Acid and Base Definitions

Acid and Base are two important substances in Chemistry. Commonly, acid is classified as sour substances and base is bitter. Three scientists have defined acid and base and which improved the knowledge in Chemistry

• Arrhenius Definition 

Acid: Anything produces Hydrogen Ion (H+) and dissolve in water 

Base: Anything produces Hydroxide Ion (OH-) and dissolve in water

Note: Dissociation is different from Solubility, for instance, Calcium Hydroxide is strong base, dissociate completely but slightly soluble. 

• Brønsted–Lowry Definition 

Acid: Anything donates proton H+ to others

Base: Anything accepts proton H+ from others 

Acid and Base strength

Stronger Brønsted acid. Weaker conjugate base

Weaker Brønsted acid. Stronger conjugate base

For equilibrium reaction in weak acid and base 

All acid base reaction favor weak acid and weak base which means the reaction will shift toward the side of weaker acid or base. 

Important Terminology

Amphoterism: species of molecule or ion that can act as acid and base

Amphoprotic molecule: is the molecule that can donate and accept proton

Periodic Trends 

The strength of binary acid (H-X) 

The weaker the bond is, the stronger the acid is

The more electronegativity X is, the more polar the bond and the stronger the acid  

The more polar the H-X bond is, the stronger acid is.

The larger the atom is, the stronger the acid is. 

Figure 1a. Binary Acid Trends

The strength of Oxoacid (H-O-Y)

The greater the electronegativity of Y is, the stronger the acid is. 

(The more Oxygen in Y is, the more electronegavity is) 

Figure 1b. Oxoacid Trends

• Lewis Definition

Acid: Any ion or molecule that can accept a pair of electrons in order to form a covalent bond.

Base: Any ion or molecule that can donate a pair of electrons in order to form a covelent bond.

MNEMONIC DEVICE: 

Arrhenius vs. Ions, 

Brønsted vs.Protons

Lewis vs. Electron pairs (Shape)  

Source:

Jespersen, Neil D. Chemistry The Molecular Nature of Matter. John Wiley & Sons. Inc. 2011. 

        (740- 766). Print