How a Neutralization Reaction Works in Salt Formation

When acids and bases react with each other, they can form a salt and (usually) water. This is called a neutralization reaction and takes the following form:

HA + BOH → BA + H₂O

Depending on the solubility of the salt, it may remain in ionized form in the solution or it may precipitate out of solution. Neutralization reactions usually proceed to completion.

The reverse of the neutralization reaction is called hydrolysis. In a hydrolysis reaction a salt reacts with water to yield the acid or base:

BA + H₂O → HA + BOH

Strong and Weak Acids and Bases

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

strong acid + strong base, e.g., HCl + NaOH → NaCl + H₂O

When 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 react with the water.

strong acid + weak base, e.g., HCl + NH₃ → NH₄Cl

The 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 + H₂O

When 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 ↔ NH₄ClO

The pH of the solution formed by 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.