Question

Classify each base as strong or weak. (a) \(\mathrm{LiOH}\) (b) \(\mathrm{NH}_{4} \mathrm{OH}\) (c) \(\mathrm{Ca}(\mathrm{OH})_{2}\) (d) \(\mathrm{NH}_{3}\)

Short answer

LiOH and Ca(OH)2 are strong bases; NH4OH and NH3 are weak bases.

Step-by-step solution

Identify the Nature of the Bases

To classify each base as strong or weak, consider the solubility of hydroxides in water and the tendency of the substance to dissociate completely into its ions.

Classifying Lithium Hydroxide

Lithium Hydroxide (LiOH) is a soluble metal hydroxide and soluble hydroxides of Group 1 elements are strong bases. Therefore, LiOH is a strong base.

Classifying Ammonium Hydroxide

Ammonium Hydroxide (NH4OH) is not a strong base, as it is the aqueous form of ammonia which has a weak tendency to dissociate in water. It is a weak base.

Classifying Calcium Hydroxide

Calcium Hydroxide (Ca(OH)2) is a soluble hydroxide of a Group 2 element. These are generally strong bases, so Ca(OH)2 is classified as a strong base.

Classifying Ammonia

Ammonia (NH3) is a weak base because it does not completely dissociate in water but instead forms an equilibrium with only a partial donation of its lone pair to form hydroxide ions.

Key concepts

Strong and Weak Bases

Understanding the differences between strong and weak bases is crucial for students studying chemistry. A strong base is one that completely dissociates into its ions in water. This means that when dissolved, a strong base will split into its constituent cations and hydroxide (OH-) ions fully. Typically, these are the hydroxides of Group 1 elements, like lithium hydroxide (LiOH), which ionizes completely to form Li+ and OH- ions.
In contrast, a weak base only partially dissociates in an aqueous solution. This results in an equilibrium between the undissociated base and the ions it forms. Ammonia (NH3), is a prime example, as it only partially donates its electrons to form hydroxide ions, leaving a significant amount of undissociated ammonia in solution.

• Strong Base Example: LiOH - it dissociates completely.
• Weak Base Example: NH3 - it only partially dissociates.

Solubility of Hydroxides

The solubility of hydroxides is an important factor in determining whether a compound can be categorized as a strong or a weak base. Solubility is a measure of how well a chemical compound dissolves in a solvent like water. Hydroxides of certain metals, particularly those in Group 1 and Group 2 of the periodic table, tend to be very soluble in water.
For example, calcium hydroxide (Ca(OH)2), a Group 2 hydroxide, is soluble in water, which makes it a strong base as it readily forms Ca2+ and OH- ions. The ease with which these hydroxides dissolve and dissociate into hydroxide ions in water correlates with their strength as bases.

• Soluble Hydroxide: Ca(OH)2 is a soluble and strong base.
• Insoluble Hydroxide: Mg(OH)2 - Less soluble, and considered a weaker base.

Base Dissociation

The concept of base dissociation is an essential part of predicting pH and understanding reactions in aqueous solutions. When bases dissolve in water, they may release hydroxide ions (OH-) into the solution through a process called dissociation. The degree to which this happens is described by the base dissociation constant (Kb).
Strong bases have a high Kb value, indicating a high degree of dissociation, while weak bases have a low Kb value. Ammonium hydroxide (NH4OH), for instance, has a relatively low Kb, denoting its weak base characteristic and suggesting limited dissociation in water.

• Complete Dissociation: Strong bases like NaOH.
• Partial Dissociation: Weak bases like NH4OH.

Acidity and Basicity

Chemists use the concepts of acidity and basicity to describe the strength of acids and bases. These terms relate to a substance's tendency to donate protons (in the case of acids) or accept protons (in the case of bases). The pH scale is a numeric scale used to determine the acidity or basicity of an aqueous solution, with lower values indicating higher acidity and higher values indicating higher basicity.
Bases increase the pH of a solution by adding hydroxide ions to the solution or by accepting hydrogen ions. Strong bases, because of their complete dissociation, raise the pH significantly, while weak bases cause a smaller increase in pH due to their partial dissociation.

• pH Increase: Caused by bases accepting protons or releasing hydroxide ions.
• pH Scale Range: Ranges from 0 to 14, with 7 being neutral.

Chemical Equilibrium

In the context of acid-base reactions, chemical equilibrium refers to the point at which the rate of the forward reaction, where the base accepts a proton or releases a hydroxide ion, equals the rate of the reverse reaction, where the formed hydroxide ion recombines with a proton. This is a dynamic process where the concentrations of reactants and products remain constant over time.
For weak bases like ammonia (NH3), an equilibrium is established between the unreacted ammonia and the products of its reaction with water (NH4+ and OH- ions). The position of the equilibrium, which is determined by the equilibrium constant (Kb for bases), gives us valuable information about the relative strengths of the bases.

• Established Equilibrium: Indicates the point where forward and reverse reactions are balanced.
• Equilibrium Constant: Kb gives insight into the relative strength of a base.