Understanding the Acetic Acid Equilibrium

The question concerns the chemical equilibrium of acetic acid (CH₃COOH) in solution. The dissociation reaction is represented as:

$$ \text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+ $$

This equilibrium involves a weak acid, acetic acid, partially dissociating into its conjugate base, the acetate ion (CH₃COO^-), and hydrogen ions (H⁺).

Applying Le Chatelier's Principle

To determine the effect of adding H⁺ ions, we apply Le Chatelier's principle. This principle states that if a change (like adding or removing a substance, changing temperature, or changing pressure) is applied to a system at equilibrium, the system will shift in a direction that counteracts the applied change.

In this specific case:

• Change Applied: Addition of H⁺ ions.
• System's Reaction: H⁺ is a product in the equilibrium reaction. Adding more of a product disturbs the equilibrium.
• Counteraction: To counteract the increase in [H⁺], the system will try to consume the excess H⁺ ions.

Effect on Equilibrium Position

The reaction consumes H⁺ ions when it proceeds in the reverse direction (from right to left). Therefore, adding H⁺ ions will cause the equilibrium to shift towards the reactants:

$$ \text{CH}_3\text{COO}^- + \text{H}^+ \rightarrow \text{CH}_3\text{COOH} $$

This shift means that the concentration of undissociated acetic acid (CH₃COOH) will increase, while the concentrations of acetate ions (CH₃COO^-) might decrease (as they react with the added H⁺) or increase less than they would have without the added H$^+$, depending on the specific concentrations and the extent of the shift.

The equilibrium position moves to the left.

Analyzing the Options

• 1. Lead to no change: Incorrect. Adding a product affects the equilibrium position.
• 2. Stop the reaction: Incorrect. Equilibrium shifts, reactions don't necessarily stop completely.
• 3. Shift the equilibrium right: Incorrect. Shifting right would mean producing more products (H⁺ and CH₃COO^-), which is the opposite of what happens when H⁺ is added.
• 4. Shift the equilibrium left: Correct. Adding H⁺ drives the reaction in reverse, consuming products and forming reactants, thus shifting the equilibrium to the left.