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Chapter No.3
S- Block Elements
Scientific Reasons
Q.1: Why alkali and alkaline earth metals easily form cations?
Ans. Alkali and alkaline earth metals (elements of IA and llA) groups easily form cations because they have:
a) Large size
b) Low ionization potential
c) Low electron affinity
Q.2: Why Li and Be markedly differ from other members of their respective group?
Ans. Li and Be are:
(i) smaller in size.
(ii) Having higher charge densities causing strong polarizing effects on their ions.
(iii) Have high heat of hydration.
Due to these facts, they are markedly differ from members of their respective group.
Q.3: Why the elements of group IA are called "Alkali Metals"?
Ans. The name Alkali is derived from the Arabic word means "ashes". These metals are present in the ashes of plants and produce strong alkaline solution in water. That's why they are called "Alkali Metals".
Q.4: Why the elements of group llA are called "Alkaline Earth Metals"?
Ans. These metals are present produce Alkaline solution in water and widely distributed in earth's crust. That's why they are called "Alkaline Earth Metals".
Q.5: Why alkali metals have largest covalent radii?
Ans. In a period nuclear charge increases subsequently atomic size or covalent radii decreases. Alkali metals are on left most side in periodic table therefore they have largest covalent radii.
Q.6: Why the first Ionization Enthalpies of alkali and alkaline earth metals are generally low. However, Ionization Enthalpies of llA group elements are higher than IA group elements?
Ans. Ionization enthalpies of s-block elements are generally low as their outer electron is effectively shielded from nucleus by inner electrons. However, i.e. of llA group is higher due to increase in nuclear charge and decrease in size than IA group in the same period.
Q.7: Why Ionization Potential decreases from Li to Cs?
Ans. Ionization Potential is inversely proportional to atomic size. In a group atomic size increases, when we go down the group that's why ionization potential decreases.
Q.8: Why alkali metals are powerful reducing agents?
Ans. Alkali metals get easily oxidized and form cations because they have:
(i) Large size
(ii) Low ionization potential
(iii) Low electron affinity
Hence they are more powerful reducing agents.
Q.9: Why alkali metals are highly reactive?
Ans. Alkali metals are highly reactive than alkaline metals because alkali metals have low ionization potential energy, greater size less nuclear charge as compared to the corresponding alkaline earth metals.
Q.10: Why cat ionic radius is smaller than that of atomic radius? OR Na+ ion is smaller than Na atom.
Ans: Cat ionic radius is smaller than that of neutral atomic radius because on removal of an electron the nuclear charge become more effective to pull the remaining electrons and causes shrinkage in radius. That is why Na+ ion is smaller than Na atom.
Q.11: Why alkaline earth metal ion are more hydrated than alkali metal ions?
Ans: The alkaline earth metal ions are more strongly hydrated than alkali metal ions because M2+ ions (alkaline earth metal) ions have a stronger electric field than M+ ions (alkali metal ions) and hydration energy increases with decreasing size of the ion.
Q.12: Why Li+ ions are more readily hydrated than K+ ions?
Ans: Hydration energy decreases with increasing size of the ion down the group. That's why Li+ ions are more readily hydrated than K+ ions?
Q.13: Why Li+ / Li couple has exceptionally high negative electrode potential?
Ans: Li+ / Li couple couple has exceptionally high negative electrode potential because of its large value for the hydration energy which promotes oxidation of Li to form Li+.
Q.14: Why alkali and alkaline earth metals cannot be used in voltaic cell?
Ans: Alkali and alkaline earth metals cannot be used in voltaic cell because their hydration energy values are based on water as solvent which rapidly helps in oxidation.
Q.15: Why alkaline earth metals are harder than alkali metals?
Ans: Alkaline earth metals are harder than alkali metals because of the presence of divalent cations in their metallic structure produce greater binding forces.
Q.16: Why in manufacture of sodium, the two electrodes are separated by iron-gauze diaphragm?
Ans: The two electrodes are separated by a cylindrical iron gauze diaphragm which screens the graphite anode from the ring shaped iron cathode to keep away the molten sodium ion to the top of the cathode compartment from gaseous chlorine formed at the anode.
Q.17: How the given reaction is avoided during the preparation of sodium hydroxide.
6 OH + Cl2 ⟶ Cl- + ClO3 + 3H2O
Ans: Chlorine gas is collected over at Titanium anodes where as NaOH is collected in the Denuder separately. hence possible reaction between NaOH and C12.Q.18: Why Na+ ions are discharged at the cathode in preference to H+ in the manufacture of NaOH?
Ans: Na+ ions are discharged at the cathode in preference to H+ ions in the manufacture of NaOH due to high voltage of H+ ions.
Q.19: Why Zinc hydroxide is soluble in excess sodium hydroxide solutions?
Ans: In excess sodium hydroxide. Zinc hydroxide forms a complex anion tetrahydroxo zincate (II) ion.
Zn(OH)2 + 2NaOH ⟶ Na2[Zn(OH)4]
Where Na2[Zn(OH)4] is Sodium tetranydroxo zinoate.Q.20: Why Plaster of Paris is used in making plaster coats and moulds?
Ans: Plaster of Paris when mixed with water sets in 5 minutes with expansion and converts into hard mass which permits it to use in the preparation of coats and moulds.
By Sir Asif Izhar (Anees Hussain)
We can easily understand with the help of these scientific reasons it's really helpful thank u so much 🥺
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