Problem 15 - Chemistry The Molecular Nature of Matter

Question

Potassium oxide is readily soluble in water, but the resulting solution contains essentially no oxide ion. Explain, using an equation, what happens to the oxide ion.

Step-by-Step Solution

Answer

Potassium oxide (K2O) dissolves in water to form potassium ions (K+) and oxide ions (O2-). The oxide ions then react with water to form hydroxide ions (OH-), leaving no free oxide ions in the solution. The reaction is: O2-(aq) + H2O(l) -> 2 OH-(aq).

Step 1: Write the Chemical Formula of Potassium Oxide

First, identify the chemical formula of potassium oxide, which is K2O. This compound is composed of potassium (K) and oxygen (O).

Step 2: Write the Dissolution Equation of Potassium Oxide in Water

When K2O is dissolved in water, it dissociates into potassium ions (K+) and oxide ions (O2-). The dissolution equation is written as: K2O(s) -> 2K+(aq) + O2-(aq).

Step 3: Explain the Reaction of Oxide Ions with Water

The oxide ion (O2-) reacts with water (H2O) to form hydroxide ions (OH-). Each oxide ion reacts with two water molecules to produce four hydroxide ions. The reaction is written as: O2-(aq) + H2O(l) -> 2 OH-(aq). This reaction explains why the solution contains no oxide ions but instead contains hydroxide ions.

Key Concepts

Chemical Formula of Potassium Oxide

Understanding the chemical formula of a compound is critical to grasping its properties and behavior. Potassium oxide, denoted by the formula K2O, consists of two potassium atoms (K) and a single oxygen atom (O). In this compound, the valency of potassium is +1, and oxygen is -2. This tells us that it takes two potassium atoms to balance the charge of one oxygen atom, hence forming a stable ionic compound.

This basic description of potassium oxide helps us anticipate its interactions with other substances, such as water. When dealing with solubility questions or chemical reactions, knowledge of the compound's formula lays the foundation for understanding its dissolution and the resulting solutions.

Dissolution Equation

During dissolution, a solid ionic compound separates into its constituent ions. For potassium oxide, the dissolution equation is a simple representation of this process. When K2O is introduced to water, it dissociates into potassium and oxide ions according to the equation: \[\begin{equation}K2O_{(s)} \rightarrow 2K^{+}_{(aq)} + O^{2-}_{(aq)}\end{equation}\]In this representation, (s) denotes a solid state while (aq) stands for aqueous, indicating the ions are dispersed in water. Since ionic compounds are made up of charged particles, their dissolution involves the separation of these ions, crucial for explaining the nature of the resulting solution. Furthermore, this process is vital for understanding how these ions interact with water and other substances.

Reaction of Oxide Ions with Water

The behavior of oxide ions in water is intriguing and consequential. Elaborating on the dissolution equation, the oxide ion's interaction with water leads to another reaction. Oxide ions (O2-) do not remain free in an aqueous medium; they quickly react with water molecules, according to the equation: \[\begin{equation}O^{2-}_{(aq)} + H2O_{(l)} \rightarrow 2OH^{-}_{(aq)}\end{equation}\]Here, the subscript (l) signifies that water is in its liquid state. This reaction forms hydroxide ions (OH-), which are far more common in aqueous solutions due to their stability. This is why, after dissolving potassium oxide in water, no oxide ions are detected; they are all converted into hydroxide ions. This transformation is a prime example of how chemical species can change forms, leading to solutions with different properties than might be initially expected.