Question

Write a chemical equation for the ionization of each base and indicate whether it proceeds \(100 \%\) to products or not. a) \(\mathrm{NH}_{3}\) b) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\) c) \(\mathrm{Mg}(\mathrm{OH})_{2}\)

Short answer

NH3 and (CH3)3N ionize partially; Mg(OH)2 dissociates completely, but is sparingly soluble.

Step-by-step solution

Examine the Ionization of Ammonia (NH3)

Ammonia, \(\mathrm{NH}_3\), is a weak base that partially ionizes in water. The ionization equation is: \[\mathrm{NH}_3 + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{NH}_4^+ + \mathrm{OH}^-\] This reaction does not proceed \(100\%\) to products, indicating partial ionization.

Write the Ionization Equation for Trimethylamine ((CH3)3N)

Trimethylamine, \((\mathrm{CH}_3)_3\mathrm{N}\), is also a weak base and partially ionizes in water. The ionization equation is:\[(\mathrm{CH}_3)_3\mathrm{N} + \mathrm{H}_2\mathrm{O} \rightleftharpoons (\mathrm{CH}_3)_3\mathrm{NH}^+ + \mathrm{OH}^-\] Similar to \(\mathrm{NH}_3\), this reaction does not proceed \(100\%\) to products.

Write the Ionization Equation for Magnesium Hydroxide (Mg(OH)2)

Magnesium hydroxide, \(\mathrm{Mg(OH)}_2\), is considered a strong base and dissociates as follows:\[\mathrm{Mg(OH)}_2(s) \rightarrow \mathrm{Mg}^{2+}(aq) + 2\mathrm{OH}^-(aq)\] Although it is a strong base, \(\mathrm{Mg(OH)}_2\) is sparingly soluble in water. The reaction is considered \(100\%\) to products for the portion that dissolves.

Key concepts

Chemical Equations

Chemical equations are a symbolic way of representing a chemical reaction. They provide valuable information about the reactants and products involved. In the context of ionization of bases, chemical equations help show how a base interacts when it comes into contact with water.

For example, when ammonia (\(\mathrm{NH}_3\)) dissolves in water, its ionization equation is written as follows: \[\mathrm{NH}_3 + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{NH}_4^+ + \mathrm{OH}^-\]
In this equation, the phase change is represented with a double arrow symbol (\(\rightleftharpoons\)), indicating that not all of the reactants convert into products.

Understanding chemical equations is crucial since they provide insight into the behavior of chemical substances under different conditions.

Weak Base

A weak base is a type of chemical base that does not fully ionize in water. This means that only a small proportion of the base forms ions, while the rest remains unchanged. Both ammonia \((\mathrm{NH}_3)\) and trimethylamine \((\mathrm{CH}_3)_3\mathrm{N})\) exemplify weak bases.

In aqueous solutions, these bases partially ionize, which reflects their inability to donate all their available hydroxide ions \((\mathrm{OH}^-)\) to the solution.

• For ammonia, the ionization can be reflected in the equation:
  \[\mathrm{NH}_3 + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{NH}_4^+ + \mathrm{OH}^-\]
• For trimethylamine, the reaction is:
  \[(\mathrm{CH}_3)_3\mathrm{N} + \mathrm{H}_2\mathrm{O} \rightleftharpoons (\mathrm{CH}_3)_3\mathrm{NH}^+ + \mathrm{OH}^-\]

These reactions help illustrate why these substances are labeled as weak bases as they do not proceed completely to the right side of the equilibrium expression.

Strong Base

Strong bases are those that completely dissociate in water, meaning they fully release their hydroxide ions \((\mathrm{OH}^-)\) into the solution. Magnesium hydroxide \((\mathrm{Mg(OH)}_2)\), although sparingly soluble, is an example of a strong base due to its high degree of ionization in solution for the dissolved portion.

When it dissolves, it follows this equation: \[\mathrm{Mg(OH)}_2(s) \rightarrow \mathrm{Mg}^{2+}(aq) + 2\mathrm{OH}^-(aq)\]
In this equation, a single arrow \((\rightarrow)\) represents a reaction that proceeds to completion, unlike weak bases that partially ionize.

Understanding the characteristics of strong bases is vital for predicting the conductivity, reactivity, and pH of their solutions.

Partial Ionization

Partial ionization occurs when only a fraction of a solute dissociates to form ions in solution. For weak bases like ammonia \((\mathrm{NH}_3)\) and trimethylamine \((\mathrm{CH}_3)_3\mathrm{N})\), this means only some molecules convert to hydroxide ions.

This process can be described using their equilibrium reactions:

• Ammonia:
  \[\mathrm{NH}_3 + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{NH}_4^+ + \mathrm{OH}^-\]
• Trimethylamine: \[(\mathrm{CH}_3)_3\mathrm{N} + \mathrm{H}_2\mathrm{O} \rightleftharpoons (\mathrm{CH}_3)_3\mathrm{NH}^+ + \mathrm{OH}^-\]

The results of partial ionization are significant as they influence the base's ability to conduct electricity and its overall reactivity in water.

Complete Dissociation

Complete dissociation is a characteristic of strong bases where all the base molecules dissociate into ions in solution. This can be seen in magnesium hydroxide \((\mathrm{Mg(OH)}_2)\), particularly for the fraction that dissolves: \[\mathrm{Mg(OH)}_2(s) \rightarrow \mathrm{Mg}^{2+}(aq) + 2\mathrm{OH}^-(aq)\]
Although \((\mathrm{Mg(OH)}_2)\) is not very soluble, the small amount that does dissolve dissociates fully.

• Complete dissociation results in maximal hydroxide ion formation.
• This also leads to high pH values in solutions.
• It means more excellent electrolyte properties, enhancing the conductivity of the solution.

Understanding complete dissociation helps chemists anticipate the reactivity and interaction of strong bases with other substances in an aqueous environment.