Question

According to the Bronsted-Lowry model, an acid is a "proton donor" and a base is a "proton acceptor." Explain.

Short answer

In the Bronsted-Lowry model, an acid is a proton donor because it has the ability to donate a proton (hydrogen ion, \(H^{+}\)) during a chemical reaction. A base, on the other hand, is a proton acceptor because it can accept a proton during a chemical reaction. The exchange of protons between an acid and a base allows chemists to understand, predict, and study the outcomes of reactions involving acids and bases. For example, water (\(H_{2}O\)) can act as both an acid and a base, demonstrating the versatility of the Bronsted-Lowry model.

Step-by-step solution

Understand Protons

Protons are subatomic particles found in an atom's nucleus and have a positive charge. In chemistry, a proton can be represented by the \(H^{+}\) ion (hydrogen ions).

Define Bronsted-Lowry Model

The Bronsted-Lowry model is a theory of acids and bases that was independently proposed by Johannes Bronsted and Thomas Lowry in 1923. This theory defines acids as "proton donors" and bases as "proton acceptors."

Understand Proton Donor

A proton donor is a species that can donate a proton (hydrogen ion, \(H^{+}\)) to another species during a chemical reaction. Acids in Bronsted-Lowry theory have the ability to donate a proton to another molecule upon forming a bond.

Understand Proton Acceptor

A proton acceptor, on the other hand, is a species that can accept a proton (hydrogen ion, \(H^{+}\)) from another species during a chemical reaction. Bases in the Bronsted-Lowry model have the ability to accept a proton from another molecule upon forming a bond.

Bronsted-Lowry Acid-Base Reactions

In a chemical reaction involving a Bronsted-Lowry acid and base, the acid donates a proton to the base. As a result, the acid becomes a conjugate base (the species that remains after the acid has lost its proton), and the base becomes a conjugate acid (the species formed after the base has accepted the proton). This exchange of protons is at the root of the Bronsted-Lowry definition of acids and bases.

Example: Water as both Acid and Base

Water (\(H_{2}O\)) can act as both a Bronsted-Lowry acid and a base. For example, when water molecules interact with ammonia (\(NH_{3}\)), water donates a proton and acts as an acid, while ammonia accepts the proton and acts as a base: \[ H_2O + NH_3 \rightarrow NH_4^+ + OH^- \] However, in the presence of hydrochloric acid (\(HCl\)), water accepts a proton and behaves as a base while \(HCl\) donates a proton and acts as an acid: \[ H_2O + HCl \rightarrow H_3O^+ + Cl^- \] In conclusion, according to the Bronsted-Lowry model, an acid is considered a proton donor because it can donate a proton during a chemical reaction, while a base is considered a proton acceptor because it can accept a proton during a chemical reaction. This allows chemists to understand the behavior of substances in various reactions, predict the outcomes of a reaction and further understand the properties of acids and bases.

Key concepts

Proton Donor

In the Bronsted-Lowry model, a proton donor plays the central role of an acid in a chemical reaction. The term "proton donor" essentially refers to any molecule that can release a proton, or more specifically, a hydrogen ion denoted as \(H^+\). This ability to donate a proton is what characterizes a substance as an acid according to this theory.

When an acid donates a proton to another substance, the resulting reaction alters the acid, typically transforming it into what is called its conjugate base. This is the substance left behind once the acid has given away its proton. It holds great relevance in chemical reactions because this transformation leads to the acid no longer being able to act in its initial capacity as a proton donor.

• The acid must be stable enough to easily lose a proton; for instance, hydrochloric acid (\(HCl\)) readily donates a proton to form chloride (\(Cl^-\)).
• Proton donation is often reversible, suggesting acids have an inherent readiness to reaccept protons under suitable conditions.

Proton Acceptor

The concept of a proton acceptor is equally vital when discussing the Bronsted-Lowry model. Bases are defined by their ability to receive a proton during their interaction with acids, thus playing the complementary role to proton donors. Therefore, a proton acceptor is any species capable of accepting a \(H^+\) ion.

This transformative process involves the base receiving a proton and converting into its conjugate acid. This change in structure grants the molecule new properties and influences its role in a chemical system. Bases like ammonia (\(NH_3\)) are classic examples of proton acceptors, converting into ammonium \(NH_4^+\) upon accepting a proton from water or another acid.

• Proton acceptance generally results in a slight positive charge increasing on the accepting molecule, such as the formation of hydronium \(H_3O^+\) in water.
• The concept also highlights the dynamic interplay between molecules, emphasizing the reversible nature of many chemical reactions where bases can once again act as donors under the right conditions.

Conjugate Acid-Base Pairs

A crucial aspect of the Bronsted-Lowry model is its focus on conjugate acid-base pairs. Understanding this relationship allows scientists to better predict and control reactions. A conjugate acid-base pair is a couple of chemical species that transform into each other by the gain or loss of a proton.

This dual relationship means any Bronsted-Lowry acid has a corresponding "conjugate base," as already stated. Likewise, any base becomes its specific "conjugate acid" when it accepts a proton. These pairs are always seen together in reactions because they showcase the reversible and interchangeable nature of proton transfer.

• An example of a conjugate acid-base pair is the hydrogen sulfate ion (\(HSO_4^-\), conjugate base) and sulfuric acid (\(H_2SO_4\), conjugate acid).
• Conjugate pairs provide insights about equilibrium states in reactions because one can visualize the direction of proton exchange, thus predicting concentrations of reactants and products over time.