How do bases react with ammonium salts?

Short Answer

When a base reacts with an ammonium salt, it produces ammonia gas, water, and a salt. The hydroxide ions (OH⁻) from the base react with the ammonium ion (NH₄⁺), releasing ammonia gas.

For example, sodium hydroxide reacts with ammonium chloride:
NH₄Cl + NaOH → NH₃ + H₂O + NaCl
This reaction is widely used to generate ammonia in laboratories and demonstrates the displacement of a weak base (NH₃) by a stronger base (OH⁻).

Detailed Explanation :

Reaction of Bases with Ammonium Salts

Ammonium salts are salts containing the NH₄⁺ ion, which is the conjugate acid of ammonia (NH₃). When a strong base like sodium hydroxide (NaOH) or potassium hydroxide (KOH) reacts with an ammonium salt, the base removes H⁺ ions from NH₄⁺, liberating ammonia gas. This is a common gas-evolution reaction in chemistry.

The general reaction can be written as:

NH₄⁺ + OH⁻ → NH₃ + H₂O

When the ammonium salt is combined with the cation of the base, a new salt forms along with water and ammonia:

Ammonium salt + Base → Salt + H₂O + NH₃

Examples of Base-Ammonium Salt Reactions

  1. Ammonium chloride + Sodium hydroxide
    NH₄Cl + NaOH → NH₃ + H₂O + NaCl
  • Produces ammonia gas, water, and sodium chloride (common salt).
  1. Ammonium sulfate + Potassium hydroxide
    (NH₄)₂SO₄ + 2KOH → 2NH₃ + 2H₂O + K₂SO₄
  • Produces ammonia gas, water, and potassium sulfate.
  1. Ammonium nitrate + NaOH
    NH₄NO₃ + NaOH → NH₃ + H₂O + NaNO₃
  • Produces ammonia gas, water, and sodium nitrate.

Mechanism of the Reaction

  1. The ammonium ion (NH₄⁺) acts as a weak acid.
  2. The OH⁻ ions from the base accept a proton (H⁺) from NH₄⁺.
  3. This forms ammonia (NH₃) and water (H₂O).
  4. The cation of the base (Na⁺ or K⁺) combines with the anion of the ammonium salt to form a new salt.

This is a proton transfer reaction, where the base removes a hydrogen ion from the ammonium ion.

Factors Affecting the Reaction

  1. Strength of Base
  • Strong bases like NaOH or KOH produce ammonia quickly.
  • Weak bases may react slowly or incompletely.
  1. Temperature
  • Heating increases the reaction rate and helps release ammonia gas more rapidly.
  1. Concentration of Reactants
  • Higher concentrations of ammonium salt and base produce more ammonia.
  1. Nature of Ammonium Salt
  • NH₄Cl, NH₄NO₃, and (NH₄)₂SO₄ react similarly, but solubility affects the speed of gas evolution.

Observations During the Reaction

  1. Ammonia gas is evolved, which has a sharp, pungent smell.
  2. Fumes may be observed, especially on heating.
  3. White smoke can appear if concentrated ammonium salts are used with strong bases like KOH.
  4. Water and a new salt remain dissolved in solution.

Applications

  1. Laboratory Preparation of Ammonia Gas
  • This reaction is a standard method to produce NH₃ gas in the lab.
  • Example: Heating NH₄Cl with NaOH in a test tube.
  1. Cleaning and Industrial Uses
  • Used in cleaning products to generate ammonia in situ.
  • NH₃ acts as a reducing agent or cleaning agent.
  1. Fertilizer Chemistry
  • Ammonium salts in soil can react with natural bases or hydroxides, releasing ammonia for plant use.
  1. Educational Demonstrations
  • Demonstrates acid-base reactions, gas evolution, and proton transfer mechanisms.

Safety Considerations

  • Ammonia gas is pungent and irritating; avoid inhalation.
  • Conduct the reaction in a well-ventilated area or under a fume hood.
  • Handle strong bases like NaOH and KOH with gloves and goggles.
Conclusion

When a base reacts with an ammonium salt, the hydroxide ions remove H⁺ ions from NH₄⁺, producing ammonia gas, water, and a new salt. The reaction is commonly used in laboratories to generate ammonia, and it demonstrates the acid-base properties of ammonium ions. Factors like strength of base, concentration, temperature, and type of ammonium salt affect the rate and completeness of the reaction. This reaction is important in laboratory demonstrations, fertilizer chemistry, and industrial applications.