Lewis theory (1923)

Scientist Gilbert Newton Lewis formulated a more comprehensive theory about acids and bases, which states that:-

Lewis acid: The substance that receives one or more electrons

Lewis base: The substance that gives away one or more electrons

When a positive hydrogen ion (H+) binds with a negative fluorine one (Fl-), Hydrogen ion is regarded as "Lewis acid" because it gives away electrons, while the fluorine ion is regarded as "Lewis base" because it takes electrons.

Acids can be classified according to the degree of their disassociation in water into:-

Strong acids: They are acids which completely ionize in water (all their molecules disassociate in water into ions). Their solutions are high conductors of electricity (strong electrolytes) due to the presence of ions in them

Examples:-

1- Nitric acid (HNO3)

2- Hydrogen iodide acid (HI)

3- Sulphuric acid (H2SO4)

4- Hydrochloric acid (HCl)

5- Hydrobromic acid (HBr)

6- Perchloric acid (HCIO4)

Weak acids: They are acids which partially ionize in water (some of its molecules disassociate into ions). Their solutions are low conductors of electricity (weak electrolytes)

Examples:-

à Acetic acid (vinegar) CH3COOH

Acetic acid ionizes in water into hydronium ion and acetate ion

CH3COOH + H2O à CH3COO− + H3O+

N.B: There's no relation between the strength of an acid and the no. of hydrogen atoms in its molecules.

Acids can be classified according to their sources into:-

Organic acids: They are acids of organic origin (animal-plant) which are extracted form living organisms

Examples:-

1- Formic acid (CH2O2)

2- Acetic acid

3- Lactic acid

4- Citric acid

5- Oxalic acid (H2C2O4)

Mineral acids: They are acids which don't have any organic origin. They are formed from nonmetal elements (chlorine, Nitrogen, Phosphorus, Oxygen…etc)3

Examples:-

1- Hydrochloric acid (HCl)

2- Phosphoric acid (H3PO4)

3- Perchloric acid (HClO4)

4- Carbonic acid (H2CO3)

5- Nitric acid (HNO3)

6- Sulphuric acid (H2SO4)

An acid can be classified according to no. of hydrogen atoms in its molecules (the basicity of acids) into:-

Monobasic acid: It gives away one proton (hydrogen ion) when reacting with one molecule of base

Examples:-

1- Hydrochloric acid (HCl)

2- Nitric acid (HNO3)

Dibasic acid: It may give away two protons (hydrogen ions) when reacting with two molecules of base

Examples:-

1- Sulphuric acid (H2SO4)

2- Carbonic acid (H2CO3)

3- Oxalic acid

Tribasic acid: It may give away three protons (hydrogen ions) when reacting with two molecules of base

Examples:-

1- Phosphoric acid (H3PO4)

2- Citric acid

Bases can be classified according to their degree of disassociation in water into:-

Strong bases: Bases which completely ionize in water and considered as strong electrolytes

Examples:-

1- Potassium hydroxide (KOH)

2- Sodium hydroxide (NaOH)

3- Barium hydroxide Ba(OH)2

Weak bases: They are bases that partially ionize in water and considered as weak electrolytes

Examples:-

1- Ammonium hydroxide (NH4OH)

Bases can be classified according to their molecular structure into:-

Metal oxides:-

1- Iron oxide (FeO)

2- lead oxide (PbO)

3- Calcium oxide (CaO)

4- Magnesium oxide (MgO)

5- Sodium oxide (Na2O)

6- Potassium oxide (K2O)

Metal hydroxides:-

1- Potassium Hydroxide (KOH)

2- Sodium hydroxide (NaOH)

3- Calcium hydroxide Ca(OH)2

4- Magnesium hydroxide Mg(OH)2

5- Barium hydroxide Ba(OH)2

Metal carbonates (or bicarbonates):-

1- Sodium bicarbonate (NaHCO3)

2- Potassium bicarbonate (KHCO3)

3- Sodium carbonate (Na2CO3)

4- Potassium carbonate (K2CO3)

à Bases which dissolve in water producing negative hydroxide ions are called " Alkalis"

à We can say that all alkalis are bases but all bases are not alkalis

There are many ways to determine the acidity or basicity of a solution

They are weak acids or bases whose colours change by the change of the kind of solution

à Acidic indicator is denoted by HIn, while basic indicator is denoted by InOH

In neutral solutions	In bases	In acids	The indicator
Orange	Yellow	Red	Methyl orange
Green	Blue	Yellow	Bromothymol blue
Colourless	Fuchsia	Colourless	Phenolphthalein
Purple	Blue	Red	Litmus paper

Litmus papers Bromothymol blue

Methyl orange

Phenolphthalein

A method for determining the type of solution in numerical values ranging from 0 to 14. Papers and digital devices are used to measure the pH of solutions

N.B: Read chapter (1) to know more about pH measurement

à If the pH of a solution is less than 7, so its acidic

à If the pH of a solution equals 7, its neutral

à If the pH of a solution is greater than 7, so its basic

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