Question

Write the following bases in either the ionized or the nonionized form to best represent an aqueous solution: (a) \(\mathrm{NaOH}(a q)\) (b) \(\mathrm{NH}_{4} \mathrm{OH}(a q)\) (c) \(\mathrm{Ba}(\mathrm{OH})_{2}(a q)\) (d) \(\mathrm{Al}(\mathrm{OH})_{3}(\mathrm{~s})\)

Short answer

(a) NaOH(aq): Na⁺(aq) + OH⁻(aq); (b) NH₄OH(aq); (c) Ba(OH)₂(aq): Ba²⁺(aq) + 2OH⁻(aq); (d) Al(OH)₃(s).

Step-by-step solution

Identify the Type of Substance

Start by identifying whether the given substance is a strong or weak base. Strong bases dissociate completely in water, while weak bases do not.

Write the Ionized Form for Strong Bases

For each strong base identified in Step 1, write the equation showing how the base dissociates into ions in water. Strong bases like NaOH and Ba(OH)2 dissociate completely. Thus, \( \text{NaOH(aq)} \) becomes \( \text{Na}^+(aq) + \text{OH}^-(aq) \), and \( \text{Ba(OH)}_2(aq) \) becomes \( \text{Ba}^{2+}(aq) + 2\text{OH}^-(aq) \).

Write the Non-Ionized Form for Weak Bases

For weak bases, do not dissociate completely, so represent them mainly in their molecular form. Ammonium hydroxide, \( \text{NH}_4\text{OH}(aq) \), is a common weak base and is best represented as \( \text{NH}_4\text{OH}(aq) \) since it doesn't dissociate fully in solution.

Consider the Solubility for Solids

If a base is not soluble or only sparingly soluble, like \( \text{Al(OH)}_3(s) \), it remains primarily in its solid non-ionized form in water. Therefore, under these conditions, it is best written as \( \text{Al(OH)}_3(s) \).

Key concepts

Strong Bases

Strong bases are substances that completely dissociate into their ions when dissolved in water. This means that every molecule of the base separates into its respective ions, which results in a high concentration of hydroxide ions (OH⁻) in the solution. A few well-known strong bases include sodium hydroxide (NaOH) and barium hydroxide (Ba(OH)₂). The complete dissociation of these bases is a defining characteristic, making them very effective in increasing the pH of a solution. For example, when sodium hydroxide is dissolved in water, it separates completely into sodium ions (Na⁺) and hydroxide ions (OH⁻). Similarly, barium hydroxide dissociates to form barium ions (Ba²⁺) and hydroxide ions.

In practice, strong bases are used extensively in industrial processes, laboratory experiments, and even in domestic cleaning products due to their ability to neutralize acids effectively.

Weak Bases

Weak bases, unlike strong bases, do not dissociate completely in water. This means that in an aqueous solution, many of the base molecules remain intact, and only a slight percentage release hydroxide ions (OH⁻). This partial ionization leads to a relatively lower pH compared to strong bases. Ammonium hydroxide (NH₄OH) is a typical example of a weak base. In water, it exists mostly as (NH₄OH), with only a small fraction breaking down into ammonium ions (NH₄⁺) and hydroxide ions.

The level of ionization of weak bases depends on their chemical structure and environmental factors like temperature. Understanding weak bases is crucial, especially when considering them in buffering solutions, where they help maintain a relatively stable pH despite additions of acids or bases.

Ionization

Ionization refers to the process by which a molecule breaks into ions, either completely or partially, when dissolved in a solvent like water. For strong bases such as sodium hydroxide, ionization is complete, resulting in distinct ions present in the solution. This is represented chemically by showing the molecules breaking down into their respective positive and negative ions.

Essentially, the extent of ionization determines the strength of the base. A strong base will fully ionize, providing a maximal number of hydroxide ions, whereas a weak base, as discussed, remains mostly in its molecular form. It's crucial for students to recognize that ionization is a reversible process, where the balance between molecular and ionic forms can shift depending on conditions like concentration and the presence of other chemicals.

Solubility of Bases

Solubility refers to how well a substance can dissolve in a solvent, forming a homogenous solution. For bases, solubility can vary widely. Strong bases such as sodium hydroxide are highly soluble in water, allowing them to dissociate fully and produce a strong basic solution. Meanwhile, the solubility of some bases like Al(OH)₃ is quite limited.

Aluminum hydroxide, for instance, is sparingly soluble in water, which means it does not completely dissolve and primarily remains in its solid form. This has implications in real-world applications; for example, the low solubility of Al(OH)₃ makes it suitable for use as an antacid, as it slowly neutralizes stomach acid. Understanding base solubility is key in predicting how bases behave in aqueous solutions and their potential for neutralizing acids effectively.