Q.(a) Why are alkali metals not found in nature? [1]

(a) Alkali metals are too reactive to exist free in nature. (b) The Solvay process makes Na2CO3 via ammoniated brine, sodium bicarbonate, then heating.

(a) Why alkali metals are not found free in nature:

Alkali metals (Li, Na, K, Rb, Cs) have very low ionization enthalpies and are strongly electropositive — they lose their single outer ns1 electron extremely easily. This makes them extremely chemically reactive: they react vigorously with atmospheric oxygen, moisture, and even nitrogen at ordinary conditions (e.g. sodium tarnishes in air and reacts violently with water). Because of this high reactivity, alkali metals are never found in the free (elemental) state in nature; they are always found combined with other elements, as compounds/minerals/ores (e.g. NaCl in seawater and rock salt, KCl in sylvite, etc.) or as dissolved salts.

(b) Solvay process for manufacture of Na2CO3:

The Solvay (ammonia-soda) process uses brine (concentrated NaCl solution) saturated with ammonia, through which CO2 is passed. The key reactions are:

Step 1 — Ammonia and CO2 react with water to form ammonium bicarbonate:

$NH_3 + H_2O + CO_2 \rightarrow NH_4HCO_3$

Step 2 — Ammonium bicarbonate reacts with sodium chloride (brine); sodium bicarbonate, being less soluble, precipitates out:

$NaCl + NH_4HCO_3 \rightarrow NaHCO_3\downarrow + NH_4Cl$

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