Question

What is an acid and what is a base? An acid-base reaction is sometimes called a proton-transfer reaction. Explain.

Short answer

An acid is a substance that can donate a proton (H⁺ ion) to another substance, while a base is a substance that can accept a proton (H⁺ ion) from another substance. A proton-transfer reaction is a chemical reaction that involves the transfer of a proton from one substance to another. In acid-base reactions, acids donate protons to bases, making these reactions a type of proton-transfer reaction. For example, in the reaction between hydrochloric acid (HCl) and ammonia (NH₃), HCl donates a proton to NH₃, forming ammonium (NH₄⁺) and chloride (Cl⁻) ions.

Step-by-step solution

Define an acid

An acid is a substance that can donate a proton (H⁺ ion) to another substance. Acids are known for their sour taste, like citric acid in lemons or acetic acid in vinegar. There are two main theories on defining acids: Arrhenius and Bronsted-Lowry. However, in both these theories, acids donate protons to other substances.

Define a base

A base is a substance that can accept a proton (H⁺ ion) from another substance. Bases have a bitter taste, like in baking soda (sodium bicarbonate) or ammonia. Similar to acids, there are two main theories for defining bases: Arrhenius and Bronsted-Lowry. In both these theories, bases accept protons from other substances.

Explain the term "proton-transfer reaction"

A proton-transfer reaction is a chemical reaction that involves the transfer of a proton (H⁺ ion) from one substance to another. In this type of reaction, one substance loses a proton while the other substance gains a proton.

Relate "proton-transfer reaction" to acid-base reactions

Acid-base reactions involve the transfer of protons from one substance (the acid) to another (the base). This is because, as we defined earlier, acids donate protons (H⁺ ions) and bases accept protons. Therefore, an acid-base reaction can also be described as a proton-transfer reaction. For example, consider the reaction between hydrochloric acid (HCl) and ammonia (NH₃): \(HCl + NH_3 \rightarrow NH_4^+ + Cl^-\) In this reaction, HCl (the acid) donates a proton to NH₃ (the base), forming ammonium (NH₄⁺) and chloride (Cl⁻) ions. Thus, an acid-base reaction occurred, involving the transfer of a proton from HCl to NH₃.

Key concepts

Proton-Transfer Reaction

A proton-transfer reaction is a fundamental concept in acid-base chemistry. It describes the process where a proton, which is a hydrogen ion (H⁺), is transferred from one molecule to another. This exchange is the hallmark of many chemical reactions in both the natural world and industrial processes.

Understanding proton-transfer reactions helps us see how molecules interact on a microscopic level. For example, in the reaction between hydrochloric acid (HCl) and ammonia (NH₃), the HCl donates a proton to the NH₃ molecule:

\[ HCl + NH_3 \rightarrow NH_4^+ + Cl^- \]

Here, the proton from HCl is transferred to NH₃, resulting in the formation of ammonium and chloride ions. This simple act of proton transfer underlines much of what makes chemical reactions work.

• Proton (H⁺) moves from one reactant to another.
• One substance acts as a donor (acid), and the other as a recipient (base).
• Essential in explaining acid-base reactions.

Bronsted-Lowry Theory

The Bronsted-Lowry theory expands on the nature of acids and bases. It defines acids specifically as proton donors and bases as proton acceptors. This definition allows us to classify a wide array of substances beyond classic "acidic" or "basic" materials.

This theory is quite versatile. Instead of being limited to reactions in water, it can apply to gas-phase reactions and those occurring in non-aqueous environments.

In the example of hydrochloric acid and ammonia, HCl donates a proton according to the Bronsted-Lowry theory, thus acting as the acid, while NH₃ accepts that proton, fulfilling the role of the base.

• More flexible definition than earlier theories.
• Applies to more types of chemical reactions.
• Broadens our understanding of what qualifies as an acid or base.

Arrhenius Theory

The Arrhenius theory gives us one of the earliest frameworks for understanding acids and bases. According to this theory, an acid is a substance that increases the concentration of hydrogen ions (H⁺) in an aqueous solution. Meanwhile, a base is something that increases the concentration of hydroxide ions (OH⁻).

This approach is practical for many common acid-base reactions, especially those occurring in water. For instance, when HCl is dissolved in water, it increases the concentration of H⁺ ions, characterizing it as an Arrhenius acid. Conversely, substances like sodium hydroxide (NaOH) increase OH⁻ ions, making them Arrhenius bases.

The Arrhenius theory is limited to aqueous solutions and does not account for non-aqueous environments. Still, it is a great foundational theory for beginners to get a grasp of how acids and bases operate in water.

• Strong focus on aqueous solutions.
• Simple yet effective framework for understanding acids and bases.
• Basis for further theories like Bronsted-Lowry.