Overview of Acids and Bases

The acid and base concepts are so prevalent that it occurs in all branches of chemistry including organic, inorganic and analytical chemistries.  It also occurs in physical and green chemistries.  From this we see the importance of this concept so that it would be reasonable to give an overview about this important topic. 

Basically, there are three definitions of acids and bases which share in common certain features.  The first definition of acid and base that I discuss here is the bronsted definition of acids and bases.  According to this definition all acids are defined as any compound that releases a proton or H+ ion. 

On the other hand, a bronsted base is defined as any compound which releases a hydroxyl group or OH- ion.  The degree of acidity and basicity is measured using a scale that is called pH.  This scale ranges from 1 to 14.  Superacids exist in which their pH is negative in value.   An example of an organic bronsted acid is acetic acid while an example of an inorganic acid would be HBr or hydrogen bromide.

According to The bronsted definition of acids and bases hydrogen peroxide can also behave like an acid but not as a base in th usual sense although it contains in its structure two hydroxyl groups. 

The strength of acids and bases depends on the stability of the conjugate counter ion.  Also the acidity according to the bronsted definition depends on the extent of solvation of the counter ion in the solvent that the acid is present in it.  Small ions that are easily solvated in water gives a stronger acid than those with a counter ion that is not well solvated in the solvent.   Hydrogen bonding can also affect the acidity of compounds.

The second definition of acids and bases that I discuss here is the definition of acids and bases by Lewis.  A Lewis acid is defined as as a compound which has a low lying empty orbital which can accept an electron lone pair.  A Lewis base is defined as a donor of electron pair. 

By this definition Silicon atom is a lewis acid due to the existence on it of a low lying empty d-orbital.  Another example of a Lewis acid is the compound BF3.  Boron has usually an empty p-orbital that it can share for bonding with electron pair donors.  By this definition BF3 is considered as a Lewis acid. 

In BF3 the fluorine lone pair can delocalize to the empty p orbital on boron.  Thus giving it an extra stability.  An example of a Lewis base is any molecule that can donate an electron pair.  According to this definition OH- or the hydroxyl group is considered a lewis base as well as a bronsted base.  Amines and alcohols are examples of Lewis bases due to the existence of lone pairs on nitrogen and oxygen respectively. 

The third definition of acids and bases is based on the type of oxides.  Metal oxides are considered bases while non metal oxides are considered acidic.  An example of an acid would be CO2 or carbon dioxide.  An exmaple of a base would be MgO or magnesium oxide.  A reaction between these to compounds would give usually a salt  MgCO3.