✅Notes on s-Block Elements:

 

✅Notes on s-Block Elements:

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. 

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)

➖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.

✅Notes on s-Block Elements:

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

✅Notes on s-Block Elements:

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.

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