Compounds Of Alkali Metals

Oxides of Metal

Oxides of Sodium: The possible oxides of Na are Na₂O and Na₂O₂.

Sodium Monoxide

Preparation

It is obtained by burning sodium at 180^o C in a limited supply of air or oxygen and distilling off the excess of sodium in vacuum, or by heating sodium peroxide(Na₂O₂), nitrate(NaNO₃) with sodium

2Na + 1/2O₂ → Na₂O

Na₂O₂ + 2Na → 2Na₂O

2NaNO₃ + 10 Na → 6Na₂O + N₂

Properties

It dissolves in water violently yielding caustic soda.

Na₂O + H₂O → 2NaOH

Sodium Peroxide (Na₂O₂)

Preparation

It is formed by heating the sodium metal in excess air or oxygen. The air should be moisture free and temperature required is 300o C.

2Na + O₂ → Na₂O₂

Properties

(i) It is pale yellow solid becoming white in air due to the formation of a film of sodium hydroxide and carbonate.

(ii) It dissolves in ice - cold water with hydrolysis, yielding hydrogen peroxide, which decomposes into water and oxygen on warming; whereas it gives oxygen and caustic soda with water at room temperature,

Na₂O₂ + 2H₂O 2NaOH + H₂O₂

Na₂O₂ + 2H₂O 2NaOH + O₂

(iii) It dissolves in ice-cold dilute mineral acid yielding H₂O₂.

Na₂O₂ + H₂SO₄ → Na₂SO₄ + H₂O₂

Oxides of Potassium

Potassium forms number of oxides namely,K₂O,K₂O₂ and KO₂ . Other two oxides can also exist which are K₂O₃ and KO₃

Potassium monoxide (K₂O)

It is solid, yellow when hot and white when cold, obtained by heating potassium nitrate with potassium.

2KNO₃ + 10K → 6K₂O + N₂

It dissolves in water to give KOH like Na₂O

Potassium Peroxide (K₂O₂)

Controlled oxidation of potassium in excess air or oxygen at 300^o C gives mainly K₂O₂. It gives H₂O₂ when dissolves in water.

K₂O₂ + 2H₂O → 2KOH + H₂O₂

Potassium dioxide (KO₂)

It is made by burning potassium in a good supply of air or oxygen. It is powerful oxidising agent. It reacts with water giving both oxygen and hydrogen peroxide.

2KO₂ + 2H₂O → 2KOH + H₂O₂ + O₂

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

Properties

(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 + 2H₂O

Al₂O₃ ↓ + 2NaOH → 2NaAlO₂ + H₂O

SiO₂ + 2NaOH → Na₂SiO₃ + H₂O

3P + 3 NaOH +3H₂O → PH₃ + 3NaH₂PO₂

2Al + 2 NaOH + 2H₂O → 3H₂ + 2NaAlO₂

Uses

(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

Properties

(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.

Carbonates of Metals (Na, K)

Sodium Carbonate (Washing soda) (Na₂CO₃)

Solvay Process

Sodium carbonate is generally prepared by a process called the ammonia – soda process or solvay process as described below:

Principle

When carbon dioxide gas is bubbled through a brine solution saturated with ammonia, it results in the formation of sodium hydrogen carbonate.

Sodium hydrogen carbonate so formed precipitates out because of the common ion effect caused due to the presence of excess of NaCl. The precipitated NaHCO3 is filtered off and then ignited to get Na2CO3.

Plant Process

The various parts for the manufacture of Na₂CO₃ by solvay process have been illustrated in figure below.

Plant used for the manufacture of washing soda

(i) Ammonia absorber

A 30% solution of brine is saturated with ammonia (from recovery tower) is introduced into absorber tower. Various impurities like calcium and magnesium salts present in the commercial NaCl precipitate out as corresponding insoluble carbonates. (which comes along with ammonia from the ammonia recovery plant)

The ammoniated brine is filtered to remove the precipitated calcium and magnesium carbonate.

(ii) Carbonation Tower

It is high tower fitted with perforated plates. Ammoniated brine solution is made to trickle down from the top of the tower while CO2 gas from the lime kiln is admitted from the base of the tower. CO2 rises through the small perforations and its interaction with ammoniated brine results in the formation of insoluble sodium hydrogen carbonate.

(iii) Filtration

The solution containing crystals of NaHCO₃ is drawn off from the base of the carbonation tower and filtered to get NaHCO₃.

(iv) The NaHCO₃ obtained from the above step is heated strongly in kiln to covert it into sodium carbonate (Na₂CO₃)

The carbon dioxide produced here is sent to carbonation tower.

(v) Ammonia recovery tower

The filtrate, after removal of NaHCO₃ contains ammonium salts such as NH₄HCO₃ and NH4Cl.

The filtrate is mixed with Ca(OH)₂ and is heated with steam in ammonia recovery tower.

The mixture of ammonia and CO₂ gases is obtained which is used for saturation of brine while calcium chloride is obtained as a by – product.

(vi) Lime kiln

Here limestone is heated at about 1300 K to obtained CO₂ and calcium oxide

The CO₂ gas goes to the carbonation tower while lime is slaked with water in tank known as slakes to form ca(OH)2.

The overall reaction taking place in solvay process is

Flow sheet diagram of Solvay process

Properties

(i) The aqueous solution absorbs CO₂ yielding sparingly soluble sodium bicarbonate.

Na₂CO₃ + H₂O + CO₂ → 2NaHCO₃

(ii) It dissolves in acids with an effervescence of carbondioxide and is causticised by lime to give caustic soda.

Na2CO₃ + 2HCl → 2NaCl + H₂O + CO₂

Na₂CO₃ + Ca(OH)₂ → 2NaOH + CaCO₃

(iii) Fusion with silica, sodium carbonate yields sodium silicate.

Na₂CO₃ + SiO₂ → Na₂SiO₃ + CO₂

(iv) Hydrolysis – being a salt of a strong base (NaOH) and weak acid (H₂CO₃), when dissolved in water sodium carbonate. Undergoes hydrolysis to form an alkaline solution

Na₂CO₃ + 2H₂O → H₂CO₃ + 2NaOH

Uses

(i) It is used for softening hard water.

(ii) A mixture of sodium carbonate and potassium carbonate is used as fusion mixture.

(iii) As an important laboratory reagent both in qualitative and quantitative analysis.

(iv) It is used in paper, paints and textile industries.

(v) It is used for washing purposes in laundry.

(vi) It is used in the manufacture of glass, borax, soap and caustic soda.

Potassium carbonate (K₂CO₃)

It is also known as pearl ash. It is made by passing CO₂ into a conc. solution of the chloride, containing hydrated mangesium carbonate in suspension at 20°C when an insoluble potassium hydrogen magnesium carbonate is precipitated.

  2KCl + 3(MgCO₃. 3H₂O) + CO₂ → 2(MgCO₃. KHCO₃⋅4H₂O) + MgCl₂

The precipitate is separated by filtration, and then decomposed either by heating with water under pressure at 140°C or by the action of magnesium oxide below 20°C.

2(MgCO₃. KHCO₃. 4H₂O) → 2MgCO3 + K2CO3 + 9H₂O + CO₂

2(MgCO₃. KHCO₃. 4H₂O) + MgO → 3(MgCO₃. 3H₂O) + K₂CO₃

Properties

(i) It is white, deliquescent solid

(ii) K2CO₃ resembles Na₂CO₃ in properties, but is more alkaline and more soluble than Na₂CO₃.

Bicarbonates of Metals (Na, K)

Sodium Bicarbonate

A concentrated solution of sodium carbonate absorbs CO2 to give sparingly soluble sodium bicarbonate.

Na₂CO₃ + CO₂ + H₂O → 2NaHCO₃

Properties

(i) It is sparingly soluble in water

(ii) When heated between 250°C and 300°C, it is converted into pure anhydrous sodium carbonate which can be used for standardising acids.

2NaHCO₃ → Na₂CO₃ + H₂O + CO₂

Potassium bicarbonate

It is made by absorbing CO₂ in moist potassium carbonate and then drying the product in a porous plate.

K₂CO₃ + H₂O + CO₂ → 2KHCO₃

Properties

KHCO₃ resembles NaHCO₃, but is much more soluble in water. The solution is strongly alkaline owing to hydrolysis.

KHCO₃ + H₂O → KOH + H₂CO₃

Chlorides of Metal (Na, K)

Sodium Chloride (NaCl)

It is also called common salt occurs abundantly in nature as rock salt or halite. The most abundant source is sea-water where sodium chloride occurs to the extent of 2.6 – 2.9 percent. The sea water is exposed to the sun and air in large shallow pits. The gradual evaporation of water leading to the crystallization of the salt. The purification is done by dissolving the salt in minimum volume of water and filtering, if necessary, to remove insoluble impurities. The solution is then saturated with a current of dry hydrogen chloride whereby crystals of pure sodium chloride separate out.

Properties

(i) NaCl is a colourless crystalline salt, almost insoluble in alcohol and highly soluble in water.

(ii) It gives rise to HCl when heated with conc. H₂SO₄ and Cl₂, with MnO₂ plus H₂SO_4.

NaCl + H₂SO₄ → NaHSO₄ + HCl↑

NaHSO₄ + NaCl → Na₂SO₄ + HCl↑

2NaCl + MnO₂ + 2H₂SO₄ → MnSO₄ + Na₂SO₄+ 2H₂O + Cl₂ ↑

Potassium Chloride

KCl is prepared from fused carnallite – nearly pure KCl separates from the melt, leaving fused MgCl₂ behind.

KCl, MgCl₂⋅6H₂O → KCl + MgCl₂⋅6H₂O

Properties

It is colourless cubic crystal like solid soluble in water. Its solubility increases almost linearly with temperature.

Sulphates of Metals: (Na, K)

Sodium Sulphate, Na₂SO₄

The anhydrous salt known as salt cake, is prepared on an industrial scale by heating strongly sodium chloride with conc. sulphuric acid.

NaCl + H₂SO₄ NaHSO₄+ HCl↑

NaCl + NaHSO₄ Na₂SO₄ + HCl↑

Glauber’s salt or hydrated sodium sulphate, Na₂SO₄⋅10H₂O is prepared from salt cake by crystallisation from water below 32°C This temperature represents the transition temperature for Na2SO4 and Na₂SO₄.10H₂O.

It is colourless salt, crystallising in large monoclinic prisms. It is exceedingly soluble in water.

Potassium Sulphate, K₂SO₄

It is obtained by strongly heating potassium chloride with conc. H₂SO₄

KCl + H₂SO₄ → KHSO₄ + HCl

KCl + KHSO₄ → K2SO₄+ HCl

It is colourless crystalline salt, m.p. 1070°C. It is less soluble in water than sodium sulphate and has no hydrate like the later.