Revision Notes on s-Block Elements:

Alkali Metals (Group I elements of modern periodic table):

*ppm (parts per million)              

** percentage by weight

Physical Properties of Alkali Metals:

• These are soft in nature and can be cut with the help of knife except the lithium.  

• The atoms of alkali metals have the largest size in their respective periods. 

• The first ionization energy of the alkali metals are the lowest as compared to the elements in the other group. 

• The alkali metals show +1 oxidation state. 

• The alkali metals have low values of reduction potential (as shown in table-I) and therefore have a strong tendency to lose electrons and act as good reducing agents. 

• The melting and boiling points of alkali metals are very low because the intermetallic bonds in them are quite weak. 

• ll the alkali metals form ionic (electrovalent) compounds. 

• The alkali metals are good conductors of heat and electricity. 

• Alkali metals (except Li) exhibit photoelectric effect 

• The alkali metals and their salts impart a characteristic colour to flame 

Hydroxides of Alkali Metals:

a)All the alkali metals, their oxides, peroxides and superoxides readily dissolve in water to produce corresponding hydroxides which are strong alkalies.

• 2Na + 2H₂O → 2NaOH + H₂                   

• Na₂O + 2H₂O 2NaOH               

• Na2O₂ + 2H₂O → 2NaOH + H₂O₂               

• 2KO₂ + 2H₂O → 2KOH + H₂O₂ + O₂    

b) The basic strength of these hydroxides increases as we move  down the group Li to Cs.

c) All these hydroxides are highly soluble in water and thermally    stable except lithium hydroxide.

d) Alkali metals hydroxides being strongly basic react with all acids forming salts.

• NaOH + HCI → NacI + H₂O               

•  2NaOH + H₂ SO₄ → Na₂SO₄ + 2H₂O      

Halides of Alkali metals:

M₂O + 2HX → 2MX + H₂O                

MOH + HX → MX + H₂O               

M₂CO₃ + 2HX → 2MX + CO₂ + H₂O (M = Li, Na, K, Rb or Cs)

(X = F, Cl, Br or I)

a) Standard enthalpies of formation in (kJ/mol^-1)

b) Covalent Character:.

• Small cation and large anion favors covalency.

• Order: LiCl > NaCl > KCl > RbCl > CsCl & . LiI > LiBr > LiCl > LiF

• Greater the charge on the cation greater is its polarizing power and hence larger is the covalent character: Na⁺CI^- < Mg⁺²CI₂ < AI⁺³ CI₃              

• Greater the charge on the anion, more easily it gets polarized thereby imparting more covalent character to the compound formed eg covalent character increase in the order. NaCI < Na₂SO₄ < Na₃PO₄ 

c) Lattice Energies: Amount of energy required to separate one mole of solid ionic compound into its gaseous ions. 

Greater the lattice energy, higher is the melting point of the alkali metals halide and lower is its solubility in water

d) Hydration Energy: Amount of energy released when one mole of gaseous ions combine with water to form hydrated ions.

• M⁺ (g) + aq → M⁺ (aq) + hydration energy

• X^- (g) + aq → X^- (aq) + hydration energy     

Higher the hydration energy of the ions greater is the solubility of the compound in water.

The solubility of the most of alkali metal halides except those of fluorides decreases on descending the group since the decrease in hydration energy is more than the corresponding decrease in the lattice energy.

Due to high hydration energy of Li⁺ ion, Lithium halides are soluble in water except LiF which is sparingly soluble due to its high lattice energy.

For the same alkali metal the melting point decreases in the order

fluoride > chloride > bromide > iodide

For the same halide ion, the melting point of lithium halides are lower than those of the corresponding sodium halides and thereafter they decrease as we move down the group from Na to Cs.

The low melting point of LiCl (887 K) as compared to NaCl is probably because LiCl is covalent in nature and NaCl is ionic.

Anomalous Behavior of Lithium and diagonal relationship with Magnesium:

Li has anomalous properties due to

• Very small size

• High polarizing Power

Lithium show diagonal relationship with magnesium because both  elements have almost same polarizing power.

• The melting point and boiling point of lithium are comparatively high.

• Lithium is much harder than the other alkali metals. Magnesium is also hard metal.

• Lithium reacts with oxygen least readily to form normal oxide whereas other alkali metals form peroxides and superoxides.

• LiOH like Mg (OH)₂  is weak base. Hydroxides of other alkali metals are strong bases.

• Due to their appreciable covalent nature, the halides and alkyls of lithum and magnesium are soluble in organic solvents.

• Unlike elements of group 1 but like magnesium. Lithium forms nitride with nitrogen.6Li + N₂ → 2Li₃N       

• LiCl is deliquescent and crystallizes as a hydrate, LiCI2H₂O. Other alkali metals do not form hydrates.  also forms hydrate, MgCI₂.8H₂O .

• Unlike other alkali metals lithium reacts directly with carbon to form an ionic carbide. Magnesium also forms a similar carbide.

• The carbonates, hydroxides and nitrates of lithium as well as magnesium decompose on heating. 

  • Li₂CO₃ → Li₂O + CO₂

  • MgCO₃ → MgO + CO₂

  • 2LiOH → Li₂O + H₂O

  • Mg (OH)₂  → MgO + H₂O

  • 4LiNO₃ → 2Li₂O + 4NO₂ + O₂

  • 2Mg ( NO₃)₂ → 2Mg + 4NO₂ +O₂ 

• The corresponding salts of other alkali metals are stable towards heat. 

Lithium nitrate, on heating, decomposes to give lithium oxide, Li₂O  whereas other alkali metals nitrate decomposes to give the corresponding nitrite.

• 4LiNO₃ → 2Li₂O + 4NO₂ + O₂  

• 2NaNO₃ → 2NaNO₂ + O₂               

• 2KNO₃ → 2KNO₂ + O₂   

• Li₂CO₃, LiOH, LiF and Li₃PO₄   are the only alkali metal salts which are insoluble in water. The corresponding magnesium compounds are also insoluble in water.

• Hydrogen carbonates of both lithium and magnesium can not be isolated in solid state. Hydrogen carbonates of other alkali metals can be isolated in solid state.

 Sodium Hydroxide (NaOH):  

a. Properties

• NaOH is stable towards heat but is reduced to metal when heated with carbon

  2NaOH + 2C → 2Na +2CO + H₂

• 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

b) Uses

• It is used in the manufacture of paper, soap and artificial silk.

• It is used in petroleum refining.

• It is used for mercerizing cotton.

• It is used for the preparation of sodium metal and many salts of sodium.

 Sodium Carbonate (Washing soda) (Na₂CO₃): 

a) Preparation:  

Solvay process:

Carbon dioxide gas is bubbled through a brine solution saturated with ammonia and itresults in the formation of sodium hydrogen carbonate.

• NH₃ + H₂O + CO₂ → NH₄HCO₃

• NaCI + NH₄HCO₃ → NaHCO₃ + NH₄CI         

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.
2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O

b) Properties

1.  The aqueous solution absorbs CO₂ yielding sparingly soluble sodium bicarbonate.

•  Na₂CO₃ + H₂O + CO₂ → 2NaHCO­₃

 2. 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

3.  Fusion with silica, sodium carbonate yields sodium silicate.

• Na₂CO₃ + SiO₂ → Na₂SiO₃ + CO_­2

4. 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

c) Uses

• It is used for softening hard water.

• A mixture of sodium carbonate & potassium carbonate is used as fusion mixture.

• As an important laboratory reagent both in qualitative and quantitative analysis.

• It is used in paper, paints and textile industries.

• It is used for washing purposes in laundry.

• It is used in the manufacture of glass, borax, soap and caustic soda.

Alkali Earth Metals (Group II elements of modern periodic table):

Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba) and Radium (Ra).

• Physical Properties of Alkali Earth Metals:

1. Alkali earth metals are almost similar in properties to the corresponding alkali metals.

• Higher melting points than alkali metals

• Higher boiling points alkali metals

• Higher densities alkali metals

• Harder than the corresponding alkali metals

2. Atomic and ionic radii

The atomic radii as well as ionic radii of the members of the family are smaller than the corresponding members of alkali metals.

3. Ionization energy: The alkaline earth metals owing to their large size of atoms have fairly low values of ionization energies as compared to the p – block elements. However with in the group, the ionization energy decreases as the atomic number increases. It is because of increase in atomic size due to addition of new shells and increase in the magnitude of screening effect of the electrons in inner shells. Because their (IE)₁ is larger than that of their alkali metal neighbours, the group IIA metals trend to the some what less reactive than alkali metals. The general reactivity trend is Ba > Sr > Ca > Mg > Be.

4. Oxidation state: The alkaline earth metal have two electrons in their valence shell and by losing these electrons, these atoms acquire the stable noble gas configuration. Thus, unlike alkali metals, the alkaline earth metals exhibit +2 oxidation state in their compounds.

M → M+2 + 2e-

     [noble gas]
5. Characteristic flame colouration: 

Difference between alkali metals and alkali earth metals:

Chemical Properties of Alkali Earth Metals:

1. Reaction with water :

• Mg + H₂O → MgO + H₂

• or,  Mg + 2H₂O → Mg (OH)₂ + H₂

• Ca + 2H₂O → Ca(OH)₂ + H₂

2. Formation of oxides and nitrides 

• Be + O₂ (air) +Δ→ 2BeO            

• 3Be + N2 (air) +Δ → Be₃N₂

• Mg + air + Δ → MgO + Ng₃N₂

3. Formation of Nitrides 

• 3M + N2 + Δ → M3N2

• Be₃N₂  + Δ  → 3Be + N₂                           

• Ba₃N₂ + 6H₂O + Δ  → 3Ba (OH)₂ + 2NH₃

• Ca₃N₂ + 6H₂O + Δ  → 3Ca (OH)₂ + 2NH₃ 

4. Reaction with hydrogen: 

M + H₂ + Δ  → MH₂ 

Both BeH₂ and MgH₂ are covalent compounds having polymeric structures in which H – atoms between beryllium atoms are held together by three 
centre – two electron (3C - 2e) bonds as shown below:

5.  Reaction with carbon – (Formation of carbides)

When BeO is heated with carbon at 2175 – 2275 K a brick red coloured carbide of the formula Be₂C is formed

It is a covalent compound and react water forming methane.

Be₂C + 4H₂O → 2Be (OH)₂ + CH₄

6. Reaction with Ammonia: 

Like alkali metal, the alkaline earth metals dissolve in liquid ammonia to give deep blue black solution from which ammoniates [ M (NH₃)₆ ]²⁺ can be recovered.

Anamolous Behaviour of Beryllium:

• Be is harder than other members of its group.

• Be is lighter than Mg.

• Its melting and boiling points are higher than those of Mg & other members.

• Be does not react with water while Mg reacts with boiling water.

• BeO is amphoteric while MgO is weakly basic.

• Be forms covalent compounds whereas other members form ionic compounds.

• Beryllium carbide reacts with water to give methane whereas carbides of other alkaline earth metals gives acetylene gas.

  • Be₂C + 4H₂O → 2Be (OH)₂ + CH₄ 

  • Mg₂C₂ + 2H₂O → Mg (OH)₂ + C₂H₂        

  • CaC₂ + 2H₂O → Ca (OH)₂ + C₂H₂  

• Beryllium does not exhibit coordination number more than four as it has four orbitals in the valence shell. The other members of this group has coordination number 

 Diagonal relationship of Be with Al:  

• Unlike groups – 2 elements but like aluminium, beryllium forms covalent compounds.

• The hydroxides of Be, [Be(OH)₂] and aluminium [Al(OH)₃] are amphoteric in nature, whereas those of other elements of group – 2 are basic in nature.

• The oxides of both Be and Al i.e. BeO and Al₂O₃ are high melting insoluble solids.

• BeCl₂ and AlCl₃ have bridged chloride polymeric structure.

• The salts of beryllium as well as aluminium are extensively hydrolysed.

• Carbides of both the metal reacts with water liberating methane gas.

  • Be₂C + 4H₂O → 2Be (OH)₂ + CH₄       

  • AI₄C₃ + 12H₂O → 4Al (OH)₃ + 3CH₄       

• The oxides and hydroxides of both Be and Al are amphoteric and dissolve in sodium hydroxide as well as in hydrochloric acid.

  • BeO + 2HCI → BeCI₂ + H₂O       

  • BeO + 2NaOH → Na₂BeO₂ + H₂O       

  • Al₂O₃ + 6HCI → 2AICI₃ + H₂O       

  • AI₂O₃ + 2NaOH → 2NaAIO₂ + H₂O   

• Like Al, Be is not readily attacked by acids because of the presence of an oxide film.

Calcium Carbonate (CaCO₃):

It occurs in nature as marble, limestone, chalk, coral, calcite, etc. It is prepared as a white powder, known as precipitated chalk, by dissolving marble or limestone in hydrochloric acid and removing iron and aluminium present by precipitating with NH₃, and then adding ammonium carbonate to the solution; the precipitate is filtered, washed and dried.

CaCl₂ + (NH₄)₂CO₃ →CaCO₃ + 2NH₄Cl
It dissolves in water containing CO₂, forming Ca(HCO₃)₂ but is precipitated from solution by  boiling.

CaCO₃ + H₂O + CO₂ ↔ Ca(HCO₃)₂

Plaster of Paris, CaSO₄.1/2 H₂O or (CaSO₄)₂.H₂O:

It occurs in nature as gypsum and the anhydrous salt as anhydride. It is prepared by precipitating a solution of calcium chloride or nitrate with dilute sulphuric acid.

The effect of heat on gypsum or the dihydrate presents a review of interesting changes. On heating the monoclinic gypsum is first converted into orthorhombic form without loss of water. When the temperature reaches 120°C, the hemihydrate or plaster of paris is the product. The latter losses water, becomes anhydrous above 200°C and finally above 400°C, it decomposes into calcium oxide.

2CaSO₄ → 2CaO + 2SO₂­↑ + O₂­↑

The addition of common salt accelerates the rate of setting, while a little borax or alum reduces it. The setting of plaster of paris is believed to be due to rehydration and its reconversion into gypsum.

2CaSO₄. 1/2 H₂O + 3H₂O → 2CaSO₄. 2H₂O

Plaster of Paris                         Gypsum

Uses

• Plaster of pairs is used for producing moulds for pottery and ceramics & casts of statues & busts.

• It is used in surgical bandages used for plastering broken or fractured bones.

• It is also used in dentistry

Industrial uses of lime and Limestone

Uses of lime

• Calcium oxide is called lime or quick lime. It main industrial uses are

• It is used in steel industry to remove phosphates and silicates as slag.

• It is used to make cement by mixing it with silica, alumina or clay.(iii) It is used in making glass.

• It is used in lime soda process for the conversion of Na₂CO₃ to NaOH & vice versa.

• It is used for softening water, for making slaked lime Ca(OH)­₂ by treatment with water and calcium carbide CaC₂.

Uses of Slaked lime [Ca(OH)₂]

• Slaked lime is used as a building material in form of mortar. It is prepared by mixing 3 – 4 times its weight of sand and by gradual addition of water. Its sets into a hard mass by loss of H₂O and gradual absorption of CO₂ from air.

• In manufacture of bleaching powder by passing Cl₂ gas.

• In making glass and in the purification of sugar and coal gas.

• It is used in softening of hard water. 

Uses of lime stone (CaCO₃)

• It is used as building material in form of marble.

• In manufacture of quick lime.

• It is used as a raw material for the manufacture of Na₂Co₃ in solvay – ammonia process.

• Comm ercial limestone contains iron oxide, alumina, magnesia, silica & sulphur with a CaO content of 22 – 56% MgO content upto 21%. It is used as such as a fertilizer.

 
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