Acids and Bases  Elmhurst College
Acids Neutralization & Salts Acid-Base Strength  Chemistry Department
Bases pH  Indicators  Virtual ChemBook

 

Bases

Introduction and Definitions:

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 the formula such as sodium hydroxide (NaOH) and magnesium hydroxide (Mg(OH)2).

Self Ionization Reaction:
NaOH ---> Na+ + OH-

Lime water: Ca(OH)2 --> Ca2+ + 2 OH-

Table of Common Bases
 Sodium Hydroxide  NaOH Lye, Caustic); make soap and textiles, oven cleaner, Drano, Liquid Plumber
Potassium Hydroxide  KOH Lye, Caustic); make soap and textiles
Magnesium Hydroxide Mg(OH)2 Milk of magnesia; laxative and antacid
Calcium Hydroxide Ca(OH)2 Lime Water; astringent -causes contraction of skin pores
Ammonium Hydroxide  NH4OH Ammonia in water; window cleaner, other cleaning solutions
 Ammonia  NH3 Gas; inhalant to revive an unconscious person, anhydrous or liquid ammonia is injected into soil as a fertilizer
Sodium Carbonate  Na2CO3 Soda Ash; Detergents
 Sodium Phosphate  Na3PO4 Sodium triphosphate;Detergents

Ammonia - Chime in new window

 
Click for larger image

 Ammonia, Ammonium Hydroxide, and Ammonium Ions:

Ammonia, Ammonium Hydroxide, and Ammonium Ions are all similarly sounding words with related but different meanings.

Ammonia (NH3)shown in the graphic on the left is a gas at room temperature. If you can smell ammonia whether from an aqueous solution or other source is in the form of this gas.

Ammonia - Chime in new window

If ammonia ( which is a polar molecule) dissolves in water, it reacts with the water to form ammonium ions (NH4+) and hydroxide ions.

  HOH +

  NH3--->
 NH4+ + OH-

Bronsted acids or bases, above?
 
Shows the presence of 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 of Salts:

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.

Do you expect that salts are capable of changing the acids or bases in water? On the basis of previously learned principles you probably should say "NO". Salts have neither H+ or OH- ions in their formulas.

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.

Note that the hydrolysis equation is a double replacement reaction.

Na2CO3 + 2 H20 ---> 2Na+ + 2 OH- + H2CO3

Water is the Bronsted Acid and sodium carbonate is the Bronsted Base. 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.

In the graphic on the left, the solution in the beaker is a mixture of Arm and Hammer Sodium Carbonate detergent. The pink hands are from the the indicator phenolphthalein which turns pink in the presence of hydroxide ions or base. Most detergents have the property of being basic which contributes to the cleaning action.