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 forward reaction is favored; equilibrium shifts towards \( \mathrm{HF}(aq) + \mathrm{OH}^{-}(aq) \).

Step-by-step solution

Identify Reactants and Products

In this equilibrium reaction, we have \( \mathrm{F}^{-}(aq) \) and \( \mathrm{H}_{2}\mathrm{O}(l) \) as the reactants, and \( \mathrm{HF}(aq) \) and \( \mathrm{OH}^{-}(aq) \) as the products.

Evaluate Acid-Base Strength

Identify that \( \mathrm{HF} \) is a weak acid and \( \mathrm{OH^{-}} \), the conjugate base of water, is the conjugate base of a very weak acid. On the other hand, \( \mathrm{F^{-}} \) is the conjugate base of \( \mathrm{HF} \), and water (\( \mathrm{H_2O} \)) is acting as a weak acid here.

Predict Equilibrium Direction

The reaction tends to favor the side with the weaker acids and bases in an acid-base reaction. \( \mathrm{HF} \) is a weaker acid compared to water acting as an acid in this context. Thus, the equilibrium will predominately shift towards the formation of \( \mathrm{HF} \) and \( \mathrm{OH}^{-} \), meaning the forward reaction is favored.

Key concepts

Weak Acids

A weak acid is an acid that does not completely dissociate in water. This means that in solution, both the acid and its dissociated ions exist in equilibrium. For example, hydrofluoric acid (HF) is a common weak acid. In water, it only partially breaks down into hydrogen ( +H^+ ) ions and fluoride ( F^- ) ions.
Why is HF considered a weak acid? - **Partial Dissociation:** Unlike strong acids, which fully ionize in solution, HF only disassociates slightly. - **Equilibrium Presence:** There is a dynamic balance between HF and its ions ( H^+ and F^- ).
The concept of weak acids is crucial in understanding acid-base equilibrium. In any reaction involving a weak acid, you must consider how the equilibrium will shift based on the relative strengths of the acids and bases involved.

Conjugate Base

The conjugate base of an acid is what remains after the acid donates a proton ( H^+ ). For every acid-base reaction, there is a corresponding conjugate acid-base pair. Understanding these pairs is key to predicting reaction behavior.
For example, in our reaction: - **HF becomes** F^- **on losing a proton** - **This makes F^- the conjugate base of HF**
To understand better: - The stronger an acid, the weaker its conjugate base. - Similarly, the weaker the acid, the stronger its conjugate base. In this situation, because HF is a weak acid, F^- is relatively strong as a conjugate base. This plays a role in determining which direction the equilibrium will shift in an acid-base reaction, as the interaction between acids and their conjugate bases helps drive that shift.

Equilibrium Direction

In any acid-base reaction, predicting equilibrium direction involves comparing the strengths of the acids and bases on both sides of the equation. Generally, the equilibrium will favor the side with the weaker acid and base because they are less likely to react further.
**Factors affecting equilibrium direction:** - **Acid and Base Strengths:** The side with the weaker acid ( HF ) and the weaker base ( OH^- ) is favored. - **Initial Concentrations and Conditions:** While not highlighted here, in more advanced scenarios initial conditions can also influence equilibrium direction.
For our reaction between F^- and H_2O , because HF is weaker compared to water acting as an acid, the equilibrium shifts to the right, towards the products ( HF and OH^- ). This is an essential principle for predicting the predominant direction of acid-base reactions.

Acid-Base Reaction

Acid-base reactions involve the transfer of protons ( H^+ ) between reactants. These reactions can be understood and predicted using the Brønsted-Lowry acid-base theory, which classifies acids as proton donors and bases as proton acceptors.
In our example: - F^- acts as the base: accepting a proton from water. - H_2O acts as an acid, donating a proton to form OH^- .
**Key aspects of acid-base reactions include:** - **Acid and Base Identification:** Determining which substances are acids and bases in the reaction. - **Conjugate Pairs:** Recognizing conjugate acid-base pairs formed during the reaction. - **Dynamic Equilibrium:** Understanding that these reactions reach a balance where the rate of the forward reaction equals the rate of the reverse reaction. Comprehending these fundamentals aids in mastering more complex acid-base interactions and predicting outcomes in varying chemical contexts.