Question

Predict the direction that predominates in this reaction: $$\mathrm{F}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{HF}(a q)+\mathrm{OH}^{-}(a q)$$

Short answer

The reaction primarily proceeds from right to left, towards the reactants.

Step-by-step solution

Identify the reactants and products

The reactants are fluoride ion (\(F^-\)) and water (\(H_2O\)). The products are hydrofluoric acid (\(HF\)) and hydroxide ion (\(OH^-\)).

Understand the acid-base properties

Fluoride ion (\(F^-\)) is the conjugate base of \(HF\) and \(OH^-\) is the conjugate base of \(H_2O\). In terms of acidity, \(HF\) is a weak acid and \(H_2O\) is even weaker (practically neutral). When comparing the basicity, \(F^-\) is a weak base while \(OH^-\) is a strong base.

Apply Le Chatelier's Principle

Le Chatelier's principle states that if a dynamic equilibrium is disturbed by changing the conditions, the system responds by opposing the change to reach a new equilibrium. Since \(OH^-\) is a stronger base than \(F^-)\), it would be expected for the reaction to shift to the left to minimize the effect, favoring the reactants. This is due to the fact that the position of equilibrium will shift towards the weaker acid-base pair to minimize the change.

Predict the reaction direction

Based on the principles described, in this reaction, the equilibrium lies to the left, in favor of the reactants. Therefore, the reaction primarily proceeds from right to left.

Key concepts

Acid-Base Reactions

Acid-base reactions are fundamental chemical processes where acids and bases interact with each other. In our reaction, we have fluoride ion (\( F^- \)) and water (\( H_2O \)) on the reactant side, and hydrofluoric acid (\( HF \)) and hydroxide ion (\( OH^- \)) on the product side.
- **Acid:** An acid is a substance that can donate a proton (H⁺). In this reaction, hydrofluoric acid (\( HF \)) acts as the acid.- **Base:** A base is a substance that can accept a proton. The fluoride ion (\( F^- \)) acts as our base here.During the reaction, the fluoride ion accepts a proton from water, forming hydrofluoric acid and hydroxide ion. This is called an equilibrium reaction because the products can react to form the reactants again.
Acid-base reactions are characterized by the transfer of protons between reactants and products. In our example, \( H_2O \) donates a proton to \( F^- \), shifting back and forth as conditions allow. The equilibrium nature of these reactions is crucial for controlling which side of the reaction is favored.

Le Chatelier's Principle

Le Chatelier's Principle helps us predict how equilibrium in a chemical system will adjust under stress. It asserts that if an external condition is altered, the system will adjust to minimize this change.
- **Reaction Shift:** In the given reaction, \( OH^- \) is a stronger base compared to \( F^- \). According to Le Chatelier, the equilibrium will shift towards the less reactive side to counteract this difference in base strength.- **Equilibrium Adjustment:** Since \( OH^- \) is significantly stronger, the reaction shifts left, favoring the reactants \( F^- \) and \( H_2O \). This adjustment reduces the formation of the stronger base by promoting the weaker acid-base pair of \( F^- \) and \( H_2O \).This principle is a vital tool in chemistry for predicting the behavior of reactions in equilibrium. It allows chemists to manipulate the conditions to either drive the reaction towards products or reactants, depending on the desired outcome.

Weak Acids and Bases

Weak acids and bases do not dissociate completely in solution. This incomplete dissociation is key to understanding equilibrium reactions like the one at hand.
- **Weak Acids:** Hydrofluoric acid (\( HF \)) is a weak acid, meaning it only partially donates protons when in solution. \( HF \) thus exists in equilibrium, promoting both its dissociation into ions and recombination back into neutral acid.- **Weak Bases:** On the flip side, the fluoride ion (\( F^- \)) is a weak base, capable of accepting protons from water, but not completely. In an environment where weak acids and bases are present, the equilibrium of reactions can be delicate. The balance between them governs the extent to which products or reactants are favored in the chemical equation. By understanding the nature of these weak counterparts, one can predict the directionality of reactions under given conditions. This grasp of chemical behavior at equilibrium helps in explaining why the equilibrium for our reaction lies towards the reactant side.