NEUTRALIZATION REACTION

Neutralization is a chemical reaction in which an acid and a base interact with the formation of a salt to make a solution neutral. With strong acids and bases the important reaction is the combination of hydrogen ions with hydroxide ions to form water.

A salt is any compound which can be derived from the neutralization of an acid and a base. The word "neutralization" is used because the acid and base properties of H⁺ and OH^- are destroyed or neutralized. In the reaction, H⁺ and OH^- combine to form HOH or H₂O or water molecules. A neutralization is a type of double replacement reaction. A salt is the product of an acid-base reaction and is a much broader term then common table salt as shown in the first reaction.

The following are some examples of neutralization reactions to form salts.

a. HCl + NaOH --> NaCl + HOH

b. H₂SO₄ + 2 NH₄OH --> (NH₄)₂SO₄ + 2 HOH

c. 2 NaOH + H₂CO₃ --> N₂CO₃ + 2 NaOH
Carbon dioxide in the breath reacts first with the water to form carbonic acid:

CO₂ + H₂O --> H₂CO₃

The carbonic acid then further reacts with the lime water in the neutralization reaction. The phenolphthalein is pink in the presence of base and turns colorless when all of the base has been neutralized and there is a slight excess of the carbonic acid.

More specifically, there are four combinations of strong and weak acids and bases:

• strong acid + strong base, e.g., HCl + NaOH --> NaCl + H₂OWhen strong acids and strong bases react, the products are salt and water. The acid and base neutralize each other, so the solution will be neutral (pH=7) and the ions that are formed will not reaction with the water.
  
• strong acid + weak base, e.g., HCl + NH₃ ---> NH₄ClThe reaction between a strong acid and a weak base also produces a salt, but water is not usually formed because weak bases tend not to be hydroxides. In this case, the water solvent will react with the cation of the salt to reform the weak base. For example:
  HCl (aq) + NH₃ (aq) <--> NH₄⁺ (aq) + Cl^- while
  NH₄^- (aq) + H₂O <--> NH₃ (aq) + H₃O⁺ (aq)
  
• weak acid + strong base, e.g., HClO + NaOH --> NaClO + H₂OWhen a weak acid reacts with a strong base the resulting solution will be basic. The salt will be hydrolyzed to form the acid, together with the formation of the hydroxide ion from the hydrolyzed water molecules.
  
• weak acid + weak base, e.g., HClO + NH₃ <--> NH₄ClOThe pH of the solution formed from the reaction of a weak acid with a weak base depends on the relative strengths of the reactants. For example, if the acid HClO has a K_a of 3.4 x 10^-8 and the base NH₃ has a K_b = 1.6 x 10^-5, then the aqueous solution of HClO and NH₃ will be basic because the K_a of HClO is less than the K_a of NH₃.