Question

Write a balanced equation for the proton transfer reaction between hydrofluoric acid (HF) and ammonia \(\left(\mathrm{NH}_{3}\right)\). Identify each conjugate acid-base pair, and rewrite the equilibrium arrows to indicate if the forward or reverse reaction is favored.

Short answer

\(\mathrm{HF} + \mathrm{NH}_{3} \rightleftharpoons \mathrm{F}^{-} + \mathrm{NH}_{4}^{+}\); conjugate pairs: (HF/F⁻) and (NH₃/NH₄⁺); reverse is slightly favored.

Step-by-step solution

Write the Balanced Proton Transfer Reaction

The proton transfer reaction between hydrofluoric acid (HF) and ammonia (NH₃) can be written as follows:\[\mathrm{HF} + \mathrm{NH}_{3} \rightleftharpoons \mathrm{F}^{-} + \mathrm{NH}_{4}^{+}\]In this reaction, HF donates a proton (H⁺), becoming the fluoride ion (F⁻), and NH₃ accepts a proton, becoming ammonium ion (NH₄⁺).

Identify Conjugate Acid-Base Pairs

In the reaction \(\mathrm{HF} + \mathrm{NH}_{3} \rightleftharpoons \mathrm{F}^{-} + \mathrm{NH}_{4}^{+}\), the pairs can be identified as follows:- \(\mathrm{HF}\) is the conjugate acid and \(\mathrm{F}^{-}\) is its conjugate base.- \(\mathrm{NH}_{3}\) is the base and \(\mathrm{NH}_{4}^{+}\) is its conjugate acid.

Rewrite Equilibrium Arrows Based on Reaction Favored

To determine which direction is favored, consider the strength of HF and NH₄⁺ as acids. HF is a weak acid while NH₄⁺ is also weak, but slightly weaker than HF. Thus, the reverse reaction is slightly favored over the forward one:\[\mathrm{HF} + \mathrm{NH}_{3} \rightleftharpoons \mathrm{F}^{-} + \mathrm{NH}_{4}^{+}\]This implies the equilibrium lies somewhat to the left.

Key concepts

Proton Transfer Reaction

In acid-base chemistry, a proton transfer reaction is a fundamental concept. It involves the movement of a proton (H⁺) from an acid to a base. In this specific reaction, hydrofluoric acid (HF) donates a proton to ammonia (NH₃).
This results in the formation of fluoride ions (F⁻) and ammonium ions (NH₄⁺). The balanced equation for this proton transfer is:

• \(\mathrm{HF} + \mathrm{NH}_{3} \rightleftharpoons \mathrm{F}^{-} + \mathrm{NH}_{4}^{+}\)

In this equation, you can see the proton (H⁺) transfers from HF to NH₃, highlighting the nature of the reaction.
Understanding which molecule donates and which one accepts the proton is key to grasping the complete reaction.

Conjugate Acid-Base Pairs

Conjugate acid-base pairs are central to understanding acid-base reactions. They are pairs of compounds that transform into each other by the gain or loss of a proton.
In the reaction between HF and NH₃:

• HF is the acid, and its conjugate base after losing a proton is F⁻.
• NH₃ is the base, and its conjugate acid after gaining a proton is NH₄⁺.

These pairs show how compounds are related through proton transfer. It's a dynamic relationship, where acid turns into its conjugate base and vice versa.
This concept of shifting roles helps to predict the outcomes of reactions and the behavior of different substances in solution.

Equilibrium in Reactions

Equilibrium describes the state where both the forward and reverse reactions take place at the same rate. In this proton transfer reaction, equilibrium details which direction is more favored:

• \(\mathrm{HF} + \mathrm{NH}_{3} \rightleftharpoons \mathrm{F}^{-} + \mathrm{NH}_{4}^{+}\)

Here, HF is a weak acid, and NH₄⁺ is a weaker acid. Since NH₄⁺ is slightly weaker than HF, the reverse reaction is favored, meaning the equilibrium lies to the left.
This implies that more of the reactants are present compared to the products. Understanding equilibrium helps in predicting concentrations and explaining why reactions do not proceed all the way to products in certain conditions.