Question

To prevent the formation of oxides, peroxides, and superoxides, alkali metals are sometimes stored in an inert atmosphere. Which of the following gases should not be used for lithium: Ne, Ar, \(\mathrm{N}_{2}, \mathrm{Kr}\) ? Why?

Short answer

\(\text{N}_2\) should not be used because it reacts with lithium to form lithium nitride.

Step-by-step solution

Understanding the Problem

We are asked to determine which inert gas should not be used to store lithium to prevent the formation of oxides, peroxides, and superoxides. The options are Ne, Ar, \(\text{N}_2\), and Kr.

Defining Inert Gases

Inert gases, such as Neon (Ne), Argon (Ar), and Krypton (Kr), are generally non-reactive due to their full electron shells. Nitrogen (\(\text{N}_2\)) is relatively inert due to its triple bond, but it can react under certain conditions.

Evaluating Reactivity

Most noble gases (Ne, Ar, Kr) are considered chemically inert, meaning they do not readily form compounds. \(\text{N}_2\) is typically non-reactive but can form lithium nitride (Li3N) when exposed to lithium metal.

Identifying Non-Ideal Gas

Use of \(\text{N}_2\) with lithium is problematic because \(\text{N}_2\) reacts with lithium to form lithium nitride (\(\text{Li}_3\text{N}\)), an undesired compound in this context.

Key concepts

Alkali Metals

Alkali metals are a group of elements found in Group 1 of the periodic table. They include lithium, sodium, potassium, rubidium, cesium, and francium. These metals are known for their high reactivity with air and water.
Here are some key characteristics of alkali metals:

• They are highly reactive and can easily lose their one valence electron to achieve a stable electron configuration.
• Alkali metals are soft and can often be cut with a knife.
• These metals have low melting points compared to other metals.
• They tend to form oxides, peroxides, and superoxides when reacting with air.

Because of their reactivity, especially with moisture, alkali metals must be stored carefully. Storing them in inert gases prevents unwanted reactions.

Lithium Storage

Storing lithium properly is crucial due to its high reactivity. When exposed to air, lithium can form oxides, which are unwanted. Inert gases like Neon (Ne), Argon (Ar), and Krypton (Kr) are often used for storage. These gases prevent lithium from reacting with oxygen.
However, one must be cautious with the choice of gas:

• Ne, Ar, and Kr are noble gases, known for their lack of reactivity.
• Nitrogen ( ext{N}_2) might seem inert because of its strong triple bond, but it can react with lithium.

Thus, while the noble gases provide a stable atmosphere for lithium storage, nitrogen is not a good choice since it can lead to the formation of lithium nitride.

Chemical Reactivity

Chemical reactivity refers to how likely a substance will undergo a chemical change. Alkali metals, including lithium, are known for their high reactivity. This is primarily because they can easily lose an electron to attain a stable electron configuration.
Some points about reactivity include:

• Alkali metals react exothermically with water to form hydroxides and hydrogen gas.
• Reactions with oxygen lead to the formation of various oxides, peroxides, or superoxides.
• Nitrogen, under certain conditions, can react with lithium, despite its general inertness.

The choice of storing reactive metals like lithium in an inert environment helps prevent unwanted chemical reactions, ensuring the safety and integrity of the metal.

Lithium Nitride

Lithium nitride is a chemical compound that forms when lithium reacts with nitrogen. Its chemical formula is ext{Li}_3 ext{N}. This compound is a reddish-brown solid and is the only stable alkali metal nitride.
Formation of lithium nitride is usually not desired in typical storage scenarios due to its reactive nature.

• This reaction removes lithium from its pure form, affecting its storage and usage.
• The compound can sometimes release ammonia and even decompose under certain conditions.
• Due to its formation during the reaction of lithium with nitrogen, environments containing ext{N}_2 are avoided when storing lithium.

Understanding the formation and implications of lithium nitride highlights why storing lithium in truly inert atmospheres like those with noble gases is especially important.