Acid and Base Strength

While acids taste sour, give sharp stinging pain in a cut or wound, turn blue litmus paper red, make phenolphthalein colorless, react with metals to produce hydrogen gas, bases taste bitter, feel slippery, turn red litmus paper blue, make phenolphthalein pink, and react with carbonates or bicarbonates to produce carbon dioxide gas

Arrhenius Definition of acids and bases covers the dissociation of acids and bases in water to produce hydrogen ions (H+)         and hydroxide ions (OH-) respectively. For example, HCl and NaOH dissociate in water to give H+ and OH- ions respectively. As the definition of acids and bases involving hydrogen and hydroxide ions, respectively is much too limiting, a broader definition was proposed by Bronsted and Lowry in 1923. The main effect of the definition is to increase the number of substances that act as bases.

The determination of a substance as a Bronsted-Lowery acid or base can only be done by observing the reaction. In the case of H2O, HOH, it is a base in the first case and an acid in the second case. To determine whether a substance is an acid or a base, one has to count the hydrogens on each substance before and after the reaction. If the number of hydrogens has decreased that substance is the acid (donates hydrogen ions). If the number of hydrogens has increased that substance is the base (accepts hydrogen ions). If the reaction is viewed in reverse a new acid and base can be identified. The substances on the right side of the equation are called conjugate acid and conjugate base compared to those on the left. Also the original acid turns in the conjugate base after the reaction is over.

At the simplest definition, an acid is any compound that yields hydrogen ions (H+) or hydronium ions (H3O+) when dissolved in water. Hydronium ions are really the combination of H+ ions and H2O molecules, i.e. H+ + H2O === H3O+. The hydrogen ion with no electrons bonds to water through what is called a coordinate covalent bond where the oxygen donates both electrons for the bond. The molecule is still short an electron so the positive charge is retained. The hydronium ion also has trigonal pyramid geometry. Even though in solution a "bare" hydrogen ion is surrounded by up to 20 water molecules and is the best representation, most of the time just a simple hydrogen ion is used to represent the acid. In the chime model, notice the close contact of the water molecules as they engage in hydrogen bonding. An acid ionizes or dissociates (breaks apart) in water to form an equilibrium mixture of its individual ions. This is how the hydrogen ions are formed.

A base is any compound that yields hydroxide ions (OH-) when dissolved in water. There are quite a few identifiable bases with hydroxide in the formula such as sodium hydroxide (NaOH) and magnesium hydroxide (Mg(OH)2). Ammonia, Ammonium Hydroxide, and Ammonium Ions are all similarly sounding words with related but different meanings. If we can smell ammonia whether from an aqueous solution or other source, it is in the form of this gas. If ammonia (which is a polar molecule) dissolves in water, it reacts with the water to form ammonium ions (NH4+) and hydroxide ions.  Ammonium Hydroxide (NH4OH) is a convenient, but possibly non-existent molecule to help define the properties of a solution of ammonia gas dissolved in water. This molecule is used to call attention to the presence of the hydroxide ion and characterize ammonia as a base. Ammonium Ions (NH4+) have a +1 charge, contain 4 hydrogen atoms, and have a tetrahedral geometry as shown on the left.

Hydrolysis reactions are general reactions in which the water molecule takes part in the reaction as one of the reactants. This reaction is observed in many reactions both in inorganic and organic chemistry. The hydrolysis of salts is really the reverse of the neutralization reaction. Other bases do not have hydroxide ions in the formula, but readily react with water to produce hydroxide ions. These bases include sodium carbonate and phosphate. It is to be note that the hydrolysis equation is a double replacement reaction. The results of the hydrolysis reaction are the formation of a weak acid and a strongly ionized base. The strongly ionized base gives the overall property of basic. Most detergents have the property of being basic which contributes to the cleaning action.

All acids and bases do not ionize or dissociate to the same extent. This leads to the statement that acids and bases are not all of equal strength in producing H+ and OH- ions in solution. The terms "strong" and "weak" give an indication of the strength of an acid or base. The terms strong and weak describe the ability of acid and base solutions to conduct electricity. If the acid or base conducts electricity strongly, it is a strong acid or base. If the acid or base conducts electricity weakly, it is a weak acid or base.

We can test the conductivity of various solutions with a light bulb apparatus. If the light bulb circuit is completed by a solution containing a large number of ions, the light bulb will glow brightly indicating a strong ability to conduct electricity as shown for HCl. If the circuit is completed by a solution containing large numbers of molecules and either no ions or few ions, the solution does not conduct or conducts very weakly as happens with acetic acid. An acid or base which strongly conducts electricity contains a large number of ions and is called a strong acid or base. An acid or base which conducts electricity only weakly contains only a few ions and is called a weak acid or base.  

The bond strengths of acids and bases are implied by the relative amounts of molecules and ions present in solution. Strong acids have mostly ions in solution, therefore the bonds holding H and A together must be weak. Strong acids easily break apart into ions. Weak acids exist mostly as molecules with only a few ions in solution, therefore the bonds holding the species together must be strong. Weak acids do not readily break apart as ions but remain bonded together as molecules. Acids or bases with strong bonds exist predominately as molecules in solutions and are called "weak" acids or bases. Acids or bases with weak bonds easily dissociate into ions and are called "strong" acids or bases. All characteristics of acids and bases are related to whether the forms that predominate are molecules and ions. We can consider examples of Hydrochloric and hydrofluoric acid as prototype representatives of strong and weak acids respectively

Hydrochloric acid is a strong acid. Strong acids are strong electrolytes, which, for most purposes, are assumed to ionize completely in water. In this case, HCl ionizes completely to hydrogen ions, H+ and chloride ions, Cl-. The reaction is better represented as hydrochloric acid donating an H+ to water, producing hydronium ion, H3O+, and chloride ion. The role of water in this reaction is not shown to simplify the presentation. We can think of H+ as a shorthand representation of H3O+.

Hydrofluoric acid is a weak acid. Weak acids are weak electrolytes, which only ionize to a limited extent in water. At equilibrium, a hydrofluoric acid solution contains mostly non-ionized acid molecules, HF, and a small amount of hydrogen ions H+, and the conjugate base ions F-. This reaction is better represented as hydrofluoric acid donating a proton to water, producing hydronium H3O+ and a fluoride ion. We should remember that weak acids only partially ionize in water. Again the role of water in this reaction is not shown to simplify the presentation. Here too we can think of H+ as a shorthand representation of H3O+.