Arrhenius Acids and Bases: Videos & Practice Problems

Acid–base chemistry is central to understanding reactions in aqueous solutions. An acid, as defined by Arrhenius, is a substance that releases H⁺ ions in water, exemplified by hydrochloric acid which dissociates into H⁺ and Cl^- ions. Conversely, an Arrhenius base produces OH^- ions in water, like potassium hydroxide which separates into K⁺ and OH^- ions. This framework, however, is applicable only to reactions in aqueous solutions, limiting its scope. While the Arrhenius definition provides a foundational understanding of acid–base reactions, it does not encompass all scenarios where acids and bases interact, highlighting the need for broader definitions to fully grasp the complexity of acid–base behavior.

Arrhenius Acids and Bases

The concept of acids and bases has evolved significantly since the initial definitions proposed by Swedish chemist Svante Arrhenius in the 1800s. According to Arrhenius, an acid is defined as a compound that produces hydrogen ions (H⁺) when dissolved in water. A prime example of this is hydrobromic acid (HBr), which, when dissolved in an aqueous solution, dissociates to release H⁺ ions. This characteristic of generating H⁺ ions is what classifies hydrobromic acid as an Arrhenius acid.

In contrast, an Arrhenius base is a compound that produces hydroxide ions (OH^-) in water. For instance, potassium hydroxide (KOH) dissolves in water to yield potassium ions (K⁺) and hydroxide ions (OH^-). The production of OH^- ions is what qualifies potassium hydroxide as an Arrhenius base.

While the Arrhenius definitions provide a straightforward understanding of acids and bases, they are limited to aqueous solutions. This means that not all acid-base reactions occur in water, and therefore, these definitions do not encompass all types of acids and bases. As a result, while the Arrhenius definitions are foundational, they are not the most comprehensive framework for understanding acid-base chemistry.

Acid-Base Dissociation Example

In the context of Arrhenius theory, nitric acid (HNO₃) and aluminum hydroxide (Al(OH)₃) can be represented in their dissociation equations as follows:

When nitric acid is dissolved in water, it dissociates to produce hydrogen ions and nitrate ions:

HNO₃(aq) → H⁺(aq) + NO₃^-(aq)

This reaction illustrates that nitric acid acts as an Arrhenius acid because it increases the concentration of H⁺ ions in solution.

On the other hand, aluminum hydroxide dissociates in water to yield aluminum ions and hydroxide ions:

Al(OH)₃(s) → Al³⁺(aq) + 3 OH^-(aq)

This equation demonstrates that aluminum hydroxide functions as an Arrhenius base, as it increases the concentration of OH^- ions in the solution.

In summary, the dissociation of nitric acid produces H⁺ ions, confirming its role as an acid, while the dissociation of aluminum hydroxide produces OH^- ions, confirming its role as a base.

Out of the following compounds, which can only be an Arrhenius acid?
a) NaCl
b) Sr(OH)₂
c) H₂O
d) CH₃COOH
e) NH₃

• A. NaCl
• B. Sr(OH)₂
• C. H₂O
• D. CH₃COOH
• E. NH₃

Answer: (D)

Which of the following is an Arrhenius base?
a) Mg(OH)₂
b) BaO
c) CH₃NH₂
d) NH₄⁺
e) H₂SO₄

• A. Mg(OH)₂
• B. BaO
• C. CH₃NH₂
• D. NH₄⁺
• E. H₂SO₄

Answer: (A)

Here’s what students ask on this topic:

Which statement describes the Arrhenius interpretation of acids and bases?

The Arrhenius interpretation of acids and bases is a chemical concept that defines acids and bases in terms of their ability to produce ions in aqueous solutions. According to the Arrhenius definition, an acid is a substance that, when dissolved in water, increases the concentration of hydrogen ions (H⁺), which are often represented as hydronium ions (H₃O⁺). On the other hand, a base is defined as a substance that, when dissolved in water, increases the concentration of hydroxide ions (OH^-). This interpretation is one of the earliest and focuses on the presence of these ions as the defining characteristic of acidic and basic properties in a solution. It is important to note that the Arrhenius definition is limited to aqueous solutions and does not account for acid-base reactions that occur in non-aqueous environments.

How can Arrhenius acids and bases be identified?

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Arrhenius acids and bases can be identified by their behavior in water. An Arrhenius acid is a substance that, when dissolved in water, increases the concentration of hydrogen ions (H<sup>+</sup>). In other words, it donates H<sup>+</sup> ions to the solution. Common examples of Arrhenius acids include hydrochloric acid (HCl), sulfuric acid (H<sub>2</sub>SO<sub>4</sub>), and nitric acid (H<sub>2</sub>NO<sub>3</sub>).

On the other hand, an Arrhenius base is a substance that increases the concentration of hydroxide ions (OH<sup>-</sup>) when dissolved in water. This means it produces OH<sup>-</sup> ions in the solution. Typical examples of Arrhenius bases are sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)<sub>2</sub>).

To summarize, an Arrhenius acid produces H<sup>+</sup> ions in water, while an Arrhenius base produces OH<sup>-</sup> ions in water. These changes in ion concentrations are what alter the pH of the solution and characterize the substance as either an acid or a base according to the Arrhenius definition.

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How can Arrhenius acids be identified?

Arrhenius acids can be identified by their chemical behavior in water. According to the Arrhenius definition, an acid is a substance that, when dissolved in water, increases the concentration of hydrogen ions (^H+) in the solution. This is often observed through the substance's ability to donate a proton (^H+) to water molecules, resulting in the formation of hydronium ions (_H3O⁺).

One way to recognize an Arrhenius acid is by its chemical formula. Acids typically start with hydrogen, like hydrochloric acid (HCl), sulfuric acid (_H2SO₄), and nitric acid (HNO₃). When these acids dissolve in water, they release ^H+ ions. For example, HCl in water dissociates to form ^H+ and Cl^- ions.

Another method is by using indicators or pH paper. Arrhenius acids will turn blue litmus paper red and will result in a pH less than 7 on pH paper, indicating an acidic solution. Additionally, these acids often have a sour taste and can react with metals to produce hydrogen gas, but these are not safe methods for identification in a lab setting.

How can an Arrhenius base be identified?

An Arrhenius base can be identified by its ability to release hydroxide ions (OH^-) when dissolved in water. This release of hydroxide ions increases the concentration of these ions in the solution, making it basic or alkaline. A classic example of an Arrhenius base is sodium hydroxide (NaOH). When NaOH is dissolved in water, it dissociates into sodium ions (Na⁺) and hydroxide ions (OH^-), the latter being responsible for the basic nature of the solution. This concept is fundamental to understanding acid-base chemistry and is characterized by the following general reaction:

Base (B) + H₂O → BH⁺ + OH^-

The presence of hydroxide ions is key, so when you add an Arrhenius base to water and it increases the pH (towards a value greater than 7), you can confirm its basic nature. Remember that not all bases are Arrhenius bases; this definition is specific to those that produce hydroxide ions in aqueous solutions.