Reactivity with Air
Monoxides, Peroxides, Superoxides, and the Kerosene Storage Rule.
Alkali metals are so highly reactive that they cannot be left exposed to the atmosphere. When exposed to air, they tarnish rapidly, reacting with oxygen and moisture. The way they react with oxygen, however, differs dramatically depending on the size of the metal atom.
1. Tarnishing in Air & Storage
Freshly cut alkali metals have a brilliant, silvery-white metallic luster. However, upon exposure to air, they tarnish instantly.
The Tarnish Sequence:
- They first react with $O_2$ to form an oxide layer.
- This oxide reacts with moisture ($H_2O$) in the air to form a hydroxide.
- The hydroxide absorbs $CO_2$ from the air to form a layer of metal carbonate.
Exception: Lithium is so light (low density) that it floats on kerosene. Therefore, it is often kept wrapped in paraffin wax.
2. Reaction with Oxygen: The Three Types of Oxides
When heated in an excess of air or oxygen, the alkali metals burn vigorously. However, they form completely different types of oxides depending on their atomic size. This is a very high-yield topic for exams.
| Metal | Oxide Formed (in excess $O_2$) | Reaction |
|---|---|---|
| Lithium (Li) | Monoxide ($Li_2O$) | $4Li + O_2 \rightarrow 2Li_2O$ |
| Sodium (Na) | Peroxide ($Na_2O_2$) | $2Na + O_2 \rightarrow Na_2O_2$ |
| K, Rb, Cs | Superoxide ($MO_2$) | $M + O_2 \rightarrow MO_2$ (e.g., $K + O_2 \rightarrow KO_2$) |
Many students answer $+4$ because Oxygen is usually $-2$. THIS IS WRONG. Alkali metals always exhibit an oxidation state of $+1$. In a superoxide, the anion is $O_2^-$, meaning each oxygen atom has an average oxidation state of $-\frac{1}{2}$.
3. Why do they form different oxides?
The formation of different oxides can be explained by the Size Compatibility Principle (Lattice Energy).
- A small cation stabilizes a small anion by forming a highly stable, tightly packed crystal lattice. Thus, the tiny $Li^+$ ion strongly stabilizes the small oxide ion ($O^{2-}$).
- A larger cation stabilizes a larger anion. As we move down the group, the cations get much larger ($Na^+$, then $K^+, Rb^+, Cs^+$).
- The larger $Na^+$ ion can stabilize the larger peroxide ion ($O_2^{2-}$).
- The largest cations ($K^+, Rb^+, Cs^+$) are big enough to stabilize the massive, bulky superoxide ion ($O_2^-$) effectively.
Properties of the Oxides:
- Monoxides & Peroxides: Pure ones are colorless/white and diamagnetic (all electrons paired).
- Superoxides ($MO_2$): They are yellow or orange in color and are paramagnetic. This is because the superoxide ion ($O_2^-$) has an unpaired electron in its $\pi^*$ antibonding molecular orbital.
4. Reaction with Nitrogen (The Lithium Anomaly)
Nitrogen gas ($N_2$) is highly inert because of its triple bond ($N \equiv N$). Most alkali metals cannot break this bond and therefore do not react directly with nitrogen gas.
However, Lithium is the only exception. Because $Li^+$ is incredibly small, its lattice energy with the small, highly charged nitride ion ($N^{3-}$) is extremely high. This massive release of energy drives the reaction forward.
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