Problem 11 - Chemistry The Science in Context

Question

In an aqueous solution of HF, which compound acts as a Bronsted-Lowry acid and which is the Bronsted-Lowry base?

Step-by-Step Solution

Answer

Answer: The Bronsted-Lowry acid in an aqueous solution of HF is HF, and the Bronsted-Lowry base is H2O.

Step 1: Write the reaction for the dissociation of HF in water

First, we need to write the reaction for the dissociation of HF in water. This will help us identify the substances involved in the reaction and their roles as a Bronsted-Lowry acid or base. HF (aq) + H2O (l) ⇌ H3O+ (aq) + F- (aq)

Step 2: Identify the proton donor and the proton acceptor

Now that we have the reaction, we need to identify the proton donor (acid) and the proton acceptor (base). An important key is to look for any hydrogen ion (H+) transfers. In the reaction, HF donates a proton (H+) to H2O, and H2O accepts the proton. Thus, we can identify the following: - Bronsted-Lowry acid: HF - Bronsted-Lowry base: H2O

Step 3: Match the substances with their roles as acid or base

Based on the above analysis, we can conclude that in an aqueous solution of HF: - The Bronsted-Lowry acid is HF, since it donates a proton (H+) to H2O. - The Bronsted-Lowry base is H2O, since it accepts a proton (H+) from HF.

Key Concepts

Chemical Dissociation

Chemical dissociation is the process by which a compound separates into smaller molecules, ions, or radicals, usually in a reversible manner. In the context of acid-base chemistry, dissociation refers to the splitting of a compound in an aqueous solution to form ions. A classic example is the dissociation of hydrofluoric acid (HF) into fluoride ions (F-) and hydrogen ions (H+ or protons). This reaction can be represented as:
HF (aq) ⇌ H+ (aq) + F- (aq)
In solution, the degree of dissociation is influenced by the strength of the acid or base. Strong acids and bases dissociate completely, while weak ones, like HF, only partially dissociate, establishing a dynamic equilibrium between the undissociated species and the resultant ions.

Proton Transfer Reactions

A proton transfer reaction is a type of chemical reaction where a hydrogen ion, which is simply a proton, is transferred between two chemical species. This transfer defines the Bronsted-Lowry concept of acids and bases, where an acid is a proton donor and a base is a proton acceptor. Taking our previous HF example, when it reacts with water, HF donates a proton to the water molecule:
HF (aq) + H2O (l) ⇌ H3O+ (aq) + F- (aq)
The water molecule, H2O, accepts the proton, forming hydronium ion (H3O+). This reaction is essential to understanding the roles of different substances in acid-base reactions. The species that gains the proton, in this case water, temporarily acts as a Bronsted-Lowry base.

Acid-Base Reactions

Acid-base reactions are a subset of chemical reactions involving the transfer of protons between reacting molecules. These reactions are crucial to understand because they are widespread in chemistry and biology. As demonstrated with HF dissolving in water:
HF (aq) + H2O (l) ⇌ H3O+ (aq) + F- (aq)
In this reaction, HF acts as the Bronsted-Lowry acid, donating a proton, while water acts as the base, accepting the proton. The corresponding conjugate base (F-) and acid (H3O+) are formed as a result. Acid-base reactions are also characterized by their equilibrium, where the rate of the forward reaction is equal to the rate of the reverse reaction, a concept particularly important for weak acids and bases.

Aqueous Solutions

Aqueous solutions are mixtures where water is the solvent. In the realm of acid-base chemistry, aqueous solutions are the media in which the dissociation of acids and bases and the subsequent proton transfer reactions occur. Water's ability to act as both an acid and a base, a property called amphiprotic, makes it a unique solvent for these reactions. In the HF example, water serves as the solvent and participates actively as a reactant:
HF (aq) + H2O (l) ⇌ H3O+ (aq) + F- (aq)
The properties of aqueous solutions, such as pH, conductivity, and reactivity, directly depend on the nature and concentration of the dissolved acids or bases, making the study of these solutions fundamental in both theoretical and practical chemistry.