Johannes Bronsted (Danish) and Thomas Lowry (English) proposed a more general definition of acids and bases.
- based on the transfer of H+ ions
- when the H+ ion is around water, it becomes hydrated = hydronium ion (H3O+)
- according to Bronsted & Lowry:
- acids are substances that can transfer a proton to another substance
- bases are substances that can accept a proton
(acids and bases always work together to transfer a proton)
Bronsted-Lowry Acid -- a molecule or ion must have a hydrogen atom that it can lose as a H+ ion
Bronsted-Lowry Base -- a molecule or ion must have a nonbonding pair of electrons that it can use to bind the H+ ion
- some substances can be acids in one reaction and a base in another -- amphoteric (water is a common amphoteric substance)
- some acids are better proton donors than others and some bases are better proton acceptors
- the more easily an acid donates a proton, the less readily its conjugate base accepts a proton
- the more readily a base accepts a proton, the less readily its conjugate acid gives up a proton
- the stronger the acid, the weaker its conjugate base
- the stronger the base, the weaker its conjugate acid
(the strength of an acid is based on the ability to donate protons)
3 groups of acids & bases based on their behavior in water
- Strong acids -- completely transfer protons to water
- Weak acids -- only partly dissociate in aqueous solution
- Neglibile acidity -- contain hydrogen but do not exhibit properties of acids in water
- (the position of the equilibrium favors transfer of the proton from the stronger acid to the stronger base -- see chart on page 598)
Autoionization of water
- (water can be either an acid or base)
- in the presence of an acid -- water is a proton acceptor
- in the presence of a base -- water is a proton donor
pH Scale
- pH -- [H+] = concentration of hydrogen ions
|_____________|____________|
1 7 14
Acid Neutral Base
- 7 Common Strong Acids
- HCl, HBr, HI, HNO3, HClO3, HClO4 (monoprotic acids), H2SO4 (diprotic acid)
(in an aqueous solution, a strong acid is usually the only source of hydrogen ions)