Question

Define Brónsted acids and bases. Give an example of a conjugate pair in an acid-base reaction.

Short answer

Brönsted acids donate protons, and bases accept them. Example: NH₄⁺/NH₃.

Step-by-step solution

Understand Brönsted Acids

A Brönsted acid is defined as a substance that donates a proton (H⁺) in a chemical reaction. This proton donation is what characterizes a Brönsted acid.

Understand Brönsted Bases

A Brönsted base is a substance that accepts a proton (H⁺) in a chemical reaction. The acceptance of a proton is what distinguishes a Brönsted base.

Recognize Conjugate Acid-Base Pairs

In an acid-base reaction, a Brönsted acid will lose a proton to become its conjugate base, and a Brönsted base will gain a proton to become its conjugate acid.

Example of a Conjugate Pair

Consider the reaction of ammonia with water: NH₃ + H₂O ↔ NH₄⁺ + OH⁻. Here, NH₃ (ammonia) is the Brönsted base, accepting a proton to become NH₄⁺, and H₂O (water) is the Brönsted acid, donating a proton to become OH⁻. NH₄⁺ and NH₃ are a conjugate pair, as are OH⁻ and H₂O.

Key concepts

Proton Donation

Brönsted acids are special because they can donate a proton, which is a hydrogen ion ( H^+ ). When a chemical substance donates a proton during a reaction, it acts as a Brönsted acid. This process transforms the acid

• from its original form
• into its conjugate base

by shedding a proton. This donation of a proton is a key step in many chemical reactions and helps in understanding how acids behave. For example, in the reaction of hydrochloric acid (HCl) with water, HCl donates a proton to H₂O. This transforms HCl into its conjugate base, Cl⁻.

Proton Acceptance

Brönsted bases play the opposite role compared to acids. They are substances that accept protons in chemical reactions. When a base accepts a proton, it becomes

• a larger molecule or ion
• designated as a conjugate acid

This acceptance of protons characterizes Brönsted bases. For instance, in the reaction with ammonia ( NH₃ ), ammonia acts as a Brönsted base. By accepting a proton from water, ammonia becomes NH₄⁺ , its conjugate acid. This acceptance process is essential to numerous reactions, particularly those involving neutralization and balancing pH.

Conjugate Acid-Base Pairs

In an acid-base reaction, conjugate acid-base pairs are the results of proton donation and acceptance. When an acid donates a proton, it becomes its conjugate base. Conversely, when a base accepts a proton, it becomes its conjugate acid. Each pair consists of two species linked by the gain or loss of a single proton.
For example, when water donates a proton to ammonia in a reaction, ammonia receives it to form ammonium ( NH₄⁺ ). Here, NH₄⁺ and NH₃ are a conjugate acid-base pair. Similarly, when water becomes hydroxide ( OH⁻ ) after donating a proton, OH⁻ and H₂O form another conjugate pair. These pairs help in understanding reaction mechanisms and the nature of the substances involved.

Acid-Base Reaction Example

A classic example of an acid-base reaction involves ammonia ( NH₃ ) and water ( H₂O ). This reaction can be shown as:

• Reactants: NH₃ + H₂O
• Products: NH₄⁺ + OH⁻

In this reaction, ammonia acts as a Brönsted base by accepting a proton from water. Water, acting as a Brönsted acid, donates a proton to ammonia, resulting in the formation of ammonium ( NH₄⁺ ) and hydroxide ( OH⁻ ).
These products form the conjugate acid ( NH₄⁺ ) and conjugate base ( OH⁻ ) of the original reactants. This example illustrates proton transfer, showcasing the interactions between acids and bases, and the equilibrium that can exist between these dynamic components.