Bronsted-Lowry Acids and Bases, Auto Ionization and Conjugate Acid [PDF]

Bronsted-Lowry Acids and Bases, Auto Ionization and Conjugate Acid/Base Pairs Basic Definitions: Bronsted-Lowry Acid: A substance that donates a proton (H+) in a chemical reaction. Bronsted-Lowry Base: A substance that accepts a proton (H+) in a chemical reaction. For example: H 2O + H 2O ! H 3O + + OH -

or

HCl + H 2O ! H 3O + + Cl-

In the first reaction, one of the water molecules is donating a proton (and turning itself into OH-) while the other is accepting a proton (and turning itself into H3O+). In the second reaction, the HCl is donating the proton and the H2O is accepting the proton. Note: H3O+ is called a “hydronium ion” and most often one can simply consider it to be a hydrated hydrogen ion (H+ + H2O  H3O+ ). It is not necessary for water to be present to have a Bronsted-Lowry type reaction: for example: HCl + NH 3 ! NH +4 + Cl- . The HCl is donating a proton while the NH3 is accepting the proton. Auto Ionization: The first reaction ( H 2O + H 2O ! H 3O + + OH - ) is an example of an “auto ionization reaction” Auto ionization reactions do not require water either: for example NH 3 + NH 3 ! NH +4 + NH -2 Conjugate Acid/Base Pairs: Since all of these reactions are equilibrium reactions and can “go backwards” there is a “double set” of Acids and Bases in each reaction:

H 2O + H 2O ! H 3O + + OH -

Forward Rxn Acid Base H 2O H 2O

HCl + H 2O ! H 3O + + Cl-

HCl

H 2O

H 3O +

HCl + NH 3 ! NH +4 + Cl-

HCl

NH 3

NH 3 + NH 3 ! NH +4 + NH -2

NH 3

NH 3

Reaction

Reverse Rxn Acid Base + OH H 3O

Conjugate Acid/Base Pairs H 2O/OH -

H 3O + / H 2O

Cl-

HCl /Cl-

H 3O + / H 2O

NH +4

Cl-

HCl /Cl-

NH +4 / NH 3

NH +4

NH -2

NH +4 / NH 3

NH 3 /NH -2

Notice that every reaction has two conjugate acid/base pairs and that in a pair the acid is always named first.