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>> Hydroxides of Alkali Metals
Hydroxides of Metals (Na, K)
Sodium Hydroxide (NaOH)
When calcium hydroxide is added to sodium carbonate solution, calcium carbonate is precipitated, leaving sodium hydroxide in solution
Na2CO3 + Ca(OH)2 →CaCO3 ↑ + 2NaOH
(i) NaOH is stable towards heat but is reduced to metal when heated with carbon
2NaOH + 2C → 2Na +2CO + H2
(ii) FeCl3 + 3NaOH →Fe(OH)3 + 3NaCl
NH4Cl + NaOH → NaCl + NH3 (pungent smell) + H2O
HgCl2 + 2NaOH → HgO (yellow powder) + 2NaCl + H2O
Zn(OH)2 ↑ + 2NaOH → Na2ZnO2 + 2H2Oh
Al2O3 ↑ + 2NaOH → 2NaAlO2 + H2O
SiO2 + 2NaOH → Na2SiO3 + H2O
3P + 3 NaOH +3H2O → PH3 + 3NaH2PO2
2Al + 2 NaOH + 2H2O → 3H2 + 2NaAlO2
(i) It is used in the manufacture of paper, soap and artificial silk.
(ii) It is used in petroleum refining.
(iii) It is used for mercerizing cotton.
(iv) It is used for the preparation of sodium metal and many salts of sodium.
Potassium Hydroxide or Caustic Potash (KOH)
Pure KOH is obtained by adding potassium sulphate to a hot saturated solution of Ba(OH)2.BaSO4 is filtered off and filtrate is evaporated in a silver dish.
K2SO4 + Ba(OH)2 → BaSO4 + 2KOH
(i) In the absorption of carbon dioxide, caustic potash is preferred to caustic soda, since the KHCO3 formed after sufficient absorption is soluble, while NaHCO3 is insoluble and may,therefore choke the tubes.
(ii) As an alkaline reagent KOH is not used while NaOH is used because NaOH is cheaper than KOH.