Question

What is an acid in the Bronsted-Lowry model? What is a base? What specific ion is the basis for the Bransted-Lowry model for acid-base reactions?

Short answer

In the Bronsted-Lowry model, an acid is a substance that donates a proton (H+) to another substance, and a base is a substance that accepts a proton (H+) from another substance. The specific ion that serves as the basis for this model in acid-base reactions is the hydrogen ion (H+). The model focuses on the transfer of the H+ ion between an acid and a base, forming conjugate acid and conjugate base pairs.

Step-by-step solution

Definitions of Acids and Bases in the Bronsted-Lowry Model

According to the Bronsted-Lowry theory, an acid is a substance that donates a proton (H+) to another substance, whereas a base is a substance that accepts a proton (H+) from another substance.

The Specific Ion in Bronsted-Lowry Acid-Base Reactions

The specific ion that is the basis for the Bronsted-Lowry model for acid-base reactions is the hydrogen ion (H+). This model focuses on the transfer of the H+ ion between an acid and a base during a chemical reaction. When an acid donates its H+ ion, it becomes a conjugate base, and when a base accepts the H+ ion, it becomes a conjugate acid.

Key concepts

Acid-Base Reactions

Understanding acid-base reactions in chemistry is integral to grasping how various substances interact in aqueous solutions. In the context of the Bronsted-Lowry acid-base theory, these reactions are viewed through the lens of proton (hydrogen ion, H+) transfer. An acid is characterized by its propensity to donate a proton to another substance, while a base is defined by its ability to accept a proton.

Consider the reaction between hydrochloric acid (HCl) and water (H2O). In this scenario, HCl acts as the acid, donating an H+ ion to H2O, which in turn acts as the base, accepting the H+ ion. As a result of this process, chloride ions (Cl-) and hydronium ions (H3O+) are formed. This exchange not only defines the nature of the reactants but also determines the products of the reaction.

Hydrogen Ion (H+)

The hydrogen ion (H+) is fundamental to the Bronsted-Lowry theory and is the specific ion that underpins all acid-base reactions in this model. It is essentially a hydrogen atom that has lost its electron, resulting in a positively charged ion. The unique nature of the H+ ion is its incredibly small size and high charge density, making it highly reactive. It does not exist freely in solution but is quickly hydrated to form the hydronium ion (H3O+).

In discussing acid strength, we often talk about a substance's ability to donate an H+ ion; the easier it can donate this ion, the stronger the acid. Conversely, the strength of a base is typically gauged by its ability to attract and hold on to an H+ ion. Understanding the behavior of H+ ions is crucial for predicting the outcome of acid-base reactions and the pH level of the resulting solution.

Conjugate Acid-Base Pairs

A concept that enhances our understanding of acid-base chemistry is that of conjugate acid-base pairs. A conjugate acid-base pair consists of two species that differ by the presence or absence of a proton. When an acid donates an H+ ion, the remainder of the molecule becomes its conjugate base. Similarly, when a base gains an H+ ion, it becomes its conjugate acid. These pairs are intimately linked because they originate from the same acid-base reaction and illustrate the reversible nature of such chemical processes.

To exemplify, if we return to our HCl and H2O reaction, the HCl, after donating an H+ ion, becomes Cl-, the conjugate base of HCl. Meanwhile, the H2O, after accepting the H+ ion, becomes H3O+, the conjugate acid of H2O. Thus, HCl/Cl- and H2O/H3O+ are two conjugate acid-base pairs. Recognizing these pairs is crucial in predicting the direction of the reaction and understands buffer solutions' capacity to resist changes in pH.