Compounds of Alkali Metals

COMPOUNDS OF ALKALI METALS

Oxides and hydroxides of alkali metals

On combustion in excess of air, lithium forms mainly the oxide, Li2O (plus some peroxide Li2O2), sodium forms the peroxide, Na2O2 (and some superoxide NaO2) whilst potassium, rubidium and caesium form the superoxides, MO2. Under appropriate conditions pure compounds M2O, M2O2 and MO2 may be prepared. The increasing stability of the peroxide or superoxide, as the size of the metal ion increases, is due to the stabilisation of large anions by larger cations through lattice energy effects. These oxides are easily hydrolysed by water to form the hydroxides according to the following reactions :

 

 

 

The oxides and the peroxides are colourless when pure, but the superoxides are yellow or orange in colour. The superoxides are also paramagnetic. Sodium peroxide is widely used as an oxidising agent in inorganic chemistry.

 

The hydroxides which are obtained by the reaction of the oxides with water are all white crystalline solids. The alkali metal hydroxides are the strongest of all bases and dissolve freely in water with evolution of much heat on account of intense hydration.

 

Halides of alkali metals

 The alkali metal halides, MX, (X=F,Cl,Br,I) are all high melting, colourless crystalline solids. They can be prepared by the reaction of the appropriate oxide, hydroxide or carbonate with aqueous hydrohalic acid (HX).

All of these halides have high negative enthalpies of formation; the ∆H0 values for fluorides become less negative as we go down the group, whilst the reverse is true for ∆H0 for chlorides, bromides and iodides. For a given metal ∆f H0 always becomes less negative from fluoride to iodide.

The melting and boiling points always follow the trend: fluoride > chloride > bromide > iodide.

All these halides are soluble in water. The low solubility of LiF in water is due to its high lattice enthalpy whereas the low solubility of CsI is due to smaller hydration enthalpy of its two ions.

Other halides of lithium are soluble in ethanol, acetone and ethylacetate; LiCl is soluble in pyridine also.

 

Salts of oxo-acids of alkali metals

Nature of carbonates and bicarbonates Li2CO3 is unstable toward heat.

 

The thermal stability of carbonates increases on moving down the group as

 

All the bicarbonates (except LiHCO3 which exists in solution) exist as solids and on heating form carbonates.

 

The solubility of the carbonates and bicarbonates increases on moving down the group due to decrease in lattice enthalpies. Thus, the order is

LiHCO3 < NaHCO3 <KHCO3 < RbHCO3 < CsHCO3

A mixture of Na2CO3 and K2CO3 is known as fusion mixture K2CO3is known as pearl ash.

 

Nature of nitrates LiNO3 on heating decomposes to give NO2 and O2, while the nitrates of the other alkali metals decompose on heating and give nitrites and O2.

 

NaNO3 is called chile saltpeter and KNO3 is called Indian saltpeter.

 

Nature of sulphates Li2SO4 is insoluble in water whereas the other sulphates, i:e., Na2SO4, K2SO4 are soluble in water.

Na2SO4 . 10H2O is called Glauber’s salt

 

Anomalous properties of lithium

 

 The anomalous behaviour of lithium is due to the :

(i) exceptionally small size of its atom and ion, and

(ii) high polarising power (i.e., charge/radius ratio).

 

Difference between lithium and other alklai metals

(i) Lithium is much harder. Its m.p. and b.p. are higher than the other alkali metals.

(ii) Lithium is least reactive but the strongest reducing agent among all the alkali metals. On combustion in air it forms mainly monoxide, Li2O and the nitride, Li3N unlike other alkali metals.

(iii) LiCl is deliquescent and crystallises as a hydrate, LiCl.2H2O whereas other alkali metal chlorides do not form hydrates.

(iv) Lithium hydrogen carbonate is not obtained in the solid form while all other elements form solid hydrogen carbonates.

(v) Lithium unlike other alkali metals forms no ethynide on reaction with ethyne.

(vi) Lithium nitrate when heated gives lithium oxide, Li2O, whereas other alkali metal nitrates decompose to give the corresponding nitrite.


 

(vii) LiF and Li2O are comparatively much less soluble in water than the corresponding compounds of other alkali metals.

 

Similarity between lithium and magnesium

1.     The atomic radius of lithium is 1.31 Å while that of magnesium is 1.34 Å.

2.     The ionic radius of Li+i on is 0.60 Å, which is very close to that of Mg2+ ion (0.65 Å).

3.     Lithium (1.0) and magnesium (1.2) have almost similar electronegativities.

4.     Both Li and Mg are hard metals.

5.     LiF is partially soluble in water like MgF2.

6.     Both combine with O2 to form monoxides, e.g., Li2O and MgO.

7.     Both LiOH and Mg(OH)2 are weak bases.

8.     Both LiCI and MgCl2 are predominantly covalent.

9.     Both Li and Mg combine with N2 to form their respective nitrides, Li3N and Mg3N2.

10.   Both lithium and magnesium nitrates on heating evolve NO2 and O2 leaving behind their oxides.

 

Compounds of sodium

Sodium Carbonate

Sodium Carbonate or Washing Soda (Na2CO3 . 10H2O)

Solvay process

CO2 gas is passed through a brine solution saturated with NH3

 

Sodium bicarbonate is filtered and dried. It is ignited to give sodium carbonate.

 

Properties

1. Sodium carbonate crystallises from water as decahydrate which effloresces on exposure to dry air forming monohydrate which on heating change to anhydrous salt (soda-ash).

Uses:

(i) It is used in water softening, laundering and cleaning.

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

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

(iv) It is an important laboratory reagent both in qualitative and quantitative analysis.

 

Sodium chloride

 

Sea water contains 2.7 to 2.9%by mass of the salt. Sodium chloride is obtained by evaporation of sea water but due to the presence of impurities like CaCl2 and MgCl2 it has deliquescens nature. It is purified by passing HCI gas through the impure saturated solution of
NaCl and due to common ion effect, pure NaCl gets precipitated. 28% NaCl solution is called brine.

 

Sodium hydroxide

Methods of preparation

(i) A 10% solution of Na2CO3 is treated with milk of lime (Causticizing process).

 

(ii) Electrolytic process involves Nelson cell and Castner-Kellner cell.

A brine solution is electrolysed using a mercury cathode and a carbon anode. Sodium metal discharged at the cathode combines with Hg to form Na-amalgam. Chlorine gas is evolved at the anode.

The amalgam is treated with water to give sodium hydroxide and hydrogen gas.

2Na-Hg + 2H2O → 2NaOH + 2Hg + H2

Physical properties

Sodium hydroxide is a white translucent solid. It is readily soluble in water. Crystals of NaOH are deliquescent.

Chemical properties

1. It is a hygroscopic, deliquescent white solid, absorbs CO2 and moisture from the atmosphere

 

 

Sodium hydrogen carbonate

Sodium Bicarbonate or Baking Soda (NaHCO3Preparation

It is obtained as an intermediate product in Solvay process.

Properties

 

Uses

1. It is used as a constituent of baking powder which is a mixture of sodium bicarbonate, starch and potassium bitartrate or cream of tartar and in medicine to remove acidity of the stomach (as antacid).
2. NaHCO3 is a mild antiseptic for skin infections.
3 It is used in fire extinguisher.

 

Biological importance of sodium and potassium

  • A typical 70 kg man contains about 90 g of Na and 170 g of K, with only 5 g of iron and 0.06 g of copper.
  • Sodium ions are primarily found on the outside of cells, in blood plasma and in the interstitial fluid surrounding cells. They play a role in transmitting nerve signals, regulating water flow across cell membranes, and transporting sugars and amino acids into cells.
  • Potassium ions are the most abundant cations within cell fluids, where they activate enzymes, participate in the oxidation of glucose to produce ATP, and with sodium, are responsible for transmitting nerve signals.
  • There is a significant variation in the concentration of sodium and potassium ions found on opposite sides of cell membranes. For example, in blood plasma, the sodium level is 143 mmol/L, while potassium is only 5 mmol/L within red blood cells.
  • These ionic gradients are maintained by a discriminatory mechanism called the sodium-potassium pump, which operates across cell membranes and consumes over one-third of the ATP used by a resting animal and about 15 kg per 24 h in a resting human.