Acids and bases

Acids and bases

The term ‘acid’ is derived from the latin word ‘acidus’ meaning sour. We have already learnt in earlier classes that acid tastes sour, turns the blue litmus to red and reacts with metals such as zinc and produces hydrogen gas. Similarly base tastes bitter and turns the red litmus to blue.

These classical concepts are not adequate to explain the complete behaviour of acids and bases. So, the scientists developed the acid – base concept based on their behaviour.

Let us, learn the concept developed by scientists Arrhenius, Bronsted and Lowry and Lewis to describe the properties of acids and bases.

Arrhenius Concept

One of the earliest theories about acids and bases was proposed by swedish chemist Svante Arrhenius. According to him, an acid is a substance that dissociates to give hydrogen ions in water. For example, HCl, H₂SO₄ etc., are acids. Their dissociation in aqueous solution is expressed as

The H⁺ ion in aqueous solution is highly hydrated and usually represented as H₃O⁺ , the simplest hydrate of proton [H(H₂O) ]⁺ . We use both H⁺and H₃O⁺ to mean the same.

Similarly a base is a substance that dissociates to give hydroxyl ions in water. For example, substances like NaOH, Ca(OH)₂ etc., are bases.

Limitations of Arrhenius concept

i. Arrhenius theory does not explain the behaviour of acids and bases in non aqueous solvents such as acetone, Tetrahydrofuran etc...

ii. This theory does not account for the basicity of the substances like ammonia (NH₃ ) which do not possess hydroxyl group.

Evaluate yourself – 1

Classify the following as acid (or) base using Arrhenius concept i)HNO₃ ii) Ba(OH)₂ iii) H₃ PO₄ iv) CH₃COOH

Answer:

acid : (i) HNO₃ iii) H₃PO₃ iv) CH₃COOH

base : ii) Ba (OH)₂

Lowry – Bronsted Theory (Proton Theory)

In 1923, Lowry and Bronsted suggested a more general definition of acids and bases. According to their concept, an acid is defined as a substance that has a tendency to donate a proton to another substance and base is a substance that has a tendency to accept a proton from other substance. In other words, an acid is a proton donor and a base is a proton acceptor.

When hydrogen chloride is dissolved in water, it donates a proton to the later. Thus, HCl behaves as an acid and H₂O is base. The proton transfer from the acid to base can be represented as

HCl +H₂O ↔ H₃O⁺ + Cl^-

When ammonia is dissolved in water, it accepts a proton from water. In this case, ammonia ( NH₃ ) acts as a base and H₂O is acid. The reaction is represented as

H₂O + NH₃ ↔ NH₄⁺ + OH^-

Let us consider the reverse reaction in the following equilibrium

HCl + H₂O ↔ H₃O⁺ + Cl^-

Proton donar (acid) + Proton acceptor (Base) ↔ Proton donar (acid) + Proton acceptor (Base)

H₃O⁺ donates a proton to Cl^- to form HCl i.e., the products also behave as acid and base.

In general, Lowry – Bronsted (acid – base) reaction is represented as

Acid₁ + Base₂ ↔ Acid₂ +Base₁

The species that remains after the donation of a proton is a base (Base₁ ) and is called the conjugate base of the Bronsted acid ( Acid ₁ ) . In other words, chemical species that differ only by a proton are called conjugate acid – base pairs.

HCl and Cl^- , H₂O and H₃O⁺ are two conjugate acid – base pairs. i.e., Cl^- is the conjugate base of the acid HCl . (or) HCl is conjugate acid of Cl- . Similarly H₃O⁺ is the conjugate acid of H₂O .

Limitations of Lowry – Bronsted theory

i. Substances like BF₃ , AlCl₃ etc., that do not donate protons are known to behave as acids.

Evaluate yourself – 2

Write a balanced equation for the dissociation of the following in water and identify the conjugate acid –base pairs. i) NH₄⁺ ii) H₂SO₄ iii) CH₃COOH.

Answer:

Lewis concept

In 1923, Gilbert . N. Lewis proposed a more generalised concept of acids and bases. He considered the electron pair to define a species as an acid (or) a base. According to him, an acid is a species that accepts an electron pair while base is a species that donates an electron pair. We call such species as Lewis acids and bases.

A Lewis acid is a positive ion (or) an electron deficient molecule and a Lewis base is a anion (or) neutral molecule with at least one lone pair of electrons.

Les us consider the reaction between Boron tri fluoride and ammonia

Here, boron has a vacant 2p orbital to accept the lone pair of electrons donated by ammonia to form a new coordinate covalent bond. We have already learnt that in coordination compounds, the Ligands act as a Lewis base and the central metal atom or ion that accepts a pair of electrons from the ligand behaves as a Lewis acid.

Lewis acids

• Electron deficient molecules such as BF₃ ,AlCl₃ ,BeF₂ etc...

• All metal ions. Examples: Fe²⁺ ,Fe³⁺ ,Cr³⁺ ,Cu²⁺ etc...

• Molecules that contain a polar double bond Examples : SO₂ ,CO₂ ,SO₃ etc...

• Molecules in which the central atom can expand its octet due to the availability of empty d – orbitals. Example: SiF₄ ,SF₄ ,FeCl₃ etc..

• Carbonium ion (CH₃ )₃ C⁺

Lewis bases

• Molecules with one (or) more lone pairs of electrons. NH₃ ,H₂O,R-O-H,R-O-R, R - NH₂

• All anions. F^- ,Cl^- ,CN^- ,SCN^- ,SO²₄^- etc...

• Molecules that contain carbon – carbon multiple bond. Examples: CH₂ =CH ₂ ,CH ≡ CH etc...

• All metal oxides. CaO,MgO,Na₂O etc...

• Carbanion. CH₃^−

Example

Identify the Lewis acid and the Lewis base in the following reactions.

Cr³⁺ + 6 H₂ O → [Cr(H₂O)₆ ]³⁺

In the hydration of ion, each of six water molecules donates a pair of electron to Cr³⁺ to from the hydrated cation, hexaaquachromium (III) ion, thus, the Lewis acid is Cr³⁺ and the Lewis base H₂O .

Evaluate yourself – 3

Identify the Lewis acid and the Lewis base in the following reactions.

i. CaO+CO₂ → CaCO₃

Answer:

i) CaO - Lewis base ; CO₂ –Lewisacid

AlCl₃ - Lewis acid

Evaluate yourself – 4

H₃BO₃ accepts hydroxide ion from water as shown below

H₃BO₃ (aq) + H₂O(l) ↔ B(OH)₄^- +H⁺

Predict the nature of H₃ BO₃ using Lewis concept

Answer: