12th Class Chemistry Chapter 2 S-Block Elements Short Question Answers

The name alkali is derived from the Arabic word means ashes Actually the metals like Na and K etc.are present in the ashes of plants.They produce strong alkaline solution in water.

The word alkaline means that they produce alkalies in water.Moreover they are widely distributed in earth’s crust so are called alkaline earths.

Sodium and potassium are the most abundant among the alkali metals.Each of them contributes about 2.4% of the earth’s crust.

Alkali metals are very reactive.They have strong tendencies to make compounds by reacting with other elements.So they are not found free in nature but in the form of ores and minerals.

Sodium is found in the form of
i) Rock salt (Halite)             NaCI
ii) Chile saltpetre                NaNO₃
iii)Natron                             Na₂CO₃.H₂O
iv)Trona                               Na₂CO₃.2NaHCO₃.2H₂O
v) Borax                               Na₂B₄O₇.10H₂O

The important ores and minerals of potassium are:-
i) Carnallite                         KCI.MgCI₂.6H₂O
ii) Sylvite                              KCI
iii) Alumite(alum stone)    K₂SO₄.Al₂(SO₄)₃.4AI(OH)₃.

The sizes increase down the group and due to decreasing polarizabilities their forces of attraction decrease.It decreases the melting and boiling points.

The alkali metals have low ionization energy value so they can give the electrons to other species very easily.In other they can decrease the oxidation number of other species and can act as reducing agents.

Elements of group II-A have low ionization potentials but greater than I-A.They can give the electrons to other species but not as efficiently as I-A.This is due to smaller sizes and greater nuclear charges.

Lithium shows crimson red sodium golden yellow potassium violet while rubidium and caesium also show violet flame tests.

Alkali and alkaline earth metals are s-block elements which mostly do not show the  colors  in their compounds,d-block elements show the colors.The compounds like KmnO₄ and K₂Cr₂O₇ show the color due to Mn and Cr but not due to K and oxygen.

The formation of peroxide and superoxide is due to greater reactivity of alkali metals.Since Na and K are more reactive than Li so they should give normal oxides,peroxides and superoxides.

The reaction of sodium with water is very fast and highly exothermic.The heat evolved compels hydrogen gas from the reaction to react with the oxygen of the atmosphere.This burning of hydrogen with oxygen is the fire.

Ionix hydrides release H^–   in the solution  which combines with other species to give its electrons and so it reduces the others.
NaH…………………………. Na⁺+H^–

Lithium is the only I-A group element which combines with nitrogen and carbon to give a nitride and a carbide.
6Li+N₂…………………….2Li₃N
4Li+C………………………Li₄C.

i) Li and Li⁺ have very small sizes.
ii) Li⁺ has high charge  density and high polarizing power.
iii) The ionization energy and electronegativity of Li are very high as compared to others.

The hydroxides of alkali metals are LiOH,NaOH KOH etc.They are all stable towards heat and this stability increases down the group.

Li₂CO₃ is sensitive to high temperature and decomposes on heating.
Li₂CO₃…………………..Li₂O + CO_2.But the carbonates of other alkali metals are stable towards heat.

LiNO₃ on heating decomposes to give Li₂O and NO₂ gas.The nitrates of sodium give NaNO₂ and oxygen.
4LiNO₃……………………..2Li₂O+3NO₂+O₂
2NaNO₃……………………….. 2NaNO₂+O₂.

Beryllium has two important minerals:
Beryl                      Be₃AI₂(SiO₃)₆
ChrysoberyI          AI₂BeO₄.

Some important minerals of magnesium are as follows:
Magnesite                   MgCO₃
Dolomite                     MgCO₃.CaCO₃.
Carnallite                     KCI.MgCI₂.6H₂O
Epsom salt                   MgSO₄.7H₂O
Soap stone (talc)        H₂Mg₃.(SiO₃)₄.7H₂O
Asbestos                      CaMg₃(SiO₃)₄.

Some important minerals of calcium are: Calcite (lime stone) CaCO3 Gypsum CaSO4.2H2O Fluorite CaF2 Phosphorite Ca3(PO4)2.

The atomic and ionic sizes go on increasing from upper the downward direction.The charge densities decrease down the group.So the heat of hydration decreases down the group.

Beryllium and magnesium do not show any color to the flame.Calcium shows brick red strontium shows crimson barium shows apple green and radium shows red color.

The reactions are as follows:
3Mg + N₂……………………… Mg₃N₂
3Ca + N₂……………………….. Ca₃ N₂.
Mg₃ N₂+6H₂O………………………..3Mg(OH)₂+2NH₃.
Ca₃ N₂+ 6H₂O………………………….. 3Ca(OH)₂+2NH₃.

The carbonates of I-A group are stable towards heat except Li₂CO₃ which decomposes to give  CO₂ just like the carbonates of alkaline earth metals.
Li₂CO₃………………… Li₂O+CO2.
CaCO₃…………………. 3Ca(OH)₂+2NH₃.

Na₂O reacts with H₂O in this solution to give a strong alkali NaOh
Na₂O+H₂O…………….. 2NaOH
Similarly other oxides of I-A are alkaline in nature.This property becomes more and more important.

Be₂C reacts with water to give CH₄ while CaC₂ reacts with water to  give C₂H₂.
Be₂C+ 4H₂O…………………. 2Be(OH)₂+CH₄.
CaC₂+ 2H₂O…………………….Ca(OH)₂+C₂H₂.

In all groups the basic characters of oxides increase down the group.Li₂O is less basic than Na₂O.Similarly BeO is ampphoteric  while the oxides of Mg Ca,Sr and Ba are basic.

Beryllium is the only member of II-A group elements which reacts with alkalies to give hydrogen.
Be+ 2NaOH………………. Na₂BeO₂+H₂

BeO + H₂SO₄…………………….BeSO₄+H₂O
BeO+2NaOH…………………….Na₂Beo+H₂O.

Actually KO₂ reacts with CO₂ to give K₂CO₃ and oxygen.So KO₂ can be used in breathing equipment for mountaineers because KO₂ absorbs  CO₂ released by the mountaineers and releases oxygen at the same time.

Na₂CO₃ is a salt made from weak acid and strong base.It undergoes hydrolysis to give NaOH and H₂CO₃.NaOH is a strong base but H₂eO₃ is a weak acid.So the  solution is alkaline.

The solubilities of oxides of alkaline earth metals increase down the group and they are converted into hydroxide BeO and MgO are insoluble in water.
Cao + H₂O……………………….. Ca(OH)₂
SrO + H₂O………………………… Sr(OH)₂
BaO + H₂O………………………..Ba(OH)₂

Be(OH)₂ and Mg(OH)₂ are insoluble in water.The hydroxides of other metals are slightly soluble.The solubilities increase down the group.

The formula of lime is Cao.When it is dissolved in water it gives Ca(OH)2 which is called lime water.Lime is found in solid state whereas the latter in the liquid state.

Milk of magnesia is the suspension of Mg(OH)₂ in water.It is used for the treatment of acidity in stomach.

Na₂CO₃is stable towards  heat just like the carbonates of other alkali metals except Li₂CO₃ which decomposes to give Li₂O and CO₂.

Anhydrous Na₂CO₃ is called soda ash while decahydrated is called washing soda.They are both prepared by Solvay’s process.

Excess CO₂ converts CaCO₃ to Ca(HCO₃)₂.It is water soluble.So milkyness of the lime water  disappears.

Lime water turns milky and CaCO₃ is produced in the lime.CaCO₃ is water insoluble so white suspension is produced and we say that solution of lime turns milky.

Lime water is Ca(OH)₂.It reacts with CO₂ of the air  and converts it into CaCO₃ which is a white solid.

They all decompose on heating to give metal oxides No₂ gas
4liNo₃…………………….2Li₂O+4NO₂+O₂
2Mg(NO₃)_{2……………………..}2MgO=4NO₂+O₂
2Ba(NO₃)₂……………………2BaO+4NO₂+O₂

BeSO₄ and MgSO₄ are fairly water soluble.CaSO₄ is slightly soluble in water.SrSO₄ and BaSO₄ are totally water insoluble.

45.What is gypsum and how it is related with plaster of Paris?
Ans:Gypsum is CaSO₄.2H₂O.When it is heated a little bit above 100ᵒC than half hydrate is produced which is called plaster fo Paris.
2CaSO₄.2H₂O…………………………….(CaSO₄)₂.H₂O+3H₂O

Metallic fog is not produced.Liquid sodium can be easily collected at 600ᵒC.The material of the cell is not attacked by the products during electrolysis.

CaCI₂ lowers the melting point of NaCI upto600ᵒC.In this way fuel is saved and the plant gets the longer life due to working at low temperature.

Hydrogen is collected at the cathode.Sodium is also collected at the cathode but reacts with water to give caustic soda.Chlorine is deposited at the anode.

Sulphur affects the growth of the plants and becomes the part of some of the proteins.It influences the chlorophyll development and improves the roots of the plants.

The formula of plaster of Paris is (CaSO₄)₂.H₂O or CaSO₄.1/2.H₂O.It means that for one molecule of CaSO₄ half water molecule is present in the crystal structure..