Reactions of Group I Elements: MDCAT Chemistry notes

General features

Group I (Li, Na, K, Rb, Cs) are the alkali metals. The name comes from the Arabic word for plant ashes, which contain sodium and potassium compounds. Each has a single $ns^1$ electron, which it loses easily to form $\mathrm{M^+}$.

Down the group, atomic size increases, so ionization energy and electronegativity decrease. Reactivity (reducing power) increases. Among Li, Zn, Ag and Au, lithium is by far the most reactive. Melting and boiling points fall down the group because metallic bonding weakens. Densities are low because the atoms are large (large atomic volume).

Reaction with oxygen: three types of oxide

Metal	Main product on burning in air	Type	O oxidation number
Li	$\mathrm{Li_2O}$	Normal oxide only	−2
Na	$\mathrm{Na_2O_2}$ (and $\mathrm{Na_2O}$ in limited oxygen)	Peroxide and normal oxide	−1
K, Rb, Cs	$\mathrm{KO_2}$, $\mathrm{RbO_2}$, $\mathrm{CsO_2}$	Superoxide	$-\tfrac12$

$$\mathrm{4Li + O_2 \to 2Li_2O}\qquad \mathrm{2Na + O_2 \to Na_2O_2}\qquad \mathrm{K + O_2 \to KO_2}$$

The small $\mathrm{Li^+}$ ion can only stabilize the small oxide ion. Larger cations stabilize the larger peroxide and superoxide ions.

All alkali metal oxides dissolve in water to give strongly alkaline hydroxide solutions, e.g. $\mathrm{Na_2O + H_2O \to 2NaOH}$.

Reaction with water

$$\mathrm{2M + 2H_2O \to 2MOH + H_2}$$

The vigour increases down the group. Li reacts steadily, Na melts and darts about, and K catches fire.

Reaction with chlorine

$$\mathrm{2M + Cl_2 \to 2MCl}$$

The products are white, ionic crystalline chlorides. They dissolve in water to give a neutral solution (pH 7), because they are salts of a strong acid and a strong base and neither ion hydrolyses.

Purifying NaCl: pass HCl gas into a saturated brine. The extra chloride ions (common-ion effect) make pure NaCl crystallize out, while soluble impurities stay in solution.

Chlorine with sodium hydroxide

• Cold, dilute NaOH (about 15 °C) gives chloride and hypochlorite: $\mathrm{2NaOH + Cl_2 \to NaCl + NaClO + H_2O}$
• Hot, concentrated NaOH gives chloride and chlorate: $\mathrm{6NaOH + 3Cl_2 \to 5NaCl + NaClO_3 + 3H_2O}$

Lithium: the odd one out

Lithium differs from the rest of the group mainly because of its small radius and high charge density. It has the highest hydration energy and ionization energy in the group. It forms only the normal oxide, reacts directly with nitrogen to give $\mathrm{Li_3N}$, and its compounds are more covalent. In many ways it resembles magnesium (diagonal relationship).

Common MDCAT traps

• Only Li gives only the normal oxide. Na can give both the normal oxide and the peroxide.
• K, Rb and Cs give superoxides, not peroxides.
• Alkali metal chloride solutions are neutral. Alkali metal oxide solutions are alkaline.
• Reactivity increases down the group while IE, EN and electron affinity decrease.
• The hypochlorite reaction needs cold alkali, not a high temperature.

Quick revision

• Li forms $\mathrm{Li_2O}$, Na forms $\mathrm{Na_2O_2}$, and K, Rb and Cs form $\mathrm{MO_2}$.
• $\mathrm{MCl}$ solutions have pH 7.
• Oxides and hydroxides of the alkali metals are strong alkalis.
• Lithium is anomalous because of its small size and high charge density.
• "Alkali" means plant ashes.