Do Insoluble Bases Like MgO Contribute to OH Ions in Solution?

The behavior of different types of bases in aqueous solutions, including insoluble bases like MgO, can sometimes be perplexing. Let's explore whether insoluble bases such as MgO contribute to hydroxide ions (OH-) in solution and how they differ from soluble bases.

Characterization of Bases

Not all bases produce hydroxide ions (OH-) directly in solution, especially when considering insoluble bases. Soluble bases like sodium hydroxide (NaOH) and potassium hydroxide (KOH) dissociate completely in water to produce OH- ions.

Insoluble bases like magnesium oxide (MgO) do not dissolve significantly in water. While they are not directly dissociated into their ions, they can still contribute to the presence of hydroxide ions in solution through a reaction with water.

Reaction of Insoluble Bases with Water

Insoluble bases like MgO react with water to form a water-soluble base, magnesium hydroxide (Mg(OH)2):

MgO H2O → Mg(OH)2

Magnesium hydroxide is only slightly soluble in water, making it an effective buffer for hydroxide ions in solution.

The Formation of IH2O

When MgO is added to water, it does not directly dissociate into Mg2 and O2- ions. This is because they form magnesium hydroxide (Mg(OH)2), which can then dissociate slightly to produce Mg2 and OH- ions:

Mg(OH)2 ? Mg2 2OH-

Acid-Base Theories and the Nature of MgO

The behavior of MgO in solution can be understood through various acid-base theories, such as the Arrhenius, Bronsted-Lowry, and Lewis theories. Each theory provides a different perspective on the nature of acids and bases.

Arrhenius Theory

According to the Arrhenius theory, bases dissociate in water to produce OH- ions. However, MgO, due to its low solubility, does not completely dissociate in water, making it not very meaningful to judge whether it is a base or not. At high temperatures, MgO reacts with water to form sparingly soluble Mg(OH)2, which then dissociates to produce OH- ions:

Mg(OH)2 H2O → Mg(OH)2

Mg(OH)2 ? Mg2 2OH-

Bronsted-Lowry Theory

The Bronsted-Lowry theory defines acids as proton (H ) donors and bases as proton (H ) acceptors. Applying this theory to MgO, we see that MgO accepts a proton (H ) from water to form Mg(OH)2, thus acting as a base:

MgO H2O → Mg(OH)2

Lewis Theory

The Lewis theory defines acids as electron pair acceptors and bases as electron pair donors. In the reaction MgO H2O → Mg(OH)2, MgO donates a pair of electrons to water, making it a base.

MgO H2O → [MgOH2 2e-]

By understanding the different acid-base theories, we can see that MgO indeed contributes to the presence of OH- ions in solution through its interaction with water.