Adding ammonium chloride (NH₄Cl) to a solution of ammonia primarily causes the equilibrium of the ammonia-water reaction to shift, leading to a decrease in the concentration of hydroxide ions (OH⁻).
Understanding Ammonia in Solution
When ammonia (NH₃) dissolves in water, it establishes a chemical equilibrium:
NH₃(aq) + H₂O(l) <=> NH₄⁺(aq) + OH⁻(aq)
This reaction shows that ammonia accepts a proton from water, forming ammonium ions (NH₄⁺) and hydroxide ions (OH⁻). The presence of OH⁻ ions makes the ammonia solution basic.
The Effect of Adding Ammonium Chloride
Ammonium chloride (NH₄Cl) is a salt that readily dissolves in water, dissociating completely into its ions:
NH₄Cl(s) -> NH₄⁺(aq) + Cl⁻(aq)
The Common Ion Effect
The key to understanding what happens is recognizing that the ammonium ion (NH₄⁺) produced from the dissolution of NH₄Cl is the same ion that is already present in the ammonia equilibrium. This is known as the common ion effect.
According to Le Chatelier's principle, if a system at equilibrium is subjected to a change, the system will shift in a direction that minimizes the effect of the change. In this case, the change is the addition of extra NH₄⁺ ions to the ammonia equilibrium system.
The provided reference confirms this: "The addition of NH4Cl to a solution of ammonia shows the effect of adding a common ion to a system at equilibrium. Addition of NH4Cl adds NH4+, driving the reaction to the left and consuming OH-."
Therefore, the equilibrium reaction shifts to the left:
NH₃(aq) + H₂O(l) <== NH₄⁺(aq) + OH⁻(aq)
This shift consumes the added NH₄⁺ ions by combining them with OH⁻ ions to form NH₃ and H₂O.
Consequences of the Shift
As the equilibrium shifts to the left, the concentration of hydroxide ions (OH⁻) in the solution decreases because they are consumed in the reverse reaction.
- The solution becomes less basic.
- The concentration of NH₃ increases slightly.
- The concentration of NH₄⁺ is significantly higher due to the added NH₄Cl, even though some is consumed by the shift.
Essentially, adding ammonium chloride acts as a buffer component, resisting significant changes in pH by providing a source of the common ion that can react with hydroxide.
Summary of Changes
| Component | Initial State (Ammonia Solution) | After Adding NH₄Cl | Change |
|---|---|---|---|
| NH₃ | Present | Concentration slightly increases | Equilibrium shifts left |
| H₂O | Solvent | Involved in reaction | Consumed by shift |
| NH₄⁺ | Present (from NH₃) | Concentration significantly increases | Added by NH₄Cl, then consumed by shift |
| OH⁻ | Present (from NH₃) | Concentration decreases | Consumed by shift |
| pH | Basic | Becomes less basic | Decreases |
This demonstrates how adding a common ion can significantly alter the concentrations of other species within an equilibrium system, a fundamental concept in chemistry.
