Plus One Chemistry Notes Chapter 10 The s Block Elements is part of Plus One Chemistry Notes. Here we have given Kerala Plus One Chemistry Notes Chapter 10 The s Block Elements.
|Text Book||NCERT Based|
|Chapter Name||The s Block Elements|
|Category||Plus One Kerala|
Kerala Plus One Chemistry Notes Chapter 10 The s Block Elements
The elements belonging to group I (alikali metals – Li, Na, K, Rb, Cs, Fr) and group II (alkaline earth metals – Be, Ca, Sr, Ba, Ra) of the periodic table are called s-block elements.
Group I: ns1 ; Group II : ns2
Some elements of certain groups in the. second period resemble with the certain elements of the next higher group in the third period. This is called diagonal relationship.
This is due to:
- Similarity in ionic sizes or charge/radius ratio.
- Similarity in electropositive character.
- Similarity in polarising power.
Atomic and Ionic Radii
The atomic and Ionic radii of alkali metals increase on moving down the group. Hence, ionization enthalpies of the alkali metals are considerably low and decrease down the group.
Melting and Boiling Points
Melting and boiling point of alkali metals are very low.
The hydration enthalpies of alkali metal ions decrease with increases in ionic sizes.
Li+> Na+> K+ > Rb+> Cs+has maximum degree of hydration and for this reason lithium salts are moslty hydrated.
eg., UCI. 2H2O.
It goes on increasing from top to bottom due to decrease in ionisation enthalpy which is due to increase in atomic size.
The reactivity of these metals increases with their size. They bum vigorously in oxygen forming oxides. Lithium forms monoxide, sodium forms peroxide, the other metals form superoxides. The superoxide
O-2 ion is stable only in the presence of large cations such as K, Rb, Cs.
The alkali metals are strong reducing agents, lithium is the most and sodium is the least powerful. With the small size of its ion, lithium has the highest hydration enthalpy which accounts for its high negative E° value and its high reducing power.
Uses of Alkali Metals
- Li metal is used to make alloys and electro chemical cells.
- Liquid sodium metal is used as a coolant in fast breeder unclear reactors.
- Potassium has a vital role in biological systems.
- Caesium is used in devising photoelectric cells.
Alkali metals react vigorously with halogens to form metal halides of the general formula,MX
2M + X2→2MX
X = F, Cl, Br or I and M = alkali metal
Reactivity of alkali metal towards halogen increase from Li to Cs. Halides of alkali metals are ionic compounds readily soluble in water. But LiF is almost insoluble due to high lattice energy.
Anomalous Properties of Lithium
The anomalous behaviour of lithium is due to the;
- exceptionally small size of its atom and ion.
- high polarising power (ie., charge/radius ratio).
As a result, there is increased covalent character of lithium compounds which is responsible for their solubility in organic solvents.
Sodium carbonate (washing soda) Na2CO3 -10H2O
2NH3 + H2O + CO2 → (NH4)2CO3
(NH4)2CO3 + H2O + CO2 → 2NH4HCO3
NH4HCO3 + NaCI → NH4CI + NaHCO3
Sodium hydrogen carbonate give sodium carbonate on heating.
2NaHCO3 →Na2CO3+ CO2+ H2O
In this process NH3 is recovered when the solution containing NH4CI is treated with Ca(OH)2.
- It is used in water softening, washing of clothes.
- It is used in the manufacture of glass, soap, borax and caustic soda.
- It is used in paper, paints and textile industries.
Sodium chloride or Common salt (NaCI)
It is generally obtained by evaporation of sea water.
- White crystalline solid.
- Thermally stable.
- It is used for the manufacture of a large number of useful chemicals/compounds (Na2O2, NaOH, etc).
- Essential ingradient in our food.
Sodium Hydroxide (NaOH)
It is prepared by the hydrolysis of saturated brine solution (sodium chloride). Two kinds of cells are used. Castner-Kellner cell.
At anode (+) : 2CL → Cl2 + 2e
At cathode (-) : Na+ + 1e → Na
Na + Hg → NaHg
Atomic & Ionic Radii of Alkaline Earth Metals
Smaller atomic and ionic radii when compared to alkali metals. It is due to the increase in nuclear charge. Within the group, the atomic and ionic radii increase with increase in atomic number.
Ionisation enthalpy of alkaline earth metals
Low value and decrease down the group from Be to Ra.
The 1st I.E of alkaline earth metals are higher than those of alkali metals due to its smaller size.
The 2nd I.E of alkaline earth metals are smaller than that of alkali metals.
Due to large -ve value of their reduction potential, alkaline earth metals are strong reducing agent less than that of alkali metals. Reducing nature of Be is due to its large hydration energy, small size and high value of atomization enthalpy.
Uses of alkaline earth metals
- Be is used in the manufacture of alloys.
- Mg powder is used in flash powders and bulbs, incendiary bombs and signals.
- Milk of magnesia (a suspension of Mg(OH)2 in water) is used as antacid in medicine.
Halides of alkaline earth metals
BeX2 is covalent while others are ionic in nature. BeCI2 has a chain structure in the solid state. In the vapour phase BeCI2 tends to form a chloro bridged dimer.
Biological importance of Mg and Ca
The main pigment for absorption of light in plants is chlorophyll which contains Magnesium and Calcium occurs generally in animal bones as phosphate Ca3(PO4)2.Calcium ions play an important role in muscle contraction.
Setting of Cement
When the cement is mixed with water, it absorbs water to form a gelatinous mass which sets to a hard mass which is very resistant to pressure. This is known as the setting of cement. This involves a number of complex hydration and hydrolysis reactions leading to the formation of -Si-O-Si- and Si-O-AI- chains.
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