S-Block Elements Notes and online test for NEET/JEE

General Consideration

Francium is highly radioactive, its longest-lived isotope Fr-223 (half-life of only 21 minutes).

Sodium, potassium, magnesium and calcium ions are found in biological fluids.

Maintenance of ion balance and nerve impulse conduction

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Physical Properties

Density:           Li < K < Na < Rb < Cs

Reactivity         Li < Na < K < Rb < Cs

I.P.                  Li > Na > K > Rb > Cs

Melting point    Li > Na > K > Rb > Cs

Boiling point     Li > Na > K > Rb > Cs

Electropositive character  Li < Na < K < Rb < Cs

Photoelectric effect:  Except Li (High I.P.) all elements show this effect, Cs used in solar cell.

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Reactivity towards air

 Because of their high reactivity towards air and water, alkali metals are normally kept in kerosene oil.

 Reactivity towards water

The alkali metals react with water to form hydroxide and dihydrogen.

 Li do not react with water due to its small size and very high hydration energy.

Reactivity towards halogens:

The low solubility of LiF in water is due to its high lattice enthalpy

The low solubility of CsI is due to smaller hydration enthalpy of its two ions

Lattice Enthalpy

The amount of energy released when an ionic solid is formed from its gaseous ionic constituents.

Lattice enthalpy is a measure of the strength of the forces between the ions in an ionic solid. The greater the lattice enthalpy, the stronger the forces.

The melting and boiling points always follow the trend:

fluoride > chloride > bromide > iodide

Hydration Enthalpy

The hydration enthalpy is the enthalpy change when 1 mole of gaseous ions dissolve in water to form hydrated ion. Hydration enthalpies are always negative.

Hydration Enthalpy: The hydration enthalpies of alkali metal ions decrease with increase in ionic sizes.

Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺

Salts of Oxo-Acids

Their carbonates (M₂CO₃) and in most cases the hydrogen carbonates (MHCO₃) also are highly stable to heat.

 As the electropositive character increases down the group, the stability of the carbonates and hydrogen carbonates increases.

 Cs₂CO₃ > Rb₂CO₃ > K₂CO₃ > Na₂CO₃ > Li₂CO₃

Reducing nature and Electrode Potential

The alkali metals are strong reducing agents, readily lose electrons

 In Gas phase: I.P determine the tendency to lose electrons

 Li < Na < K < Rb < Cs

 In aqueous phase: Electrode Potential determine the tendency to lose electrons

Li > K > Rb > Cs > Na

Solutions in liquid ammonia:  The alkali metals dissolve in liquid ammonia giving deep blue solutions which are conducting in nature.

Sodium Carbonate (Washing Soda) Na₂CO₃.10H₂O

Prepared by:  Solvay Process

Solvay process cannot be extended to the manufacture of potassium carbonate because potassium hydrogen carbonate is too soluble to be precipitated by the addition of ammonium hydrogen carbonate to a saturated solution of potassium chloride. 

2NH3 + H2O + CO2 (NH4)2CO3

(NH4)2CO3 + H2O + CO2 2NH4HCO3

NH4HCO3 + NaCl NH4Cl + NaHCO3

2NaHCO3 Na2CO3 + CO2 + H2O

2NH4Cl + Ca(OH)2 2NH3 + CaCl2 + H2O

Properties:

1.Sodium carbonate is a white crystalline solid (washing soda).
2.On heating, the decahydrate loses its water of crystallisation to form monohydrate.
3.Above 373K, the monohydrate becomes completely anhydrous and changes to a white powder called soda ash.

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