Question

Which of the following are Arrhenius bases? (a) \(\mathrm{H}_{3} \mathrm{~A} \mathrm{sO}_{4}\) (b) \(\mathrm{Ba}(\mathrm{OH})_{2}\) (c) \(\mathrm{HClO}\) (d) KOH

Short answer

The Arrhenius bases are \(\mathrm{Ba}(\mathrm{OH})_{2}\) and KOH.

Step-by-step solution

Understanding Arrhenius Bases

Arrhenius bases are compounds that, when dissolved in water, increase the concentration of hydroxide ions (OH^-).

Analyze \(\mathrm{H}_{3} \mathrm{~A} \mathrm{sO}_{4}\)

This is arsenic acid, which is an acid, not a base as it donates protons (H+).

Analyze \(\mathrm{Ba}(\mathrm{OH})_{2}\)

Barium hydroxide is an Arrhenius base because it dissociates in water to produce barium ions (Ba^{2+}) and hydroxide ions (OH^-). \[\mathrm{Ba}(\mathrm{OH})_{2} \rightarrow \mathrm{Ba}^{2+} + 2\mathrm{OH}^{-}\]

Analyze \(\mathrm{HClO}\)

Hypochlorous acid (HClO) dissociates in water to form hydrogen ions (H+) and hypochlorite ions (ClO^-), indicating it is an acid.

Analyze KOH

Potassium hydroxide (KOH) is an Arrhenius base because it dissociates in water to produce potassium ions (K+) and hydroxide ions (OH^-). \[\mathrm{KOH} \rightarrow \mathrm{K}^{+} + \mathrm{OH}^{-}\]

Key concepts

Dissociation of Bases

When a base dissolves in water, it undergoes a process called dissociation. This means the base separates into its individual ions. For instance, when \(\text{KOH}\) (potassium hydroxide) dissolves, it splits into \(\text{K}^+\) (potassium ions) and \(\text{OH}^-\) (hydroxide ions). The same happens with \(\text{Ba(OH)}_2\) (barium hydroxide), which dissociates into \(\text{Ba}^{2+}\) (barium ions) and \(\text{OH}^-\) ions. A base must produce \(\text{OH}^-\) ions in water to be considered an Arrhenius base. Understanding dissociation is crucial because it reveals how substances behave in water- how they break apart and what ions they form.

Hydroxide Ion Concentration

In the context of Arrhenius bases, we focus on the concentration of hydroxide ions (\(\text{OH}^-\)) produced when the base dissolves in water. The more \(\text{OH}^-\) ions generated, the stronger the base. For example, \(\text{Ba(OH)}_2\) produces twice as many \(\text{OH}^-\) ions as potassium hydroxide (\(\text{KOH}\)). This is because each \(\text{Ba(OH)}_2\) molecule dissociates into two \(\text{OH}^-\) ions. This concept helps us compare the strength of different bases:

• \(\text{KOH} \rightarrow \text{K}^+ + \text{OH}^-\)
• \(\text{Ba(OH)}_2 \rightarrow \text{Ba}^{2+} + 2\text{OH}^-\)

Knowing this allows us to predict how a base will affect the pH of a solution.

Acid-Base Theory

Arrhenius bases form a part of the broader acid-base theory. According to Svante Arrhenius, an acid is a substance that increases the concentration of hydrogen ions (\(\text{H}^+\)) in water, while a base increases the concentration of hydroxide ions (\(\text{OH}^-\)). This definition helps us categorize substances based on their behavior in water. For example:

• Arsenic acid (\(\text{H}_3\text{AsO}_4\)) increases \(\text{H}^+\) ions, so it's an acid.
• Hypochlorous acid (\(\text{HClO}\)) increases \(\text{H}^+\) ions, so it's also an acid.
• \(\text{KOH}\) and \(\text{Ba(OH)}_2\) increase \(\text{OH}^-\) ions, so they are bases.

This theory is fundamental for understanding chemical reactions and predicting the behavior of substances in different environments.