Question

Which are classified as strong bases when dissolved in water? (a) \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N},\) (b) \(\mathrm{Ba}(\mathrm{OH})_{2}\), (c) KOH, (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) (e) \(\mathrm{Cs}_{2} \mathrm{O}\) (f) \(\mathrm{N}_{2} \mathrm{O}_{5}\)

Short answer

The strong bases in the list when dissolved in water are: (b) \(\mathrm{Ba}(\mathrm{OH})_{2}\), (c) KOH, and (e) \(\mathrm{Cs}_{2} \mathrm{O}\).

Step-by-step solution

Definition of a Strong Base

Understand that a strong base is a compound that completely dissociates in water into its ion components, typically containing hydroxide ions (OH-).

Classify Each Compound

Evaluate each compound to determine if it completely dissociates into hydroxide ions and a metal ion when dissolved in water.

Compound Analysis (a) - \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N}\)

Pyridine (\(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{N}\)) is a weak base, not a strong base, because it does not completely dissociate in water.

Compound Analysis (b) - \(\mathrm{Ba}(\mathrm{OH})_{2}\)

Barium hydroxide (\(\mathrm{Ba}(\mathrm{OH})_{2}\)) is a strong base as it completely dissociates into \(\mathrm{Ba}^{2+}\) and \(2 \mathrm{OH}^{-}\) ions in solution.

Compound Analysis (c) - KOH

Potassium hydroxide (KOH) is a strong base as it completely dissociates into \(\mathrm{K}^{+}\) and \(\mathrm{OH}^{-}\) ions in solution.

Compound Analysis (d) - \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\)

Aniline (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\)) is a weak base as it does not fully dissociate in water.

Compound Analysis (e) - \(\mathrm{Cs}_{2} \mathrm{O}\)

Cesium oxide (\(\mathrm{Cs}_{2} \mathrm{O}\)) reacts with water to form cesium hydroxide (\(\mathrm{CsOH}\)), which fully dissociates into \(\mathrm{Cs}^{+}\) and \(\mathrm{OH}^{-}\) making it a strong base.

Compound Analysis (f) - \(\mathrm{N}_{2} \mathrm{O}_{5}\)

Dinitrogen pentoxide (\(\mathrm{N}_{2} \mathrm{O}_{5}\)) is not a base at all; it is an acidic oxide that forms nitric acid in water.

Key concepts

Acid-Base Dissociation

Understanding acid-base dissociation is essential for figuring out why certain substances are considered strong acids or bases. Dissociation is the process by which molecules split into smaller particles. In the context of acids and bases dissolved in water, dissociation refers specifically to the separation of hydrogen ions (\textbf{H}\(^{+}\)) from acids and hydroxide ions (\textbf{OH}\(^{-}\)) from bases.

When a strong base dissolves in water, it dissociates completely, which means there are no intact molecules of the base left in solution. The complete dissociation maximizes the concentration of \textbf{OH}\(^{-}\) ions, leading to a higher pH level of the solution. The pivotal point here is that 'complete dissociation' is the key characteristic that defines a strong base, contrasting with weak bases which only partially dissociate, leaving a mixture of intact molecules and ions.

Hydroxide Ions in Solution

Hydroxide ions (\textbf{OH}\(^{-}\)) are the hallmark of basicity in aqueous solutions. When strong bases dissolve in water, they release these ions, which are then free to react. The presence and concentration of hydroxide ions are what give a solution its basic properties.

For example, barium hydroxide (\textbf{Ba}(\textbf{OH})\(_{2}\)) and potassium hydroxide (KOH), when dissolved, release \textbf{OH}\(^{-}\) ions into the solution, leading to an increased pH - a measure of how basic a solution is. The more hydroxide ions present, the higher the pH and the stronger the base. In the case of cesium oxide (\textbf{Cs}\(_{2}\)\textbf{O}), it reacts with water to create cesium hydroxide, which then fully dissociates into \textbf{Cs}\(^{+}\) and \textbf{OH}\(^{-}\), indicating strong base behavior. Understanding the role of hydroxide ions helps in predicting the pH of solutions and the strength of the base.

Classification of Bases

Bases can be categorized based on their strength - how well they dissociate in water to produce hydroxide ions. Strong bases, such as \textbf{Ba}(\textbf{OH})\(_{2}\), KOH, and \textbf{CsOH} (from \textbf{Cs}\(_{2}\)\textbf{O}), dissociate completely, making them very effective at raising the pH of a solution.

On the other hand, compounds like pyridine (\textbf{C}\(_{5}\)\textbf{H}\(_{5}\)N) and aniline (\textbf{C}\(_{6}\)\textbf{H}\(_{5}\)\textbf{NH}\(_{2}\)) do not dissociate completely and are thus considered weak bases. They are only able to partially increase the pH of a solution. Lastly, substances like dinitrogen pentoxide (\textbf{N}\(_{2}\)\textbf{O}\(_{5}\)) that do not produce hydroxide ions but rather produce acidic solutions are not classified as bases at all. Classifying the strength of bases is critical for predicting the outcomes of acid-base reactions and for understanding the behavior of substances in various chemical environments.