Chapter 22.1, Problem 4R

To determine

To compare: The formation of ${\text{OH}}^{-}$ ions in water on reaction with ${\text{NH}}_3$ and with ${\text{Ca(OH)}}_2$ .

Explanation of Solution

Introduction: Bases dissociate in water to give ${\text{OH}}^{-}$ ions.

The dissociation of ${\text{NH}}_3$ in water can be expressed as the following chemical equation:

  ${\text{NH}}_3+{\text{H}}_2\text{O}\to {\text{NH}}_4^{+}+{\text{OH}}^{-}$

The interaction of a ${\text{NH}}_3$ molecule with a water molecule produces an ${\text{NH}}_4^{+}$ ion and a ${\text{OH}}^{-}$ ion.

The dissociation of ${\text{Ca(OH)}}_2$ in water can be expressed as the following chemical equation:

  ${\text{Ca(OH)}}_2({\text{s)+H}}_2\text{O}\to {\text{Ca}}^{2+}(\text{aq)}+2{\text{OH}}^{-}$

When a calcium hydroxide molecule dissociates in water, it produces a calcium ion and two hydroxide ions.

In the reaction of ${\text{Ca(OH)}}_2$ with water, the molecules of water restrain from combining with the ions formed from the base. However, when ${\text{NH}}_3$ reacts with water, taking the hydrogen ion from a water molecule forms an ammonium ion and a hydroxide ion.

A ${\text{Ca(OH)}}_2$ molecule dissociates in water to give two hydroxide ions whereas, a ${\text{NH}}_3$ molecule gives only one hydroxide ion.

Conclusion: The dissociation of ${\text{NH}}_3$ in water produces an ammonium ion and a hydroxide ion, by taking the hydrogen ion from water. However, when a ${\text{Ca(OH)}}_2$ molecule reacts with water, the water molecules do not combine with the ions formed from the base, thereby forming a calcium ion and two hydroxide ions.