Question

Which of the following alkali metals when burnt in air forms a mixture of oxide as well as nitride? (a) \(\mathrm{K}\) (b) \(\mathrm{Na}\) (c) \(\mathrm{Li}\) (d) \(\mathrm{Cs}\)

Short answer

Lithium (Li) is the alkali metal that burns in air forming both an oxide and a nitride.

Step-by-step solution

Understanding the question

We need to identify which alkali metal forms both an oxide and a nitride when it reacts with air. Air is composed of both oxygen and nitrogen, so the metal in question must react with both elements.

Knowledge of alkali metal reactivity

Alkali metals react with oxygen to form oxides. Among them, lithium is unique as it also reacts with nitrogen in the air to form lithium nitride (Li_3N) along with lithium oxide (Li_2O) when burnt.

Choosing the correct option

Considering the information that lithium reacts with not just oxygen but also nitrogen in the air, we can conclude that option (c) lithium (Li) is the correct answer.

Key concepts

Burning of Alkali Metals

The burning of alkali metals in air is a visually striking demonstration of chemical reactivity. When exposed to air, alkali metals such as potassium (K), sodium (Na), and lithium (Li) undergo combustion, but the products of this reaction vary depending on the metal. Alkali metals react vigorously with oxygen, and the heat generated from this reaction can ignite the metal.

These metals have one electron in their outer shell, which they readily lose, resulting in a reaction that forms metal oxides. The general equation for this reaction is as follows:
\( 4M + O_2 \rightarrow 2M_2O \)
where \(M\) represents an alkali metal. The reactivity increases down the group in the periodic table, meaning lithium reacts less violently than cesium. However, despite being less reactive, lithium's ability to also react with nitrogen makes it unique among its peers.

Formation of Oxides and Nitrides

Upon burning in air, alkali metals mainly form oxides, compounds that consist of metal ions and oxide ions (O^2-). In the context of the given exercise, when lithium is burnt in air, it uniquely generates both oxides and nitrides, due to its exceptional reactivity with nitrogen found in the air. Nitrides are compounds that consist of metal ions and nitrogen ions (N^3-).

The formation of nitrides, particularly in the case of lithium, can be represented by the chemical equation: \( 6Li + N_2 \rightarrow 2Li_3N \)
This propensity for lithium to form nitrides in addition to oxides is not as common in other alkali metals, showcasing lithium's distinctive chemical behavior.

Lithium Chemical Reactions

Lithium, the lightest of the alkali metals, shows some unique chemistry compared to its heavier counterparts. Lithium's reactions with air components are dual-faceted—forming lithium oxide with oxygen and lithium nitride with nitrogen.

In addition to these reactions, lithium also reacts with water, though not as vigorously as sodium or potassium, forming lithium hydroxide (\(LiOH\)) and hydrogen gas. The equation for this reaction is: \( 2Li + 2H_2O \rightarrow 2LiOH + H_2 \)
Lithium's compounds also exhibit less solubility in water compared to those of other alkali metals and it forms a stable carbonate (\(Li_2CO_3\)), making it suitable for various applications, including its use in lithium-ion batteries.