Arrhenius Acids and Bases - Online Tutor, Practice Problems & Exam Prep

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 vs Bases

Now, the general definition of acids and bases was proposed by the Swedish chemist Esvante Arrhenius in the 1800s. According to him, an Arrhenius acid was a compound that produces H+ ions when dissolved in water. Here we have a prime example of hydrochloric acid. Hydrochloric acid exists in an aqueous solution, meaning it's dissolved in water. When it's dissolved in water, it produces H+ ion plus Cl- ion. The fact that we create H+ is why hydrochloric acid itself is an Arrhenius acid.

Now with an Arrhenius acid, we have an Arrhenius base. An Arrhenius base is a compound that produces OH- ions when dissolved in water. Here we have potassium hydroxide. When it's placed in water, it dissolves into K+, so potassium ion plus the hydroxide ion. Because the hydroxide ion is produced, that's what makes potassium hydroxide an Arrhenius base.

Now this is a quick and easy way to understand acids, but it does have its own limitations. We're going to say here the Arrhenius definition of acids and bases is limited to aqueous solutions. Now it's hard to believe, but not all acids and bases reactions happen within aqueous solution. And for those, these definitions would not fit.

So although the Arrhenius definition of acids and bases is the first type of definition people rely on or recall, it is not the most inclusive in terms of all types of acids and bases. OK, but now that we've talked about this, let's move on to the next video where we take a look at an example problem.

Arrhenius Acids and Bases Example

Write a dissociation equation showing how nitric acid and aluminum hydroxide act as Arrhenius acid and base respectively. Right. So for nitric acid we have HNO₃ aqueous since it's being dissolved in water. When it's dissolved in water it's going to produce here an H⁺ ion and a nitrate ion. The fact that we're making an H⁺ ion is why it is an acid, Arrhenius acid. And then we have aluminum hydroxide. With aluminum hydroxide, it would dissolve into the aluminum ion, aqueous, plus we have 3 hydroxide ions being produced. So, this is what shows how aluminum hydroxide represents an Arrhenius base.

Equations:

HNO 3 (aq) → H + (aq) + NO 3 - (aq) Al (OH) 3 (s) → Al 3+ (aq) + 3 OH - (aq)

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

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

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.