Acid Base Theory - Pharmaceutical Inorganic Chemistry B. Pharma 1st Semester

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Acid Base Theory

Learning Objectives

At the end of this lecture, the student will be able to:

Classify:

• Acids

• Bases and

• Salts based on various theories

History of Acids and Bases

In the early days of chemistry chemists were organizing
physical and chemical properties of substances. They discovered that many substances could be placed in two different property categories:

Substance A	Substance B
1. Sour taste	1. Bitter taste
2. Reacts with carbonates to make CO2	2. Reacts with fats to make soaps
3. Reacts with metals to produce H2	3. Do not react with metals
4. Turns blue litmus pink	4. Turns red litmus blue
5. Reacts with B substances to make salt and water	5. Reacts with A substances to make salt and water
6. pH < 7	6. pH >7

Arrhenius was the first person to suggest a reason why substances are in A or B due to their ionization in water

Arrhenius Theory

The Swedish chemist Svante Arrhenius proposed the first
definition of acids and bases (1887)

According to the Arrhenius model:

“Acids are substances that dissociate in water to produce H⁺ ions and bases are substances that dissociate in water to produce OH^– ions”

NaOH (aq) > Na⁺ (aq) + OH^–(aq) Base

HCl (aq) > H⁺ (aq) + Cl^–(aq) Acid

Arrhenius theory: Neutralization reactions

• Arrhenius acids and bases react with each other to form
water and aqueous salts in neutralization reactions

H⁺ (aq) + A^–(aq) + M⁺ (aq) + OH^–(aq) > H₂O (l) + M⁺(aq) + A^–(aq)

• The net ionic equation is

H⁺ (aq) + OH^–(aq) > H₂O (l)

Classification of acid and base based on an Arrhenius concept:

ACID	BASE
Strong acid	Strong base
weak acid	Weak base
Mono basic acid	Mono acidic base
Dibasic acid	Di acidic base
Tribasic acid	Tribasic base

Limitations of Arrhenius concept

• The definitions are only in terms of aqueous solution and
not in terms of substance

• The theory is not able to explain acidic and basic properties of substances in non-aqueous solvents

Example: Ammonium nitrate in liquid ammonia acts as an acid though it does not give H⁺ ions

• The basic nature of substances like ammonia or sodium
carbonate which do not contain OH^– ions was not explained by this concept

• The acidic nature of carbon di oxide, sulphur di oxide
which do not contain H⁺ ions was not explained by this concept

• The neutralisation of acid and base in absence of solvent
could not be explained

Bronsted Lowry Theory (1923)

Johannes Bronsted and Thomas Lowry revised Arrhenius’s
acid-base theory to include this behavior. They defined acids and bases as follows:

“An acid is a hydrogen containing species that donates a
proton. A base is any substance that accepts a proton”

HCl (aq) + H₂O (l) > Cl^–(aq) + H₃O⁺(aq)

In the above example what is the Brønsted acid? What is the
Brønsted base?

In reality, the reaction of HCl with H₂O is an equilibrium and occurs in both directions, although in this case the equilibrium lies far to the right.

HCl (aq) + H2O (l) > Cl^–( aq) + H₃O⁺(aq)

For the reverse reaction Cl^– behaves as a Bronsted base and H₃O⁺behaves as a Bronsted acid.

The Cl- is called the conjugate base of HCl. Bronsted acids and bases always exist as conjugate acid-base pairs.

Bronsted-Lowry Theory of Acids & Bases Conjugate Acid-Base Pairs

General Equation

Bronsted-Lowry Theory of Acids & Bases: Example

Notice that water is both an acid & a base = amphoteric

Bronsted-Lowry Theory of Acids & Bases Proton transfer reactions

• Pairs of compounds are related to each other through
Bronsted-Lowry acid-base reactions. These are conjugate acid-base pairs.

• Generally, an acid HA has a conjugate base A^–(a proton hastransferred away from the acid). Conversely, a base B has a conjugate acid BH⁺ (a proton has transferred toward the base).

Classification:

Bronsted acid:

Mono protic acid: Capable of donating one proton only

Example: HF, CH3COOH

Poly protic acid: Capable of donating more than one proton

Example: H2S, H2O

Bronsted Base:

Mono protic base: Which can accept one proton

Example: water

Poly protic base: Which can accept two or more proton

Example: Sulphate ion, Phosphate ion

Lewis acids and bases

Gilbert Newton Lewis (1875-1946) influential American
chemist. His theories include the Lewis dot structure taught in Chem120 and covalent bond theories.

Lewis acids are electrophils: H+, Na+, BF3,

Lewis bases are nucleophils: NH3, H2O, PH3

Acid base reactions:

BF3 + :NH3 > F3B:NH3