Acids and Bases
Despite the everyday familiarity and the importance of acids and bases, it has not been a simple matter for chemists to define exactly what they are.
Acid-Base Definitions
The first definition of acids and bases was probably a phenomenological one, that is one that is based on what acids and bases do. For example, solutions of acids have a sour taste, such as vinegar or lemon juice. Acids also cause certain vegetable dyes to turn characteristic colors, they also dissolve some metals with the liberation of hydrogen, and they react with bases to form salts. Bases on the other hand form solutions with a bitter taste and feel slippery to the touch. Bases also cause certain dyes to change color and they react with acids to form salts.
Arrhenius Acid-Base Concept
Although useful, these phenomenological definitions do not relate the properties of acids and bases to their composition and molecular structure. The first structural acid-base definition was provided by Svante Arrhenius. According to Arrhenius, an acid is a substance that provides H+ ions when it is dissolved in water, and a base produces OH- ions when it is dissolved in water.
HCl(aq) ---> H+(aq) + Cl-(aq) an acid
NaOH(aq) ---> Na+(aq) + OH-(aq) a base
A hydrogen ion from an acid is not in the form of a bare proton in water as the Arrhenius suggests. Instead the proton is attached to a water molecule to form a hydronium ion, H3O+. So in our example above of hydrochloric acid a more accurate equation would be
HCl(aq) + H2O(aq) ---> H3O+(aq) + Cl-(aq)
Having said that, be aware that chemists often write H+(aq) as a shorthand notation for a hydrated proton or hydronium ion.
Bronsted-Lowry Acid-Base Concept
The Bronsted-Lowry acid-base concept is more encompassing than the Arrhenius concept. Any substance that is an acid by the Arrhenius concept will still be an acid according to the Bronsted-Lowry concept. The same for a base. However the Bronsted-Lowry concept will classify some substances as acids and bases which would not be classified as such by the Arrhenius concept.
According to the Bronsted-Lowry concept, an acid is a proton ( H+ ) donor and a base is a proton acceptor. The Bronsted-Lowry concept says that in order for a substance to act as an acid there must be a base present because if a substance is to donate a proton, there must be another substance present to accept it. So an acid-base reaction is simply the transfer of a proton from an acid to a base.
Strong and Weak Acids and Bases
A strong acid is an acid that ionizes completely in water, it is a strong electrolyte. A strong base is a base that is completely ionized in water.
| Strong Acids | Strong Bases |
| HClO4 | LiOH |
| H2SO4 | NaOH |
| HI | KOH |
| HBr | Ca(OH)2 |
| HCl | Sr(OH)2 |
| HNO3 | Ba(OH)2 |
A weak acid is an acid that is only partly ionized in water, it is a weak electrolyte. An example is
HF(aq) + H2O(aq) <==> H3O+(aq) + F-(aq)
A weak base is an base that is only partly ionized in water, it is a weak electrolyte. An example is
NH3(aq) + H2O(aq) <==> NH4+(aq) + OH-(aq)
Neutralization Reactions
One of the fundamental chemical properties of acids and bases is that they neutralize one another. A neutralization reaction is a reaction of an acid and a base with the production of an ionic compound and usually water. The ionic compound that forms is called a salt. Most ionic compounds other than hydroxides and oxides are salts.
| HCl(aq) + NaOH(aq) <==> NaCl(aq) + H2O(l) |
| acid base salt |