrolysis constant for anionic salts hydrolysis
From hydrolysis equation Kh can be obtained.
HYDROLYSIS CONSTANT (Kh)
Definition:
Kh is the ratio of product of molar concentration of the products to that concentration reactant raised to their powers which is equal to the balancing number in a hydrolysis equation.
During hydrolysis the weak acid formed and water molecules will also dissociate.
The weak acid dissociation:
where
- ka = Dissociation constant of acid (CH3COOH)
- kw = Dissociation constant of water
- kh = Hydrolysis constant
Hydrolysis constant for cationic salt hydrolysis
Consider the hydrolysis of NH4Br:
During hydrolysis the weak base is formed and water molecules will dissociate.
Weak base dissociation:
where
- kb = Dissociation constant of base
Example
With examples in each briefly write short note on the following:
- Cationic salt hydrolysis
- Anionic salt hydrolysis
Solution:
Cationic salt hydrolysis is a reaction between water and salt with weak cation and strong anion whereby the weak cation reacts with water to produce acid and base.
Anionic salt hydrolysis is a reaction between water and salt with strong cation and weak anion in which the weak anion reacts with water to produce acid and base.
1. With an example of salt and type of hydrolysis derive the relationship between ka, kw and kh.
Solution:
Consider the hydrolysis of Sodium Oxalate.
During hydrolysis acid formed together with water molecules ionize in solution.
Acid ionization:
Example
During Hydrolysis the base and water molecule also ionize.
Base ionization:
Water ionization:
NOTE:
pH = -LOG[H+]
This is the equation for anionic salt hydrolysis.
b) FOR CATIONIC HYDROLYSIS
This is for the salt undergoing cationic salt hydrolysis.
OR
Example
1. Briefly differentiate cationic salt hydrolysis and anionic salt hydrolysis.
Solution:
Cationic salt hydrolysis is a reaction between water and salt with weak cation and strong anion whereby the weak cation reacts with water to produce acid and base, while anionic salt hydrolysis is a reaction between water and salt with weak anion and strong cation in which the weak anion reacts with water to produce acid and base.
1. With an example of salt and type of hydrolysis derive the relationship between ka, kw and kh.
Solution:
Consider the hydrolysis of Sodium Oxalate.
During hydrolysis acid formed together with water molecules ionize in solution.
Acid ionization:
Example
During Hydrolysis the base and water molecule also ionize.
Base ionization:
Water ionization:
NOTE:
pH = -LOG[H+]
This is the equation for anionic salt hydrolysis.
b) FOR CATIONIC HYDROLYSIS
This is for the salt undergoing cationic salt hydrolysis.
OR
Example
1. Briefly differentiate cationic salt hydrolysis and anionic salt hydrolysis.
Solution:
Cationic salt hydrolysis is a reaction between water and salt with weak cation and strong anion whereby the weak cation reacts with water to produce acid and base, while anionic salt hydrolysis is a reaction between water and salt with weak anion and strong cation in which the weak anion reacts with water to produce acid and base.
1. With an example of salt and type of hydrolysis derive the relationship between ka, kw and kh.
Solution:
Consider the hydrolysis of Sodium Oxalate.
During hydrolysis acid formed together with water molecules ionize in solution.
Acid ionization:
Example
During Hydrolysis the base and water molecule also ionize.
Base ionization:
Water ionization:
NOTE:
pH = -LOG[H+]
This is the equation for anionic salt hydrolysis.
b) FOR CATIONIC HYDROLYSIS
This is for the salt undergoing cationic salt hydrolysis.
OR
Example
1. Briefly differentiate cationic salt hydrolysis and anionic salt hydrolysis.
Solution:
Cationic salt hydrolysis is a reaction between water and salt with weak cation and strong anion whereby the weak cation reacts with water to produce acid and base, while anionic salt hydrolysis is a reaction between water and salt with weak anion and strong cation in which the weak anion reacts with water to produce acid and base.
1. With an example of salt and type of hydrolysis derive the relationship between ka, kw and kh.
Solution:
Consider the hydrolysis of Sodium Oxalate.
During hydrolysis acid formed together with water molecules ionize in solution.
Acid ionization:
Example
During Hydrolysis the base and water molecule also ionize.
Base ionization:
Water ionization:
NOTE:
pH = -LOG[H+]
This is the equation for anionic salt hydrolysis.
b) FOR CATIONIC HYDROLYSIS
This is for the salt undergoing cationic salt hydrolysis.
OR
Example
1. Briefly differentiate cationic salt hydrolysis and anionic salt hydrolysis.
Solution:
Cationic salt hydrolysis is a reaction between water and salt with weak cation and strong anion whereby the weak cation reacts with water to produce acid and base, while anionic salt hydrolysis is a reaction between water and salt with weak anion and strong cation in which the weak anion reacts with water to produce acid and base.
1. With an example of salt and type of hydrolysis derive the relationship between ka, kw and kh.
Solution:
Consider the hydrolysis of Sodium Oxalate.
During hydrolysis acid formed together with water molecules ionize in solution.
Acid ionization:
Example
During Hydrolysis the base and water molecule also ionize.
Base ionization:
Water ionization:
NOTE:
pH = -LOG[H+]
This is the equation for anionic salt hydrolysis.
b) FOR CATIONIC HYDROLYSIS
This is for the salt undergoing cationic salt hydrolysis.
OR
Example
1. Briefly differentiate cationic salt hydrolysis and anionic salt hydrolysis.
Solution:
Cationic salt hydrolysis is a reaction between water and salt with weak cation and strong anion whereby the weak cation reacts with water to produce acid and base, while anionic salt hydrolysis is a reaction between water and salt with weak anion and strong cation in which the weak anion reacts with water to produce acid and base.
1. With an example of salt and type of hydrolysis derive the relationship between ka, kw and kh.
Solution:
Consider the hydrolysis of Sodium Oxalate.
During hydrolysis acid formed together with water molecules ionize in solution.
Acid ionization:
Example
During Hydrolysis the base and water molecule also ionize.
Base ionization:
Water ionization:
NOTE:
pH = -LOG[H+]
This is the equation for anionic salt hydrolysis.
b) FOR CATIONIC HYDROLYSIS
This is for the salt undergoing cationic salt hydrolysis.
OR
Example
1. Briefly differentiate cationic salt hydrolysis and anionic salt hydrolysis.
Solution:
Cationic salt hydrolysis is a reaction between water and salt with weak cation and strong anion whereby the weak cation reacts with water to produce acid and base, while anionic salt hydrolysis is a reaction between water and salt with weak anion and strong cation in which the weak anion reacts with water to produce acid and base.
1. With an example of salt and type of hydrolysis derive the relationship between ka, kw and kh.
Solution:
Consider the hydrolysis of Sodium Oxalate.
During hydrolysis acid formed together with water molecules ionize in solution.
Acid ionization:
Example
During Hydrolysis the base and water molecule also ionize.
Base ionization:
Water ionization:
NOTE:
pH = -LOG[H+]
This is the equation for anionic salt hydrolysis.
b) FOR CATIONIC HYDROLYSIS
This is for the salt undergoing cationic salt hydrolysis.
OR
Example
1. Briefly differentiate cationic salt hydrolysis and anionic salt hydrolysis.
Solution:
Cationic salt hydrolysis is a reaction between water and salt with weak cation and strong anion whereby the weak cation reacts with water to produce acid and base, while anionic salt hydrolysis is a reaction between water and salt with weak anion and strong cation in which the weak anion reacts with water to produce acid and base.
1. With an example of salt and type of hydrolysis derive the relationship between ka, kw and kh.
Solution:
Consider the hydrolysis of Sodium Oxalate.
During hydrolysis acid formed together with water molecules ionize in solution.
Acid ionization:
Example
During Hydrolysis the base and water molecule also ionize.
Base ionization:
Water ionization:
NOTE:
pH = -LOG[H+]
This is the equation for anionic salt hydrolysis.
b) FOR CATIONIC HYDROLYSIS
This is for the salt undergoing cationic salt hydrolysis.
OR
Example
1. Briefly differentiate cationic salt hydrolysis and anionic salt hydrolysis.
Solution:
Cationic salt hydrolysis is a reaction between water and salt with weak cation and strong anion whereby the weak cation reacts with water to produce acid and base, while anionic salt hydrolysis is a reaction between water and salt with weak anion and strong cation in which the weak anion reacts with water to produce acid and base.
1. With an example of salt and type of hydrolysis derive the relationship between ka, kw and kh.
Solution:
Consider the hydrolysis of Sodium Oxalate.
During hydrolysis acid formed together with water molecules ionize in solution.
Acid ionization:
Example
During Hydrolysis the base and water molecule also ionize.
Base ionization:
Water ionization:
NOTE:
pH = -LOG[H+]
This is the equation for anionic salt hydrolysis.
b) FOR CATIONIC HYDROLYSIS
This is for the salt undergoing cationic salt hydrolysis.
OR
Example
1. Briefly differentiate cationic salt hydrolysis and anionic salt hydrolysis.
Solution:
Cationic salt hydrolysis is a reaction between water and salt with weak cation and strong anion whereby the weak cation reacts with water to produce acid and base, while anionic salt hydrolysis is a reaction between water and salt with weak anion and strong cation in which the weak anion reacts with water to produce acid and base.
1. With an example of salt and type of hydrolysis derive the relationship between ka, kw and kh.
Solution:
Consider the hydrolysis of Sodium Oxalate.
During hydrolysis acid formed together with water molecules ionize in solution.
Acid ionization:
Example
During Hydrolysis the base and water molecule also ionize.
Base ionization:
Water ionization:
NOTE:
pH = -LOG[H+]
This is the equation for anionic salt hydrolysis.
b) FOR CATIONIC HYDROLYSIS
This is for the salt undergoing cationic salt hydrolysis.
OR
Example
1. Briefly differentiate cationic salt hydrolysis and anionic salt hydrolysis.
Solution:
Cationic salt hydrolysis is a reaction between water and salt with weak cation and strong anion whereby the weak cation reacts with water to produce acid and base, while anionic salt hydrolysis is a reaction between water and salt with weak anion and strong cation in which the weak anion reacts with water to produce acid and base.
1. With an example of salt and type of hydrolysis derive the relationship between ka, kw and kh.
Solution:
Consider the hydrolysis of Sodium Oxalate.
During hydrolysis acid formed together with water molecules ionize in solution.
Acid ionization:
Example
During Hydrolysis the base and water molecule also ionize.
Base ionization:
Water ionization:
NOTE:
pH = -LOG[H+]
This is the equation for anionic salt hydrolysis.
b) FOR CATIONIC HYDROLYSIS
This is for the salt undergoing cationic salt hydrolysis.
OR
Example
1. Briefly differentiate cationic salt hydrolysis and anionic salt hydrolysis.
Solution:
Cationic salt hydrolysis is a reaction between water and salt with weak cation and strong anion whereby the weak cation reacts with water to produce acid and base, while anionic salt hydrolysis is a reaction between water and salt with weak anion and strong cation in which the weak anion reacts with water to produce acid and base.
1. With an example of salt and type of hydrolysis derive the relationship between ka, kw and kh.
Solution:
Consider the hydrolysis of Sodium Oxalate.
During hydrolysis acid formed together with water molecules ionize in solution.
Acid ionization:
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