Question

State the postulates of Arrhenius theory. Explain with examples. What are the limitations of Arrhenius theory?

Short answer

Short Answer: The Arrhenius theory has two main postulates: 1) the formation of ions when an electrolyte is dissolved in water, and 2) electrical conductivity of electrolytes due to the movement of ions. However, this theory has limitations, such as its narrow definition of acids and bases, which does not include substances that exhibit acidic or basic properties in non-aqueous solvents or without the presence of water, and it does not account for the proton transfer process during acid-base reactions.

Step-by-step solution

Postulate 1: Formation of Ions

According to Arrhenius theory, when an electrolyte (acid, base, or salt) is dissolved in water, it dissociates into positive and negative ions. For example, when sodium chloride (NaCl) is dissolved in water, it dissociates into sodium ions (Na+) and chloride ions (Cl-).

Postulate 2: Electrical Conductivity

The second postulate states that the electrical conductivity of electrolytes is due to the movement of ions. When an electric potential is applied across the electrolyte solution, the positive ions (cations) move toward the negative electrode (cathode), and the negative ions (anions) move toward the positive electrode (anode). This ion movement allows the conduction of electricity.

Example 1: Acids

Let's consider an example of an acid according to the Arrhenius theory. Hydrochloric acid (HCl) is an Arrhenius acid because it dissociates in water to form hydrogen ions (H+) and chloride ions (Cl-): HCl (aq) -> H+ (aq) + Cl- (aq)

Example 2: Bases

Sodium hydroxide (NaOH) is an example of an Arrhenius base. When it dissolves in water, it forms hydroxide ions (OH-) and sodium ions (Na+): NaOH (aq) -> Na+ (aq) + OH- (aq)

Limitations of Arrhenius Theory: Narrow Definition of Acids and Bases

The Arrhenius theory defines acids and bases as substances that produce hydrogen ions (H+) and hydroxide ions (OH-) when dissolved in water, respectively. This definition is too narrow and does not include substances that exhibit acidic or basic properties in non-aqueous solvents or without the presence of water. Examples of such substances include ammonia (NH3) and sulfur trioxide (SO3).

Limitations of Arrhenius Theory: Proton Transfer Process

The theory does not account for the proton transfer process that occurs during acid-base reactions, which is better explained by the Brønsted-Lowry and Lewis theories. It is important to understand that Arrhenius's ideas were groundbreaking for their time, but subsequent theories have expanded upon his initial ideas to provide a more comprehensive understanding of acids, bases, and their behavior in solution.

Key concepts

Electrolyte Dissociation

Arrhenius theory provides a fundamental explanation for the process of electrolyte dissociation, which is crucial for understanding how substances conduct electricity in solution. When an electrolyte, such as salt, acid, or base, dissolves in water, it separates into its constituent ions. For example, if we take table salt (sodium chloride, NaCl) and dissolve it in water, it dissociates into sodium (Na⁺) and chloride (Cl⁻) ions.

This process is vital because only free-moving ions in solution can carry an electric charge. Imagine pouring salt into a glass of water: before the salt dissolves, the water cannot conduct electricity. But once the salt dissociates into ions, the solution becomes capable of conducting an electric current, which is pivotal for numerous applications, including the functioning of batteries and the biological processes within our bodies.

Acids and Bases

The Arrhenius theory also classifies substances as acids or bases based on the types of ions they produce in water. According to this theory, an acid is a substance that releases hydrogen ions (H⁺) when dissolved in water. For instance, hydrochloric acid (HCl) in aqueous solution dissociates to form H⁺ and Cl⁻ ions.

In contrast, a base produces hydroxide ions (OH⁻) when dissolved. A common example is sodium hydroxide (NaOH), which yields OH⁻ and Na⁺ ions in solution. These definitions tie directly to the ability of acids and bases to neutralize each other: when an acid and base are mixed, the H⁺ and OH⁻ ions can combine to create water (H₂O), exemplifying their characteristic reactions.

Electrical Conductivity

Electrical conductivity in solutions is intimately linked with the presence of dissolved ions, as articulated in the Arrhenius theory. When an electrolyte solution is subject to an electric potential, the ions move towards electrodes of opposite charge: positive ions (cations) towards the cathode, and negative ions (anions) towards the anode.

This movement of ions is what enables the solution to carry an electric current. Without free-moving ions, such as in pure water or in a solid salt crystal, electrical conductivity would be extremely low or non-existent. Thus, the degree of dissociation of an electrolyte in water significantly impacts the solution's ability to conduct electricity.

Limitations of Arrhenius Theory

Although the Arrhenius theory was a monumental step forward in the study of acids, bases, and electrolytes, it does have its limitations. One major restriction is the requirement for substances to be dissolved in water to be classified as acids or bases. This narrow definition excludes acidic or basic behavior in non-aqueous solutions. For example, ammonia (NH₃) acts as a base in the absence of water, but Arrhenius's definition does not account for this.

Moreover, the theory overlooks the proton transfer processes that are central to many acid-base reactions, such as those described by the Brønsted-Lowry theory. Instead, it focuses on the formation of ions, which, while useful, does not tell the full story of acid-base chemistry.