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Salt Formation

Chemistry of Neutralization and Hydrolysis

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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

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 reaction 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 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 Ka of 3.4 x 10-8 and the base NH3 has a Kb = 1.6 x 10-5, then the aqueous solution of HClO and NH3 will be basic because the Ka of HClO is less than the Ka of NH3.

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