Question

Given the general equation illustrating the reaction of the acid HA in water, $$ \mathrm{HA}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{H}_{3} \mathrm{O}^{+}(a q)+\mathrm{A}^{-}(a q) $$ explain why water is considered a base in the Bronsted-Lowry model.

Short answer

In the reaction \( \mathrm{HA}(aq)+\mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{H}_{3} \mathrm{O}^{+}(aq)+\mathrm{A}^{-}(aq) \), water acts as a base in the Bronsted-Lowry model because it accepts a proton (H⁺) from the acid HA to form hydronium ion \( (H_{3}O^{+}) \).

Step-by-step solution

Define the Bronsted-Lowry model

According to the Bronsted-Lowry model, an acid is a substance that donates protons (H⁺ ions), while a base is a substance that accepts protons. In this exercise, we will apply these definitions to the given reaction.

Analyze the given reaction of the acid in water

We are given the following reaction: \[ \mathrm{HA}(aq)+\mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{H}_{3} \mathrm{O}^{+}(aq)+\mathrm{A}^{-}(aq) \] First, let's focus on the acid, HA. In the course of the reaction, it loses a proton to form the conjugate base A⁻: \[ \mathrm{HA}(aq) \rightarrow \mathrm{H}^{+}(aq) + \mathrm{A}^{-}(aq) \]

Identify the role of water in the reaction

Next, let us turn our attention to water (H₂O). Water combines with a proton (H⁺), which is released by the acid HA: \[ \mathrm{H}_{2} \mathrm{O}(l) + \mathrm{H}^{+}(aq) \rightarrow \mathrm{H}_{3} \mathrm{O}^{+}(aq) \] In this step, water accepts a proton from HA, indicating that it is acting as a base since it is accepting a proton.

Conclusion

Based on the Bronsted-Lowry model, water is considered a base in the given reaction \( \mathrm{HA}(aq)+\mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{H}_{3} \mathrm{O}^{+}(aq)+\mathrm{A}^{-}(aq) \) as it accepts a proton from the acid HA.

Key concepts

Proton Donation

In the context of the Bronsted-Lowry acid-base theory, proton donation refers to the process in which an acid loses a proton. A proton, in this framework, is essentially a hydrogen ion denoted as H⁺. When we look at the reaction \( \mathrm{HA}(aq) + \mathrm{H}_2\mathrm{O}(l) \rightarrow \mathrm{H}_3\mathrm{O}^+(aq) + \mathrm{A}^-(aq) \), we see that the acid, symbolized by HA, donates a proton (H⁺) to water. This donation is what transforms HA into \( \mathrm{A}^- \), its conjugate base, as the proton is transferred to the water.

• The proton donation essentially starts the acid-base reaction.
• The original substance losing the proton (HA) is termed as the acid in the Bronsted-Lowry model.

Through this perspective, an acid is defined by its ability to donate protons to surrounding bases, completing its role in the acid-base interplay.

Proton Acceptance

Proton acceptance is the complementary process to proton donation in the Bronsted-Lowry theory, where a base accepts a proton. In this reaction scheme, the base provides a site for the proton to bind.Continuing with our example, in the equation \( \mathrm{HA}(aq) + \mathrm{H}_2\mathrm{O}(l) \rightarrow \mathrm{H}_3\mathrm{O}^+(aq) + \mathrm{A}^-(aq) \), water acts as a base by accepting a proton from HA. This results in the formation of \( \mathrm{H}_3\mathrm{O}^+ \), commonly known as the hydronium ion.

• Proton acceptance is crucial because it stabilizes the proton released by the acid.
• The substance that gains the proton (in this case, water) is considered a base.

This acceptance mechanism is vital in preserving the balance of reactions and maintaining the movement of protons throughout the system.

Conjugate Acid-Base Pair

In any acid-base reaction following the Bronsted-Lowry model, you'll encounter what's known as a conjugate acid-base pair. This pair consists of two species that transform into each other by the gain or loss of a proton.Looking back at our equation \( \mathrm{HA}(aq) + \mathrm{H}_2\mathrm{O}(l) \rightarrow \mathrm{H}_3\mathrm{O}^+(aq) + \mathrm{A}^-(aq) \), we can identify the conjugate pairs:- \( \mathrm{HA} \) and \( \mathrm{A}^- \) is one pair. When HA donates a proton, it becomes \( \mathrm{A}^- \), its conjugate base.- \( \mathrm{H}_2\mathrm{O} \) and \( \mathrm{H}_3\mathrm{O}^+ \) forms another pair. Once water gains a proton, it turns into \( \mathrm{H}_3\mathrm{O}^+ \), the conjugate acid.

• The acid part of the pair loses the proton and becomes the conjugate base.
• The base part gains the proton and shifts to its conjugate acid form.

Understanding conjugate acid-base pairs helps visualize the dynamic shifts during the reaction, illustrating how substances transition through proton exchanges.