1. The Chemical Reaction:
* Ammonium Chloride (NH₄Cl) Hydrolysis: When ammonium chloride dissolves in water, it undergoes hydrolysis. This means the ammonium ion (NH₄⁺) reacts with water to produce hydronium ions (H₃O⁺) and ammonia (NH₃):
```
NH₄⁺(aq) + H₂O(l) ⇌ H₃O⁺(aq) + NH₃(aq)
```
* The Effect on pH: The production of hydronium ions (H₃O⁺) increases the acidity of the solution, lowering the pH.
* Sodium Hydroxide (NaOH) and Phenolphthalein: Phenolphthalein is a pH indicator that turns pink in basic solutions and is colorless in acidic solutions. The presence of NaOH makes the initial solution basic, turning the phenolphthalein pink.
2. The Shift in Equilibrium:
* Le Chatelier's Principle: When a stress is applied to a system at equilibrium, the system will shift in a direction that relieves the stress. In this case, the addition of NH₄Cl is the stress.
* Shift to the Left: The addition of NH₄Cl increases the concentration of NH₄⁺ ions. To relieve this stress, the equilibrium shifts to the left, favoring the formation of NH₄⁺ and H₂O and consuming H₃O⁺ and NH₃.
* Decreasing pH: The consumption of H₃O⁺ ions makes the solution less acidic, decreasing the pH.
3. Observable Evidence:
* Color Change: As the pH decreases, the solution will become less basic and eventually acidic. This will cause the pink color of the phenolphthalein to fade and eventually disappear.
* Temperature Change: The reaction of NH₄⁺ with water is exothermic. The shift in equilibrium towards the reactants (NH₄⁺ and H₂O) will release heat, possibly resulting in a slight temperature increase.
In Summary:
The addition of ammonium chloride to a solution of sodium hydroxide and phenolphthalein causes a shift in equilibrium. This shift results in a decrease in pH, evidenced by the fading of the pink color of the phenolphthalein. The shift is driven by Le Chatelier's principle, as the system attempts to relieve the stress of added ammonium ions.
