Acids and bases

We all know them: acids

Acids are very familiar substances. They are characterized by having hydrogen atoms bonded to very electronegative atoms such as oxygen or chlorine.

CH₃-COOH	(HOOC-CH₂)2C(OH)COOH	HCl	CH₃CH(OH)COOH

Acetic acid is in vinegar	Citric acid is in lemon or orange juices	Hydrochloric acid is in the gastric juice	Lactic acid is in milk

They have a characteristic flavor. Some are too strong and dangerous to try. Hydrogen atoms bonded to electronegative atoms break away easily. That is the chemical characteristic that makes an acid so.

Forearm with a simulated chemical burn — Simulated acid burn

A bond between two atoms of a molecule can be broken in two ways:

Homolytic cleavage

The two electrons of the bond are divided between the two initially bonded atoms. This cleavage is generally very difficult.

Heterolytic cleavage

The two electrons in the bond stay on the side of the more electronegative atom of the two that formed the bond. This is the most common way of bond cleavage.

Animation of the homolytic cleavage of a C–H bond of methane: each atom keeps one electron

When hydrogen is bonded to an atom of moderate electronegativity, the cleavage is homolytic (usually very difficult).

Animation of the heterolytic cleavage of an O–H bond of water: the oxygen keeps both electrons and a proton is released

When hydrogen is bonded to an atom of high electronegativity, the cleavage is heterolytic (usually much easier).

A bond that is broken very easily in a heterolytic way is the one that unites the electronegative atoms of a molecule with hydrogen. The electronegative atom keeps the electrons (heterolytic cleavage) and a "proton" is released, that is, a hydrogen atom "naked" of electrons. Molecules that lose "protons" easily belong to substances called acids.

Space-filling model of the sulfuric acid molecule — Model of **sulfuric acid** (H₂SO₄) losing a proton

Sulfuric acid loses a proton: the oxygen keeps both electrons of the O–H bond and becomes negatively charged

Sulfuric acid (H₂SO₄) is a very strong acid that loses a "proton" easily. When the molecule "keeps" the two electrons from the bond, it acquires a negative charge that offsets the positive charge of the "proton."

We all know them: bases

Bases are also very familiar substances. They are characterized by having very electronegative atoms such as oxygen and nitrogen that can accept protons, even if they are already bonded to some.

NaOH	NH3	NaCO₃H	CaCO₃

Sodium hydroxide or "soda" is used to unclog pipes	Ammonia is used as a cleaning product	Sodium bicarbonate is a stomach antacid	Calcium carbonate is the main constituent of marble

A bond that forms very easily is the one established between a "proton" and an atom, of a molecule, with unshared pairs of electrons. Molecules that accept "protons" easily belong to substances called bases.

It is logical to think that acids and bases react easily with each other, mutually transferring a "proton."

Acids react with bases by transferring a proton. The product of the reaction is a salt, formed by an anion (negatively charged molecule) and a cation (positively charged molecule).

Reaction of acetic acid with ammonia: the acid transfers a proton to the base, forming the salt ammonium acetate

The acid loses a proton and the acid molecule (originally neutral) becomes negatively charged and becomes a base. The base accepts a proton and the base molecule (originally neutral) becomes positively charged and becomes an acid. The combination of anion and cation forms the resulting salt.