Reactions of Alkali Metals with Oxygen

When lithium burns in oxygen, it primarily forms lithium oxide (Li₂O), a normal oxide.

4Li + O₂ → 2Li₂O

Sodium reacts with oxygen to form sodium peroxide (Na₂O₂).

2Na + O₂ → Na₂O₂

These alkali metals react with oxygen to form superoxides of the type MO₂.

M + O₂ → MO₂ (where M = K, Rb, Cs)

Formation of Normal Oxides

Alkali metals, when reacting with oxygen, do not generally form normal oxides (M₂O) directly. Lithium is an exception as it forms lithium oxide (Li₂O) directly by reacting with oxygen. For other alkali metals, such as sodium, the formation of normal oxides requires indirect methods. This can be achieved by reducing peroxides (M₂O₂), nitrites (MNO₂), or nitrates (MNO₃) with the metal itself.

The general reactions are as follows:

1. Reduction of Peroxides:
$2Na_2O_2 + 4Na → 4Na_2O$

2. Reduction of Nitrites:
$2NaNO_2 + 6Na → 4Na_2O + N_2$

3. Reduction of Nitrates:
$2NaNO_3 + 10Na → 6Na_2O + N_2$

These reactions highlight the reactivity and reducing power of alkali metals, facilitating the formation of normal oxides through indirect methods.

Properties and Reactions of Alkali Metal Oxides

Alkali metal oxides can be categorized into normal oxides, peroxides, and superoxides, each with distinct properties and reactivity.

1. Normal Oxides (O²⁻):

• Normal oxides of alkali metals, such as Li₂O, contain the monoxide ion (O²⁻). These oxides exhibit a structure similar to that of anti-fluorite and are ionic in nature.
• When these normal oxides react with water, they undergo proton exchange to form hydroxides:
  $O^{2-} + H_2O → 2OH^-$
• This reaction involves the oxide ion (O²⁻) gaining a proton (H⁺) from water, resulting in the formation of two hydroxide ions (OH⁻).

2. Peroxides (O₂²⁻) and Superoxides (O₂⁻):

• • Peroxides contain the peroxide ion (O₂²⁻), characterized by an [O-O]²⁻ structure. An example is sodium peroxide (Na₂O₂).
  • Superoxides contain the superoxide ion (O₂⁻), which has a unique three-electron bond. For example, the superoxide ion features a three-electron bond that involves one bonding electron pair (shared between the two oxygen atoms) and one additional unpaired electron. This unpaired electron makes the superoxide ion paramagnetic and colored. An example is potassium superoxide (KO₂).
  • Both peroxides and superoxides are strong oxidizing agents and react vigorously with water to produce hydrogen peroxide (H₂O₂) and oxygen (O₂). The reactions are as follows:
    $2O_2^{2-} + 2H_2O → 2H_2O_2 + O_2$
    This reaction demonstrates the high reactivity of peroxides and superoxides, as they release oxygen upon interaction with water.

Simple oxides, such as lithium oxide (Li₂O), react with water to form metal hydroxides:

X₂O + H₂O → 2XOH

For example, lithium oxide reacts with water to give a colorless solution of lithium hydroxide:

Li₂O + H₂O → 2LiOH

Simple oxides react with acids to produce a salt and water. For instance, sodium oxide reacts with dilute hydrochloric acid to yield sodium chloride and water:

X₂O + 2HCl → 2XCl + H₂O

Specifically:

Na₂O + 2HCl → 2NaCl + H₂O

When peroxides react with water, they form metal hydroxides and hydrogen peroxide. This reaction is highly exothermic:

X₂O₂ + 2H₂O → 2XOH + H₂O₂

For example, sodium peroxide reacts with water:

Na₂O₂ + 2H₂O → 2NaOH + H₂O₂

Peroxides react exothermically with acids, producing a salt, hydrogen peroxide, and oxygen. This can be hazardous due to the violent nature of the reaction:

X₂O₂ + 2HCl → 2XCl + H₂O₂

For instance:

Na₂O₂ + 2HCl → 2NaCl + H₂O₂

Superoxides react with water to form metal hydroxides, hydrogen peroxide, and oxygen gas:

2XO₂ + 2H₂O → 2XOH + H₂O₂ + O₂

For example, potassium superoxide reacts with water:

2KO₂ + 2H₂O → 2KOH + H₂O₂ + O₂

Superoxides react even more violently with acids than peroxides do, producing a salt, hydrogen peroxide, and oxygen:

2XO₂ + 2HCl → 2XCl + H₂O₂ + O₂

For instance:

2KO₂ + 2HCl → 2KCl + H₂O₂ + O₂