Question

Water is the most common amphoteric substance, which means that, depending on the circumstances, water can behave either as an acid or as a base. Using HF as an example of an acid and \(\mathrm{NH}_{3}\) as an example of a base, write equations for these substances reacting with water, in which water behaves as a base and as an acid, respectively.

Short answer

In the reaction between water and hydrofluoric acid (HF), water acts as a base and the equation is: \[ \mathrm{HF} + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{H}_{3}\mathrm{O}^{+} + \mathrm{F}^{-} \] In the reaction between water and ammonia (\(\mathrm{NH}_{3}\)), water acts as an acid and the equation is: \[ \mathrm{NH}_{3} + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{NH}_{4}^{+} + \mathrm{OH}^{-} \]

Step-by-step solution

Reaction with HF (Acid)

First, let's write the reaction between water molecules (\(\mathrm{H}_{2}\mathrm{O}\)) and hydrofluoric acid (HF). In this case, water will act as a base, meaning it will accept protons from the acid. The reaction can be represented as: \[ \mathrm{HF} + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{H}_{3}\mathrm{O}^{+} + \mathrm{F}^{-} \] Water molecules have accepted a proton from hydrofluoric acid, forming hydronium ions (\(\mathrm{H}_{3}\mathrm{O}^{+}\)) and fluoride ions (\(\mathrm{F}^{-}\)).

Reaction with NH₃ (Base)

Now, let's write the reaction between water molecules (\(\mathrm{H}_{2}\mathrm{O}\)) and ammonia (\(\mathrm{NH}_{3}\)). In this case, water will act as an acid, meaning it will donate protons to the base. The reaction can be represented as: \[ \mathrm{NH}_{3} + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{NH}_{4}^{+} + \mathrm{OH}^{-} \] Water molecules have donated a proton to ammonia, forming ammonium ions (\(\mathrm{NH}_{4}^{+}\)) and hydroxide ions (\(\mathrm{OH}^{-}\)).

Key concepts

Acid-Base Reactions

Understanding acid-base reactions is crucial for deciphering many chemical processes, including those in biological systems. An acid is a substance that can donate a proton (H⁺), while a base is a substance that can accept a proton. This proton transfer fundamentally changes the nature of the molecules involved.

For instance, in the reaction between hydrofluoric acid (HF) and water (H₂O), HF donates a proton to H₂O, resulting in the formation of hydronium ions (H₃O⁺) and fluoride ions (F^-). This illustrates the transformative power of acid-base reactions, where new ions are created, altering the chemical makeup of the solution.

Proton Transfer

Proton transfer is the defining mechanism of an acid-base reaction. The ease with which a molecule can give up (for acids) or receive (for bases) a proton is what determines its strength in an acid-base reaction.

For example, when ammonia (NH₃), a base, reacts with water, which acts as an acid in this context, water donates a proton to ammonia. This transfer converts NH₃ into an ammonium ion (NH₄⁺) and converts the water molecule into a hydroxide ion (OH^-). The ability to track proton movement is key to understanding chemical reactions and predicting the behavior of substances in different environments.

Chemical Equilibrium

Chemical equilibrium is the state in a chemical reaction where the rate of the forward reaction equals the rate of the reverse reaction. At this point, the concentrations of reactants and products remain constant over time. It’s crucial to note, however, that being at equilibrium does not mean the reactants and products are present in equal amounts, but rather that their ratios don’t change.

In the context of acid-base reactions, equilibrium plays a significant role. It helps predict the extent of the reaction and the concentrations of various species in solution at equilibrium. Scientists use the concept of equilibrium constants to understand how likely a substance is to donate or accept protons under certain conditions.

Water as an Acid and Base

Water is a unique molecule due to its ability to act as both an acid and a base, a characteristic known as amphoteric. When water encounters a stronger base, it donates a proton, acting as an acid. Conversely, when it meets a stronger acid, water accepts a proton, functioning as a base.

In the reactions we're examining, water plays both roles. It accepts a proton from HF, acting as a base, and it donates a proton to NH₃, acting as an acid. This dual capability is fundamental for maintaining the pH balance in many natural systems, including the human body. Understanding water’s amphoteric nature is key in environmental science, chemistry, and biochemistry.