The equilibrium equation for phenol (C₆H₅OH) in water is: C₆H₅OH(aq) ⇄ C₆H₅O⁻(aq) + H⁺(aq).
This equation illustrates the slight dissociation of phenol, an organic solvent, when dissolved in water, behaving as a weak acid.
Understanding Phenol's Acidity
Phenol is a fascinating organic compound that, unlike typical alcohols, exhibits acidic properties. Its acidity stems from the delocalization of the negative charge on the phenoxide ion (C₆H₅O⁻) across the benzene ring, which stabilizes the conjugate base. This stabilization makes the deprotonation more favorable compared to simple aliphatic alcohols.
Key Characteristics of Phenol's Dissociation:
- Weak Acid: Phenol is a weak acid, meaning it only partially dissociates in water.
- Aqueous Solution: The dissociation occurs when phenol is dissolved in an aqueous (water) solution.
- Reversible Reaction: The double arrow (⇄) signifies a reversible reaction, indicating that the forward (dissociation) and reverse (recombination) reactions occur simultaneously, leading to an equilibrium state.
Components of the Equilibrium
Let's break down the species involved in the equilibrium:
| Species | Formula | Role in Equilibrium |
|---|---|---|
| Phenol (undissociated) | C₆H₅OH(aq) | Weak Acid |
| Phenoxide Ion | C₆H₅O⁻(aq) | Conjugate Base |
| Hydronium Ion (Proton) | H⁺(aq) | Acidic Product |
For a more in-depth understanding of acid-base chemistry, you can explore resources like LibreTexts Chemistry or Wikipedia's article on Phenols.
Le Chatelier's Principle and the Addition of HCl
Le Chatelier's principle states that if a change of condition is applied to a system in equilibrium, the system will shift in a direction that relieves the stress. Let's consider the effect of adding hydrochloric acid (HCl) to a solution of phenol at equilibrium.
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Addition of HCl: Hydrochloric acid (HCl) is a strong acid, meaning it completely dissociates in water to produce hydrogen ions (H⁺) and chloride ions (Cl⁻):
HCl(aq) → H⁺(aq) + Cl⁻(aq) -
Increase in H⁺ Concentration: The addition of HCl directly increases the concentration of H⁺ ions in the solution.
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Shift in Equilibrium: According to Le Chatelier's principle, the phenol equilibrium system (C₆H₅OH(aq) ⇄ C₆H₅O⁻(aq) + H⁺(aq)) will react to counteract this increase in H⁺. To consume the excess H⁺, the equilibrium will shift to the left.
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Consequences of the Shift:
- The reverse reaction (C₆H₅O⁻(aq) + H⁺(aq) → C₆H₅OH(aq)) will be favored.
- The concentration of C₆H₅OH (undissociated phenol) will increase.
- The concentration of C₆H₅O⁻ (phenoxide ion) will decrease as it combines with H⁺ to form C₆H₅OH.
Therefore, as a result of the addition of HCl, the concentration of the C₆H₅OH form of phenol will increase.
Practical Implications
Understanding phenol's equilibrium is crucial in various chemical contexts, including:
- pH Regulation: Phenol's acidic nature influences the pH of solutions containing it.
- Chemical Synthesis: In organic synthesis, controlling the protonation state of phenol is vital for reactions involving its hydroxyl group or the aromatic ring.
- Environmental Chemistry: The behavior of phenolic compounds in water bodies affects their toxicity and degradation pathways.
