Question

Classify each of the following Arrhenius bases as strong or weak given the degree of ionization: (a) lithium hydroxide, \(\mathrm{LiOH}(a q), \sim 100 \%\) (b) calcium hydroxide, \(\mathrm{Ca}(\mathrm{OH})_{2}(a q), \sim 100 \%\) (c) barium hydroxide, \(\mathrm{Ba}(\mathrm{OH})_{2}(a q), \sim 100 \%\) (d) ammonium hydroxide, \(\mathrm{NH}_{4} \mathrm{OH}(a q), \sim 1 \%\)

Short answer

(a) Strong, (b) Strong, (c) Strong, (d) Weak.

Step-by-step solution

Understand 100% Ionization

If a base is nearly 100% ionized in solution, it means it dissociates completely into its ions. For Arrhenius bases, this is indicative of a strong base because the base fully dissociates to release hydroxide ions (OH⁻) into the solution.

Analyze Strong Bases

Check each base given: (a) Lithium hydroxide, \(\mathrm{LiOH}(a q)\) is ~100% ionized. A strong base.(b) Calcium hydroxide, \(\mathrm{Ca(OH)_{2}}(a q)\) is ~100% ionized. A strong base.(c) Barium hydroxide, \(\mathrm{Ba(OH)_{2}}(a q)\) is ~100% ionized. A strong base.

Understand 1% Ionization

If a base is only ~1% ionized in solution, it means it does not dissociate completely, and only a small fraction of it forms ions. For Arrhenius bases, this indicates a weak base because it releases fewer hydroxide ions (OH⁻) into the solution.

Analyze Weak Base

For the base: (d) Ammonium hydroxide, \(\mathrm{NH_{4}OH}(a q)\) is ~1% ionized. A weak base.

Key concepts

Strong Base

A strong base is a substance that completely dissociates in water, releasing hydroxide ions and corresponding cations.
These bases ionize completely, meaning they separate into their ions almost 100% of the time when dissolved in water. For example, lithium hydroxide LiOH(aq), calcium hydroxide Ca(OH)_2(aq), and barium hydroxide Ba(OH)_2(aq) are strong bases because they nearly fully ionize in water. They have a high concentration of hydroxide ions, OH⁻, in the solution.
Strong bases are highly effective at increasing the pH of a solution due to their full ionization. In practical terms, this means they are effective for applications requiring a high pH environment.

Weak Base

A weak base is one that does not ionize completely in an aqueous solution. These bases only partially dissociate into ions, contributing fewer hydroxide ions to the solution.
For instance, ammonium hydroxide NH_4OH(aq) is a weak base, as it ionizes to a mere 1%. Weak bases like this exist in equilibrium between the un-dissociated base and the ions formed.
Due to their low ionization, weak bases have a lower concentration of hydroxide ions compared to strong bases. This results in a less drastic increase in the solution's pH.

Ionization

Ionization refers to the process by which compounds break apart to form ions in a solution. In the context of Arrhenius bases, ionization involves a base dissociating into hydroxide ions and a corresponding cation.
The degree of ionization is crucial to determine whether a base is strong or weak. Bases that ionize nearly 100%, like lithium hydroxide, calcium hydroxide, and barium hydroxide, are classified as strong bases. They contribute a high concentration of hydroxide ions to the solution.
Conversely, a base with a low ionization degree, such as ammonium hydroxide, classifies as a weak base.

Hydroxide Ions

Hydroxide ions, represented as OH⁻, play a significant role in the chemistry of bases. When a base dissolves in water, it releases hydroxide ions into the solution. These ions are responsible for the increase in pH, making the solution more basic or alkaline.
The concentration of OH⁻ ions directly influences how basic a solution becomes. A strong base, with its full ionization, contributes a large number of hydroxide ions. This leads to a higher pH level.
On the other hand, a weak base releases significantly fewer hydroxide ions, resulting in a relatively lower pH increase. Understanding hydroxide ions is key to grasping the behavior and strength of different bases.