Animated Solution for Chemistry - s and p-Block Elements: Which of the following liberates ${\text{O}}_2$ upon hydrolysis?

Imagine you are standing in a chemistry laboratory, holding four different vials containing mysterious powders: ${\text{Pb}}_3{\text{O}}_4$, ${\text{KO}}_2$, ${\text{Na}}_2{\text{O}}_2$, and ${\text{Li}}_2{\text{O}}_2$. Your mission is simple yet profound: drop each powder into a beaker of water and observe which one magically conjures bubbles of life-giving oxygen gas.

This process of a compound reacting with water is known as hydrolysis. While it might seem like a simple dissolution to the untrained eye, at the molecular level, water molecules are aggressively attacking the ionic bonds, tearing the crystal lattice apart, and forcing the atoms to rearrange into entirely new molecules. To predict the victor of our experiment, we must dive deep into the architecture of oxygen ions.

Normal, Peroxides, and Superoxides

Oxygen is a versatile element, capable of forming several distinct types of anions depending on how many electrons it manages to snatch from its metallic partners.

Most commonly, we encounter the normal oxide ion, ${\text{O}}^{2-}$. When normal oxides react with water, they typically just form hydroxides. No drama, no gas.

Then we have the peroxides, featuring the ${\text{O}}_2^{2-}$ ion. Here, two oxygen atoms share a single covalent bond, and the entire unit carries a -2 charge. When peroxides meet water, they undergo hydrolysis to form a hydroxide and hydrogen peroxide (${\text{H}}_2{\text{O}}_2$). Still, no oxygen gas is liberated.

Finally, we reach the crown jewels of this problem: the superoxides. These contain the highly reactive ${\text{O}}_2^{-}$ ion. Superoxides are fascinating because they possess an unpaired electron, making them paramagnetic. When a superoxide is thrown into water, it doesn't just form a hydroxide and hydrogen peroxide; it undergoes a spectacular disproportionation reaction, releasing pure oxygen gas (${\text{O}}_2$) in the process!

Lead, Sodium, and Lithium

Let's put our candidates on the stand. First up is ${\text{Pb}}_3{\text{O}}_4$, commonly known as red lead. This is actually a mixed oxide, structurally represented as $2\text{PbO}\cdot {\text{PbO}}_2$. It is incredibly stable and practically insoluble in water. If you drop it in our beaker, it will simply sink to the bottom and stare back at you. No reaction, no hydrolysis, and certainly no oxygen gas.

Next, we examine ${\text{Na}}_2{\text{O}}_2$ (Sodium peroxide) and ${\text{Li}}_2{\text{O}}_2$ (Lithium peroxide). Both sodium and lithium are relatively small alkali metals, and they form stable peroxides. When we introduce them to water, the following reactions occur:

As predicted by our chemical laws, the hydrolysis of these peroxides yields their respective hydroxides and hydrogen peroxide. The oxygen atoms transition smoothly without needing to release gaseous ${\text{O}}_2$.

Potassium Superoxide

Finally, we arrive at ${\text{KO}}_2$, Potassium superoxide. Why does potassium form a superoxide while sodium and lithium prefer peroxides? It all comes down to lattice energy and ionic size. Potassium is a large cation ($K^{+}$), and large cations are exceptionally good at stabilizing large, complex anions like the superoxide ion (${\text{O}}_2^{-}$) in a crystal lattice.

When ${\text{KO}}_2$ hits the water, chaos ensues. The superoxide ion is unstable in an aqueous environment and immediately undergoes disproportionation. In a disproportionation reaction, the same element is simultaneously oxidized and reduced. The oxidation state of oxygen in the superoxide ion is $-1/2$. During hydrolysis, some of these oxygen atoms are reduced to an oxidation state of $-1$ (forming ${\text{H}}_2{\text{O}}_2$), while others are oxidized to an oxidation state of $0$ (forming pure ${\text{O}}_2$ gas).

And there it is! The beaker fizzes and bubbles violently as oxygen gas escapes into the atmosphere. Potassium superoxide is the undisputed champion of our experiment.

Real-World Superpowers

This isn't just a neat trick for a chemistry exam; it is a life-saving chemical property. Because potassium superoxide reacts with moisture (and carbon dioxide) to release oxygen, it is heavily utilized in closed-environment life support systems.

If you are ever in a submarine deep beneath the ocean, or an astronaut floating in the vacuum of space, there is a good chance that canisters of ${\text{KO}}_2$ are quietly working in the background. They absorb the ${\text{CO}}_2$ you exhale and replace it with the fresh ${\text{O}}_2$ you need to survive. Chemistry isn't just equations on a page; it is the invisible architecture that keeps us alive!