Acid and Base Theories

By: James Slattery

&

Jaime Wornick

The Theorist

• Svante Arrhenius • J. N. Bronsted and T. M. Lowry• Gilbert Lewis

Svante Arrhenius Theory

• The Arrhenius theory defines acids and bases as isolated species in solution giving rise respectively to hydrogen ions H+ and hydroxyl ions OH-. The acid produces the H+ and the base produces the OH-.

• The Interaction with solvents is ignored.

• The theory cannot begin to explain acidic and alkaline properties in non-aqueous solvents.

• This theory is widely used to calculate equilibrium data for weak electrolytes.

• Example: HCl (g)---> H(aq) + Cl-(aq)

Personal Arrhenius

• Svante August Arrhenius was a Swedish Chemist who lived from 1859-1927

• He wrote many books on Physics and his theories

• He was awarded the Nobel Prize for his work in 1903

• He was elected Foreign member of the Royal Society in 1911, and was awarded the Society’s Davy medal and also the Faraday Medal of the Chemical Society (1914)

Bronsted-Lowry Theory

• Using the words of Bronsted, “…acids and bases are substances that are capable of splitting off or taking up hydrogen ions, respectively.”

• The 90’s way of saying it:

– An acid is a substance from which a proton can be removed

– A base is a substance that can remove a proton from an acid

• Though it can be defined slightly different using words as “donor” and “acceptor”.

The Lowry Speech

• Lowry made an example of H3O, which is commonly used today. He stated,

“"It is a remarkable fact that strong acidity is apparently developed only in mixtures and never in pure compounds. Even hydrogen chloride only becomes an acid when mixed with water. This can be explained by the extreme reluctance of a hydrogen nucleus to lead an isolated existence.... The effect of mixing hydrogen chloride with water is probably to provide an acceptor for the hydrogen nucleus so that the ionization of the acid only involves the transfer of a proton from one octet to another.

– HCl + H2O [an equilibrium sign] Cl¯ + OH3+

– (Lowry also draws this equilibrium with all the electron "dots" to show the full octets)

– The ionized acid is then really an ionized oxonium salt."

Example?

• HCl + H2O ---> H3O + Cl– HCl - is an acid, because it’s proton is

available to be transferred

– H2O - is a base, since it gets the proton the acid is lost

– H2O - is an acid, because it can give a proton

– Cl - is a base, since it has the capacity to receive a proton

• This theory is nice though it fails to explain acid/base behavior in aprotic solvents such as benzene and dioxane

Bronsted hard at work

Lewis Theory

• Acid - can accept a pair of electrons from another atom to form a new bond

• Base - can donate a pair of electrons to another atom to from a new bond

• The product of Lewis acid/base referred to as an adduct• Example - NH3 + BF3 ---> NH3 :BF3

Personal Lewis

• Gilbert Newton Lewis was born in Weymouth Massachusetts, on October 23. 1875

• He had his PhD at age 24

• He took a job at MIT where he would only work with the acceptionally gifted students and refused to work with the average ones.

• He wrote books which titles include, Thermodynamics and the Free Energy of Chemical Substances and Valence and the Structure of Atoms and Molecules.

• He unexpectedly died in a laboratory on March 23, 1946

The Final Remark

• "I have attempted to give you a glimpse...of what there may be of soul in chemistry. But it may have been in vain. Perchance the chemist is already damned and the guardian the blackest. But if the chemist has lost his soul, he will not have lost his courage and as he descends into the inferno, sees the rows of glowing furnaces and sniffs the homey fumes of brimstone, he will call out-: 'Asmodeus, hand me a test-tube.” -Gilbert Lewis

• The End