Question

Classify each of the following as an Arrhenius acid, an Arrhenius base, or a salt: (a) \(\mathrm{H}_{2} \mathrm{SO}_{4}(a q)\) (b) \(\mathrm{CaSO}_{4}(a q)\) (c) \(\mathrm{Ca}(\mathrm{OH})_{2}(a q)\) (d) \(\mathrm{CaCO}_{3}(a q)\)

Short answer

(a) Acid, (b) Salt, (c) Base, (d) Salt.

Step-by-step solution

Identify the Characteristics of Each Substance

The goal here is to classify each substance as an Arrhenius acid, Arrhenius base, or salt. An Arrhenius acid produces hydrogen ions (H⁺) in aqueous solution. An Arrhenius base produces hydroxide ions (OH⁻) in aqueous solution. A salt typically does not produce significant amounts of H⁺ or OH⁻ ions in solution and results from the reaction of an acid and a base.

Classify \(\mathrm{H}_{2}\mathrm{SO}_{4}(aq)\) as Acid or Base

\(\mathrm{H}_{2}\mathrm{SO}_{4}(aq)\) is sulfuric acid, a well-known Arrhenius acid. In water, it dissociates to form \(\mathrm{H}^+\) ions: \(\mathrm{H}_{2}\mathrm{SO}_{4} \rightarrow 2\mathrm{H}^+ + \mathrm{SO}_{4}^{2-}\). Therefore, it is classified as an Arrhenius acid.

Classify \(\mathrm{CaSO}_{4}(aq)\) as Acid, Base, or Salt

\(\mathrm{CaSO}_{4}(aq)\) is calcium sulfate. In water, it does not produce \(\mathrm{H}^+\) or \(\mathrm{OH}^-\) ions. It is a neutral salt formed from the reaction of sulfuric acid and calcium hydroxide. Therefore, it is a salt.

Classify \(\mathrm{Ca(OH)}_{2}(aq)\) as Acid or Base

\(\mathrm{Ca(OH)}_{2}(aq)\) is calcium hydroxide. It is a classical Arrhenius base as it dissociates in water to produce \(\mathrm{OH}^-\) ions: \(\mathrm{Ca(OH)}_{2} \rightarrow \mathrm{Ca}^{2+} + 2\mathrm{OH}^-\). Therefore, it is classified as an Arrhenius base.

Classify \(\mathrm{CaCO}_{3}(aq)\) as Acid, Base, or Salt

\(\mathrm{CaCO}_{3}(aq)\) is calcium carbonate. It does not dissociate to produce \(\mathrm{H}^+\) or \(\mathrm{OH}^-\) ions in water directly, as it's relatively insoluble. It is a salt formed typically from the reaction of carbonic acid and calcium hydroxide or calcium oxide. Therefore, it is a salt.

Key concepts

Arrhenius Acid

The concept of an Arrhenius acid is one of the fundamental principles in chemistry. It was introduced by Svante Arrhenius in the late 19th century. An Arrhenius acid is defined as a substance that increases the concentration of hydrogen ions (\(\mathrm{H}^+\)) when dissolved in water. This increase in hydrogen ions leads to a decrease in the pH of the solution, making it acidic.

A common example is sulfuric acid (\(\mathrm{H}_{2}\mathrm{SO}_{4}\)). When sulfuric acid dissolves in water, it dissociates completely, releasing two hydrogen ions per molecule:

• \(\mathrm{H}_{2}\mathrm{SO}_{4} \rightarrow 2\mathrm{H}^+ + \mathrm{SO}_{4}^{2-}\)

These hydrogen ions are responsible for the acid's properties. Arrhenius acids are not only limited to strong acids like sulfuric acid but also include weaker acids that partially dissociate in water. Understanding this concept is essential for explaining reactions that involve proton transfer and for identifying substances as acids in aqueous solutions.

Arrhenius Base

Arrhenius bases are substances that increase the concentration of hydroxide ions (\(\mathrm{OH}^-\)) when dissolved in water. This was also part of Arrhenius's theory, formulated to explain the behavior of substances in aqueous solutions.

Arrhenius bases are essential for understanding the pH of solutions and how they can turn basic or alkaline. A quintessential example of an Arrhenius base is calcium hydroxide, \(\mathrm{Ca(OH)}_{2}\). In water, \(\mathrm{Ca(OH)}_{2}\) dissociates to produce:

• \(\mathrm{Ca(OH)}_{2} \rightarrow \mathrm{Ca}^{2+} + 2\mathrm{OH}^-\)

The release of hydroxide ions is what gives the solution its basic nature by raising the pH level. Arrhenius bases often feel slippery to the touch and can neutralize acids by reacting with \(\mathrm{H}^+\) ions to form water, demonstrating the classic acid-base reaction. This concept helps in identifying compounds that can be used to balance pH in various chemical and industrial processes.

Salts

Salts are ionic compounds formed from the neutralization reaction between an acid and a base. In a typical neutralization, the hydrogen ions (\(\mathrm{H}^+\)) from an acid react with hydroxide ions (\(\mathrm{OH}^-\)) from a base to form water, while the remaining ions combine to form a salt.

The formation of a salt may not always lead to a neutral pH, as the resulting solution's characteristics depend on the strengths of the original acid and base. For instance, calcium sulfate (\(\mathrm{CaSO}_{4}\)) and calcium carbonate (\(\mathrm{CaCO}_{3}\)) are both examples of salts. Calcium sulfate is the product of the reaction between sulfuric acid (\(\mathrm{H}_{2}\mathrm{SO}_{4}\)) and calcium hydroxide (\(\mathrm{Ca(OH)}_{2}\)), whereas calcium carbonate typically results from the combination of carbonic acid with calcium hydroxide. In aqueous solutions, these salts do not alter the concentration of \(\mathrm{H}^+\) or \(\mathrm{OH}^-\) ions significantly, remaining neutral in nature.

Salts play a crucial role in various applications, from flavoring foods to being fundamental components in batteries and other technologies. Recognizing and understanding salts provides clarity into many chemical processes and reactions.